Crop and Pest Management Guidelines

A Cornell Cooperative Extension Publication
34119

3.1 Pest Information - Diseases

Several important insects and diseases that occur in New York and Pennsylvania vineyards are described below to help growers manage these pests with practices and pesticides appropriate for their production systems.

34239

ANGULAR LEAF SCORCH

ANGULAR LEAF SCORCH was first described in 1985. Symptoms of this fungal disease are similar to those of rotbrenner, a disease of grapevines found in the cool grape-growing regions of Europe, which is caused by a very closely related fungus. Angular leaf scorch occurs sporadically, and is most likely to become a problem in years when high rainfall occurs between bud break and early summer, especially if this should happen in consecutive years. Riesling appears to be a particularly susceptible cultivar.

Disease symptoms occur mainly on the leaves and first appear as faint chlorotic spots. As these lesions grow larger, they change from yellow to reddish-brown and the margin often becomes sharply defined (depending on the cultivar, the margin may be yellow, red, or absent). Lesions are confined by major veins, becoming "angular" or wedge-shaped. They eventually kill the infected tissue, sometimes crossing the major veins in the process and often causing diseased leaves to fall prematurely.

The fungus survives winter in infected leaves on the vineyard floor. Mature spores are ready for discharge in spring when grape buds begin to grow. During rainfall, spores are released into the air from fruiting structures, and susceptible tissue is infected.

Cultural practices that increase air circulation through the canopy can shorten periods of leaf wetness that favor disease development. Destruction of leaf litter by cultivation, before bud break, can also reduce disease pressure. Where needed, effective fungicides applied from the 3-inch stage and continuing through fruit set will provide significant control. Although there are no specific labels for the control of this disease, mancozeb products (e.g., Dithane, Penncozeb) applied according to label directions to control Phomopsis, black rot, and downy mildew also have provided good control of angular leaf scorch in commercial experience. In Europe, the strobilurin fungicides (Abound, Flint, Pristine, Sovran) have provided good control of the closely related disease, rotbrenner. However, experience with angular leaf scorch is lacking. The DMI fungicide, difenoconazole (Revus Top, Quadris Top; CAUTION: Quadris Top causes injury to Concord and some other cultivars) is labeled for control of rotbrenner and should provide significant post-infection control of this disease in addition to moderate protective activity (excellent protective activity for Quadris Top). See Table 3.1.1 for varietal susceptibility to this disease.

34240

ANTHRACNOSE

ANTHRACNOSE is a disease that occurs most commonly in years that are wet during the first half of the growing season, with damage typically limited to a few highly susceptible cultivars. In NY/PA, most outbreaks historically occurred on Vidal Blanc and a few seedless table grape varieties, especially Reliance. In recent years, however, there have been regular outbreaks on some of the newer cold-hardy cultivars that are gaining in popularity and expanding the geographical range of grape production. Marquette appears to be particularly susceptible, although Frontenac and La Crescent also have been affected. Some older cold-hardy cultivars (Edelweiss, Esprit, Brianna, St. Pepin, and Swenson White) also can be problematic. In some Midwestern states, Concord, Catawba, and Leon Millot have been reported as encountering problems, although such occurrences are rare in NY and PA. Symptoms occur on leaves, green shoots, and clusters. On leaves, numerous small, circular brown spots appear which later turn gray in the center and develop dark brown to black margins. In severe attacks, lesions may coalesce and cause large dead zones, distortion of the leaf blade, and eventually death of the entire leaf. Infected shoots develop dark, noticeably sunken lesions, typically on the first several internodes near the base of the new shoot. These lesions resemble the internode lesions typical of Phomopsis cane and leaf spot but they usually are more aggressive, expanding farther along the shoot and deeper into its center than those caused by Phomopsis. On berries, spots approximately 0.25-in in diameter develop, with whitish-gray centers surrounded by reddish brown to black margins, sometimes producing an appearance that superficially resembles a bird's eye. Severely affected berries may shrivel and dry into mummies.

The fungus overwinters primarily on infected canes, although the previous year's berries can also be a source. In spring, spores are produced from the fungal structures on these sources and are dispersed by splashing raindrops to young, susceptible tissues, where they cause infection if wetness persists for a sufficient length of time. Temperatures in the mid-70s to mid-80s Fahrenheit (25-30°C) are optimal and require only 3 to 4 hours of leaf wetness for infection to occur. However, infection can also occur across a much wider range of temperatures, including those that typically prevail during the early growing season in upstate NY, if it remains wet for long enough. Additional spores, which also are splash dispersed, are produced from new infections, and these can rapidly spread the disease through multiple repeating cycles of new infection and additional spore production. Hence, outbreaks occur most frequently in years with multiple rain events early and mid-season. Young tissues are most susceptible, becoming resistant as they are mature; for example, berries become relatively resistant by about 7 weeks post-bloom.

Diseased canes should be pruned during the dormant season and removed from the vineyard or destroyed. If numerous infected berries remain on the vineyard floor, the spores originating from them can be largely neutralized by covering the berries with soil through cultivation or, if practical, covering them with mulch. Early-season sprays of mancozeb, captan, or ziram targeted against Phomopsis also provide significant control of anthracnose, although this latter disease is not listed as a target on most labels. Some DMI (Group 3) fungicides, e.g., difenoconazole (Revus Top, Quadris Top, and Inspire Super), myclobutanil (Rally), and tetraconazole (Mettle) are specifically labeled for anthracnose control. A "delayed dormant" application of lime sulfur can be useful in vineyards where the disease has become established and problematic to control and/or in "organic" vineyards where traditional fungicides are not used. This treatment limits the production of infectious spores from overwintered cankers but does not protect new growth from any spores that survive the application or are produced after its effects wear off.

34120

Table 3.1.1 Relative disease susceptibility and sensitivity to sulfur and copper among grape varieties1.

Disease susceptibility or chemical sensitivitya

Variety

BR

DM

PM

Bot

Phom

Eu

CG

ALS

Sc

Cd

Arandell

+++

+

+

+

+++

?

++

?

Yes

 ?

Aromella

+

++

++

+

?

?

++

?

No

 ?

Aurore

+++

++b

+++

+++

++

+++

++

+++

No

 ++

Baco noir

+++

+

++

+++

+

++

++

++

No

?

Cabernet Franc

+++

+++

+++

+

?

?

+++

?

No

+

Cabernet Sauvignon

+++

+++

+++

+

+++

+++

+++

?

No

+

Canadice

+++

+

+

++

?

?

++

++

Slight

?

Cascade

+

+

++

+

++

++

+

?

No

?

Catawba

+++

+++

++

+

+++

+

+

+

No

++

Cayuga White

+

++

+

++

+

+

++

++

No

+

Chambourcin

++

++

++

++

++

?

++

?

Yes

?

Chancellor

+

+++

+++

+

+++

+++

++

+++

Yes

+++

Chardonel

++

++

++

++

?

?

++

++

No

?

Chardonnay

+++

+++

+++

+++

+++

+++

+++

++

No

+

Chelois

+

+

+++

+++

+++

+++

++

+++

Slight

+

Concord

+++

+

++

+

+++

+++

+

++

Yes

+

Corot noir (NY70.0809.10)

++

++

+

+

+++

?

+

?

No

?

DeChaunac

+

++

++

+

+++

+++

++

+++

Yes

+

Delaware

++

+++b

++

+

+++

+

+

+

No

+

Dutchess

+++

++

++

+

++

+

++

+

No

?

Elvira

+

++

++

+++

+

+

+

++

No

++

Einset Seedless

+++

+++

++

+

?

?

+

?

No

?

Foch

++

+

++

+

?

+++

+

+

Yes

?

Fredonia

++

+++

++

+

++

?

+

+

No

?

Frontenac

++

+

++

+

?

?

+

?

Slight

?

Frontenac gris

++

+

++

+

?

?

+

?

Slight

?

Gewurztraminer

+++

+++

+++

+++

?

?

+++

+

No

+

GR7

+

++

++

++

+

+

+

?

No

?

Gruner Veltliner

+++

+++

+++

?

?

?

+++

?

No

+

Himrod

+++

++

++

+

?

?

?

+

No

?

Ives

+

+++

+

+

?

++

+

+

Yes

?

La Crescent

++

++

++

+

?

?

+

?

No

?

Marquette

+

+

++

+

+

?

+

?

Slight

?

Marquis

+++

++

++

+

+

?

?

?

Slight

?

Melody

+++

++

+

+

?

?

+

+++

No

?

Merlot

++

+++

+++

++

+++

++

+++

?

No

++

Moore's Diamond

+++

+

+++

++

?

++

?

?

Slight

?

Niagara

+++

+++

+

+

+++

+

++

+

No

+

Noiret (NY73.0136.17)

+

++

+

+

+++

?

++

+++

No

?

Pinot blanc

+++

+++

+++

++

?

?

+++

?

No

+

Pinot gris

+++

+++

+++

+++

?

?

+++

+++

No

+

Pinot noir

+++

+++

+++

+++

?

?

+++

++

No

+

Riesling

+++

+++

+++

+++

++

++

+++

+++

No

+

Rosette

++

++

+++

+

++

++

++

++

No

+++

Rougeon

++

+++

+++

++

+++

+

++

+++

Yes

+++

Sauvignon blanc

+++

+++

+++

+++

?

+++

+++

?

No

+

Seyval

++

++

+++

+++

+++

+

++

++

No

+

Steuben

++

+

+

+

?

?

+

++

Yes

?

Traminette

++

++

+

++

?

?

+

?

No

?

Valvin Muscat (NY62.0122.01)

++

+

++

+

?

?

+

?

No

?

Vanessa

+++

++

++

+

+

?

+

?

No

?

Ventura

++

++

++

+

+

?

+

+++

No

?

Vidal 256

+

++

++

+

+

+

++

+

No

+

Vignoles

+

++

+

+++

+++

++

++

++

No

?

 Key:

 + Slightly susceptible or sensitive

 ++ Moderately susceptible or sensitive

 +++ Highly susceptible or sensitive

 No Not sensitive

 ? Relative susceptibility or sensitivity not established

 1The relative ratings in this chart apply to an average growing season in NY and PA. Under conditions favorable for disease development, any given variety may be more severely affected.

 a.BR=Black rot, DM=Downy mildew, PM=Powdery mildew, Bot=Botrytis, Phom=Phomopsis, Eu=Eutypa, CG=Crown gall, ALS=Angular Leaf Scorch, S=Sulfur, C=Copper

 b. Berries only weakly susceptible

 c. Slight to moderate sulfur injury may occur even on tolerant varieties when temperatures are 85F or higher during or immediately following the application

 d. Copper is most likely to cause injury when applied under slow-drying conditions (cool or very humid).

34241

BITTER ROT and RIPE ROT

BITTER ROT and RIPE ROT are two diseases that are common in states to the south of PA where temperatures are consistently warmer, although they also occur sporadically in NY and PA, especially in the southern regions of our two states. The two diseases are caused by different fungi, but both have similar biologies and respond to the same management practices. Bitter rot is active at somewhat lower temperatures than ripe rot and is the more regular threat in the mid-Atlantic region, although their geographical ranges overlap. In NY, bitter rot has been most problematic on Long Island, primarily in years with wet summer and pre-harvest periods (particularly on Chardonnay). It has also been problematic in southeast PA in some years, and both diseases occur further north on occasion. Usually, bitter rot symptoms first develop after veraison, when the causal fungus moves into the berry from the infected berry stem and turns the diseased portion brown (on white varieties) or a dull purple. Once the berry is completely rotted, it becomes covered with numerous prominently raised black pustules (the fungal fruiting bodies, called acervuli). Within a few days, diseased berries soften and may drop; others shrivel into mummies that resemble those caused by black rot and Phomopsis. This late onset of symptom appearance is one feature that distinguishes bitter rot from black rot (virtually all black rot infections should be apparent by veraison); others include (i) the appearance of the fungal fruiting bodies on infected fruit (those of black rot are relatively small, round and uniform in size, whereas those of bitter rot are larger and much more pronounced, providing a bumpy texture to the berry); (ii) the tendency of black-rot infected fruit to remain attached to the berry stem when pulled gently, whereas those with bitter rot are more easily detached; and (iii) the tendency of fruit infected with bitter rot to leave hands sooty black if handled when wet, whereas those infected with black rot will leave hands clean. Berries infected with Phomopsis also tend to first appear during the preharvest period but are detached very easily when pulled, and the black fruiting bodies produced on rotten berries are relatively few and inconspicuous. Also, significant Phomopsis fruit rot typically occurs in association with significant visible infections of the cluster stem and of the shoots and petioles (leaf stems) near the bottom three to five leaf positions on shoots that bear diseased berries. An absence of these additional symptoms when multiple berry infections are present suggests that Phomopsis is not the cause.

Symptoms of ripe rot do not develop until after veraison and become increasingly prevalent by harvest (hence, the disease name). Infected fruit initially develop circular, reddish brown lesions on their skin, which eventually expand to affect the entire berry. Under humid conditions, small "dots" of slimy, salmon-colored spores may develop across the rotten berry, and serve to spread the disease to healthy fruit if rains continue. Infected fruit shrivel and mummify, and may either remain attached or fall to the ground. No foliar symptoms are produced.

Both bitter rot and ripe rot are favored when abundant, warm rains occurr between veraison and harvest (72° to 77°F while berries remain wet is optimum for infection by bitter rot, 77° to 86°F while wet is optimum for ripe rot), although the first infections typically are initiated much earlier and remain "dormant" (latent) until berries begin to ripen.

The bitter rot and ripe rot fungi overwinter in mummified fruit, infected berry stems, and dead bark and cankers. Spores are produced from these sites in the spring and are distributed by splashing and blowing rain. After flowering, some spores of the bitter rot fungus infect the young berry stems; these remain latent (dormant) until veraison, then resume growth into the berry and rot it. When the berries mature, the fungus resumes growth and advances into them, causing the fruit to rot. Similarly, the ripe rot fungus may infect fruit at any stage of their development, but these infections remain latent until the berries begin to ripen. Once berries have become rotted in the post-veraison/pre-harvest period, spores produced upon them can spread the disease rapidly during warm rainy periods, with fruit becoming increasingly susceptible to infection as they continue to ripen.

Both diseases frequently are controlled in the early- to mid-summer by fungicide sprays targeted against other diseases, such as downy mildew and black rot. Captan and the strobilurins are very effective (as is mancozeb while it can still be applied), but not all fungicides used for control of other diseases will provide control of bitter rot and ripe rot; check current recommendations to determine which products may be appropriate. However, with the exception of Flint and Pristine, most fungicides used later in the season for Botrytis management provide little control of bitter rot or ripe rot. Thus, exclusive use of such Botrytis-specific fungicides for fruit rot control during wet preharvest seasons can lead to outbreaks of bitter and/or ripe rot in regions where they are not common and, therefore, are not consciously managed. Sprays targeted against these diseases may be needed in the late season if the weather is warm and wet, especially if they are observed in the vineyard or have occurred there in the past. In southerly regions where bitter/ripe rot are consistent problems, it is typically necessary to apply protectant fungicides on a 2-week schedule from bloom until harvest, except during periods of drought. Fruit are especially vulnerable to both diseases in their final stages of ripening; thus, pre-harvest sprays are particularly important if the diseases are active. French-American hybrids are generally more resistant to bitter rot than are cultivars of V. vinifera, within which Chardonnay is among the more susceptible of the cultivars widely grown in more northerly climes.

34242

BLACK ROT

BLACK ROT is one of the most potentially serious diseases of grapes in the eastern United States. Although relatively easy to manage with the aid of conventional fungicides, it can cause substantial crop loss in wet years if these are not used or if they are used incorrectly. Fruit rot is the most damaging phase of the disease, but all green tissues of the vine are susceptible to infection.

Leaf symptoms are very characteristic, consisting of relatively small, brown circular lesions surrounded by distinct dark margins; black, pimple-like fruiting bodies ("pycnidia") are scattered within these spot-like lesions. Under heavy disease pressure, black, elongated lesions may develop on petioles (leaf stems), causing affected leaves to wilt and drop; large, black, elliptical lesions may also develop on green shoots under these severe conditions, and contribute to breakage by wind. However, such infections are uncommon in conventionally managed commercial vineyards, and the disease is most frequent and damaging on berries. These appear chocolate brown when first infected, but soon become dark brown with numerous black, pimple-like pycnidia on the surface. They eventually shrivel into hard, black raisin-like mummies, most of which remain firmly attached to the berry stem.

The black rot fungus overwinters primarily in mummified fruit on the vineyard floor or in mummies retained within the vine. It can also overwinter within cane lesions should these develop. Rain triggers the release of infective spores from all sources, and infection occurs if susceptible tissues remain wet for a sufficient length of time, which depends on temperature (Table 3.1.2). Spores within cane lesions are available for infection starting at bud break. However, the majority of overwintering spores in most vineyards (i.e., those within mummified fruit on the ground, see caution below) first become available about 2-3 weeks after bud break, reach peak levels about 1-2 weeks before bloom, and are largely depleted soon after fruit set, depending on the season. However, in dry spring seasons when only a few rains occur, the fungus does not discharge all of its spores as early as usual, and significant spore discharge may extend several weeks beyond bloom if this is when rains finally develop.

34121

Table 3.1.2 Hours of leaf wetness required for black rot infection to occur, at various temperatures following a rain

Temp (°F)

Hoursa

50

24

55

12

60

9

65

8

70

7

75

7

80

6

85

9

90

12

Source: R. A. Spotts. The Ohio State University

a. Hours of continual wetness from rain

CAUTION: Most mummified fruit remain firmly attached to the berry stems and are not removed during machine harvesting. If these are not dropped to the ground during dormant pruning operations, they will produce large numbers of spores within the canopy throughout the period of berry development the following year. Research and experience have shown that this extended period of high spore production during the period of maximum berry susceptibility, combined with the closeness of the spores to newly developing berries, greatly increases the pressure for new berry infections. Therefore, removal of mummies from the canopy is a major component of black rot management programs, and is critical in "organic" or other production systems where conventional fungicides are not used.

New fruiting bodies (pycnidia) develop within the lesions caused by current season infections and begin releasing a new generation of spores about 2 weeks after infection first occurs. Unless protection is provided, these spores then cause additional infections under favorable rainy conditions. Such repeating rounds of new spore production, release and re-infection are responsible for disease spread, and are the cause of most economic loss when it occurs. Fruit are highly susceptible to infection for the first 2-3 weeks after cap fall. They become progressively less susceptible as they continue to develop, finally becoming highly resistant about 5-8 weeks after bloom, depending on the variety and year. In general, "Concord" fruit appear to become resistant about 1-2 weeks earlier than those of V. vinifera varieties. Thus, the most critical time to control berry infections is during the first few weeks after the start of bloom.

Removal of mummified clusters from the canopy during pruning significantly reduces disease pressure for the coming season; burying mummies on the ground before or soon after budbreak, by cultivation or covering them with mulch when practical, also can significantly reduce inoculum if disease was severe the previous season. All spray products currently approved for "organic" production are weak against black rot, although copper has moderate efficacy if applied very regularly. Thus, black rot is often the "Achilles heel" of organic grape production in eastern viticulture and organic growers should pay strict attention to the above sanitation procedures, which are their most important defenses against this disease. Organic growers may also benefit from an application of lime sulfur (calcium polysulfide) just before bud break if black rot shoot lesions have developed in recent years. Cultural practices that open the canopy also are beneficial because they promote drying and improve spray coverage. See Table 3.1.1 for varietal susceptibility to black rot.

Spray Timing: Fungicide applications once were recommended to begin at about 3- to 5-inch shoot growth and continue at regular intervals of 10-14 days until veraison. However, most disease control is provided by the immediate prebloom and first postbloom fungicide sprays, and virtually complete control has been provided in most vineyards when these sprays have been followed by a second postbloom application. A second post-bloom spray is potentially critical if black rot symptoms are visible on either leaves or fruit at that time, and it should be considered prudent even in most apparently "clean" vineyards if this portion of the season is reasonably wet. If significant black rot developed during the previous year, or if the vineyard has a history of consistent black rot development, an additional early spray, 2 weeks before the prebloom application, may be beneficial, particularly under wet conditions. Because damaging levels of black rot usually result from spread within the cluster, sprays should continue through 4 to 5 weeks postbloom if more than a trace level of berry rot is present and weather conditions are suitable for infection (see Table 3.1.2).

Several of the DMI (Group 3) fungicides provide excellent control of black rot, including the difenoconazole products (Revus Top, Quadris Top, and Inspire Super); the flutriafol products (†Rhyme, †Topguard EQ); Mettle; Rally; and the various tebuconazole products. In addition to providing some residual protective activity, all of these materials provide excellent post-infection activity when good spray coverage is provided, although only Rally and Mettle are specifically labeled for post-infection application (up to 72 hours after the start of the infection event). Research indicates that their effective period of postinfection activity is one to several days longer than this, although delaying postinfection applications beyond 72 hr should be employed only on an emergency basis, not as a deliberate strategy. The strobilurin fungicides (Abound, Flint, Pristine, Sovran, Quadris Top, †Topguard EQ) are excellent protectants but provide only very limited postinfection activity; to date, loss of activity due to resistance development has NOT been observed for black rot, unlike other diseases against which the strobilurin fungicides have been used. Mancozeb and ziram are good protectants but are more subject to wash-off than the strobilurins, so must be reapplied more frequently during high-rainfall conditions, when black rot pressure also is typically maximum. Fungicides other than those discussed above should not be relied upon for black rot management. The one exception to this is a new product, ^Miravis Prime (discussed in the section on fungicides), that derives its black rot efficacy from a succinate dehydrogenase inhibitor, pydiflumetofen (aka adepidyn, Group 7). Copper fungicides are the most effective organically-approved materials, but must be applied frequently (e.g., 1-wk intervals) under even moderately high disease pressure conditions.

34243

BOTRYOSPAHERIA DIEBACK

BOTRYOSPAHERIA DIEBACK (see "Eutypa Dieback")

34244

BOTRYTIS

BOTRYTIS is a fungus that causes a bunch rot of berries and also may blight blossoms, leaves, and shoots. The bunch rot phase of the disease can cause severe economic losses, particularly on tight-clustered French hybrid and Vitis vinifera cultivars and clones. Ripe berries are susceptible to direct attack and are particularly susceptible to infection through wounds such as those caused by insects, hail, or rain cracking. Once established, infections can spread rapidly throughout the cluster during the preharvest period (especially if berries are tightly compressed), causing extensive loss in yield and quality. This disease can be distinguished from other causes of bunch rot by the characteristic masses of gray "fuzzy" spores produced by the Botrytis fungus on infected plant parts, especially during humid weather.

The fungus overwinters in debris on the vineyard floor or on the vine. Old cluster stems (rachises) are a particularly important source of carry-over between seasons. Spores are produced throughout the growing season, although their numbers are reported to be greatest near bloom and after veraison. Production of spores and subsequent infection by them are greatly favored by prolonged periods of wetness or very high humidity, particularly at moderate temperatures (59-77°F).

The Botrytis fungus can attack highly succulent young growth (new leaves, tender shoots, young emerging clusters) during prolonged wet periods but it most commonly infects injured or senescing tissues. Hence, cluster infections usually occur as blossom parts wither, as fruit ripen, or through berry wounds. Wounds caused by rain cracking, berry splitting in tightly compressed clusters, and the grape berry moth are particularly common sites of infection. Under wet conditions, the fungus can infect aging blossom parts or the scar left from the falling cap at the end of bloom, and then grow into the newly developing berry. Such infections typically remain latent (dormant) until veraison or later, then can become active and rot the berry as the fruit begins to ripen.Although direct losses from these early infections appear to be modest, they often provide a starting point for sudden and significant spread of the disease within clusters if wet weather occurs during the pre-harvest period. Initial infections can also develop from "blossom trash" - old blossom parts that becomes infected during bloom and are subsequently trapped within the cluster as they fall - eventually growing into the ripening berries that they contact as these become susceptible near harvest. Berries infected by powdery mildew between fruit set and bunch closing can serve as yet another starting point for a Botrytis epidemic; hence, good control of powdery mildew during this period is an important component of a good Botrytis management program.

Botrytis management is best accomplished through a program that integrates cultural and chemical components. In fact, perhaps more than for any other common disease, consistent control of Botrytis requires the conscientious use of cultural management practices even when the best fungicides are applied. Any practice that improves air circulation and thereby reduces humidity within the canopy is very beneficial. Such practices include site selection to avoid fog pockets and proximity to heavily wooded areas, which restrict sun exposure and air movement; management of canopy densities through pruning, shoot positioning, and selectively removing leaves in the cluster zone immediately after fruit have set; removal of any old rachises (cluster stems) from the canopy during pruning, as these are important sources of Botrytis spores during the new season; and avoiding excessive nitrogen fertilization. Loose clusters can dramatically reduce the spread of Botrytis by limiting berry-to-berry contact, and the use of clones (e.g., the 'Mariafeld' clone of cv. Pinot noir) or viticultural techniques that provide loose clusters can aid in its control. Well-timed sprays of an effective fungicide also can be important in many years. Multiple spray timing trials conducted in New York over the past quarter century have shown that the optimum timing of fungicide applications varies among seasons and is largely dependent on weather patterns in any given year. Potentially important times are (i) late bloom, to protect against the initial establishment of latent infections; (ii) pre-bunch closure, the last opportunity to provide coverage of all berry and rachis surfaces; (iii) veraison, as berries transition into the ripening period with an increased susceptibility to new infections; and (iv) pre-harvest, to protect against new infections and limit the spread of any infections already active during this period of maximum susceptibility. Experience with particular varieties and blocks, the current presence or absence of active disease as determined through scouting, and a recognition of climate effects on disease development are all useful factors in assessing the desirability of a Botrytis fungicide spray at one or more of the timings referenced above.

A large number of fungicide products representing seven different fungicide classes (FRAC 2, 7, 9, 11, 12, 17, and 19) are now labeled for control of Botrytis and have shown efficacy in local trials: Elevate, Endura, Flint/Flint Extra, Inspire Super, ^Intuity, *NY†Luna Experience, *NY†Luna Sensation, ^Miravis Prime, Ph-D and OSO/Tavano (polyoxin-D), Pristine, Rovral and other iprodione products (e.g., Meteor), Scala, †Switch, and Vangard. Several of these contain the same active ingredient found in one or more of the others - e.g., Inspire Super/Vangard/†Switch and Endura/Pristine - and most are related to one or more other products in the same fungicide class (resistance group). The Botrytis fungus can develop resistance to all of these materials, and growers should not rely on a single compound or class of compounds for control over time. Note that for rotational purposes, Inspire Super, Vangard, Scala, and †Switch all contain an active ingredient in the same fungicide class (Group 9); Flint/Flint Extra, ^Intuity, and Pristine contain an active ingredient in the same class (Group 11); Switch and ^Miravis Prime contain an active ingredient in the same class (Group 12); and Pristine, Endura, *NY†Luna Sesation, and *NY†Luna Experience contain an active ingredient in the same class (Group 7). Similarly, some individual active ingredients are sold under various trade names, e.g., iprodione (Rovral, Meteor, others) and polyoxin-D (Ph-D, Oso/Tavano). Note also that for Endura, Flint/Flint Extra, *NY†Luna Experience, and Pristine, the labeled rate for Botrytis control is higher than that for control of other diseases against which these materials are active. Lastly, there are two biopesticides that have provided poor to good (Botector) and good (Fracture) control of Botrytis in a number of NY and PA fungicide trials. Fracture also provided moderate to good control of powdery mildew, a disease that can create entry points for invasion by Botrytis and other decay organisms near harvest.

34245

CROWN GALL

CROWN GALL is a bacterial disease whose characteristic symptom is fleshy galls produced mostly on the lower trunk, but which may form anywhere on the trunks where injuries occur. Large galls may develop rapidly and completely girdle young vines in one season. Galled vines frequently produce inferior shoot growth, and portions of the vine above the galls may die. Current-season galls are first apparent in early summer as white, fleshy, callus growth. Galls turn brown by late summer, and in the fall become dry and corky. The crown gall bacterium is systemically present in the vast majority of grapevines but seldom causes disease unless the vine is injured. Budding and grafting cause injuries that occasionally elicit disease development at those wounding sites, as does "tractor blight", but freeze injury is by far the most important factor responsible for crown gall development in the Northeast. Therefore, management practices that minimize the risk of cold injury are currently the only practical techniques for limiting disease development. These include careful site selection for cold-sensitive cultivars and cultural practices that promote winter hardiness. Hilling above the union of grafted vines protects buds from freezing and ensures the development of new scion shoots that may be needed for trunk renewal. The above practices, combined with the use of multiple-trunk training systems and regular replacement of diseased or dead trunks with renewals, help to manage the disease at commercially viable levels.

34246

DOWNY MILDEW

DOWNY MILDEW is caused by a fungus-like organism that can infect berries, leaves, and young shoots. Leaf lesions appear as yellow or reddish-brown areas on the upper surface, with corresponding white, downy, or cottony growth of the causal organism directly opposite on the lower surface (note that downy mildew growth appears only on the lower leaf surface and looks cottony, whereas powdery mildew growth can occur on both sides of the leaf and looks more like baby powder, especially on the upper surface). Leaf lesions in the late spring and early summer are often circular or "blotchy", whereas those in late summer or early fall are often smaller and restricted to areas between or along the veins. Leaf lesions become brown and dead with age, and severely infected leaves fall prematurely. Young, infected shoots and cluster stems may curl and are characteristically covered with the white, "downy" growth of the pathogen on mornings following rain or dew the night before. Berries on infected cluster stems may fail to set or can turn brown and eventually shrivel, depending on the time of infection. Berries that are directly infected soon after fruit set may become entirely covered with the fuzzy white growth of the pathogen similar to that on diseased leaves and shoots. Cluster infections that occur later in the season cause berries to remain hard, with a mottled light green to brown appearance on white-fruited cultivars whereas those on purple-fruited cultivars turn red prematurely.

Frequent rainfall and high humidity are the most important environmental factors promoting downy mildew epidemics. The downy mildew organism overwinters as dormant spores within infected leaves on the vineyard floor or (more commonly) within the upper soil layer, and typically becomes active in the spring about 2-3 weeks before bloom. Infective spores are produced during rainy periods of approximately 0.1" or more when temperatures are above 52°F, and these are splashed from the soil onto susceptible green tissues to cause the season's first ("primary") infections. (Note that inoculum for such early-season infections comes strictly from within the vineyard.) Epidemic disease development can then result from repeated cycles of new "secondary" infections, which are caused by new spores produced within the white "downy" growth visible on diseased tissues. These spores are produced only at night when the relative humidity is extremely high (>95%; dew usually forms in the morning). They can be blown on air currents to cause infection when they land on susceptible tissues if those remain wet for just a few hours thereafter. (Note that such disease spread can also originate from nearby vines outside the vineyard.) The generation time or latent period for the downy mildew organism (the time required from spore germination and infection until the production of a visible infection bearing a new "crop" of secondary spores) is only 4 to 5 days at optimum temperatures in the mid- to upper-70s (°F), allowing explosive disease development during extended or recurring periods of muggy weather punctuated by periodic rain showers. On some cultivars, including all V. vinifera varieties, this can be particularly destructive during a several-week period from prebloom through early berry development, when fruit clusters are highly susceptible to infection. Young leaves remain highly susceptible to infection so long as they continue to be produced, although even older leaves can become diseased under high-pressure conditions. Poorly controlled leaf infections can cause extensive defoliation in wet years, limiting both fruit ripening and vine winter hardiness. Winter kill of buds or even entire vines is not uncommon when spraying stops too early on susceptible varieties in a bad downy mildew season and a very cold winter event follows. Disease can develop at a wide range of temperatures, from the low 50s to the mid-80s (°F), although the rate of spread is slower while temperatures are at these edges of this range.

Any practice that improves air circulation and speeds drying within vine canopies will help to limit downy mildew. However, properly timed fungicides are still necessary for acceptable disease management on susceptible cultivars. Mancozeb, captan, and copper fungicides provide good protection when applied at 7-10 day intervals (tighten spray intervals when very rainy - wash-off plus high disease pressure - or during periods of rapid shoot growth). Ridomil is a systemic fungicide that provides outstanding control of downy mildew, but it is not effective against any other disease of grapes. (The mancozeb or copper that is prepackaged with commercial Ridomil formulations will provide some benefit against other diseases; however, copper is only moderately effective against other diseases and the rate of mancozeb in the MZ 58 formulation is low, i.e., the 2.5-pound rate of Ridomil MZ 58 provides the same amount of mancozeb as 2 pounds of a standard mancozeb product such as Dithane or Penncozeb). Ridomil also has significant post-infection activity, but it is highly prone to resistance development and should be applied no more than once (ideally) or twice in any particular season, in rotation with other DM products. Products representing four other fungicide classes new to grapes have been registered within the past few years, and all have provided good to excellent results when applied at 14-day intervals. These are Revus and Revus Top (mandipropamid); *NYPresidio (fluopicolide [NOTE: Valent has pulled the grape use pattern from the *NYPresidio label. See the discussion of *NYPresidio for more details]); Ranman (cyazofamid); and *NY†Zampro. Ranman is primarily a protectant fungicide whereas the other materials provide protective plus limited post-infection activity; Revus and Revus Top appear to provide the least post-infection control among these other materials. Since the early 2000s, several different products containing phosphorous acid (also called "phosphite" or "phosphonate") have come onto the market; some (e.g., ProPhyt, Phostrol, AgriFos, Rampart) are labeled for control of downy mildew whereas others are not. (Be sure to note that this is a different chemical than phosphoric acid or phosphate [note the "-ic" or "-ate" suffix], which is the form of P found in most commercial fertilizers.) The phosphite products have been used widely throughout eastern viticulture regions over this time, with generally good results. Research in NY has shown that these materials provide both protective and postinfection activity, although the period of residual (protective) activity is limited, particularly in older leaves. Traditionally, post-infection sprays have been very effective at controlling disease when applied within the first few days after a rain-induced infection event, before symptoms occur. This activity is most pronounced at higher rates, especially when applied in "back to back" applications a week or 10 days apart. Sprays applied to active leaf lesions will not eradicate them. After prolonged use, the phosphites can begin losing some of their activity, particularly when lower rates are applied (this is similar to what has happened with the DMI fungicides for control of powdery mildew). Such a situation appears to be developing in some NY vineyards. Thus, it is critical to limit the use of these materials and to apply them in rotation with other effective downy mildew fungicides, as you would for any fungicide at risk for resistance development. Azoxystrobin products (Abound, Azaka, Quadris Top, †Topguard EQ) and Pristine once provided excellent control of downy mildew when applied at 14-day intervals; Sovran, another strobilurin fungicide, was good. *NY†Reason (fenamidone) is another Group 11 fungicide with this same mode of action, and it also gave excellent control in a limited number of trials several years ago. Unfortunately, this group of fungicides is highly prone to resistance development, and downy mildew resistance to them is now common throughout the eastern half of the United States. Although they may still be effective in some vineyards, especially when cultivars are moderately (e.g., Concord) to highly resistant, it is risky to depend on these "Group 11" materials for downy mildew control any longer. Experience shows that once they have been used for a "long enough" period of time, resistance can develop suddenly and without warning in any specific vineyard - they can be effective one year and fail the next, particularly when multiple infection periods occur. Therefore, growers wishing to continue using these fungicides on a limited basis for control of other diseases should tank mix them with an unrelated, effective downy mildew fungicide (see entry for *NYDexter Max below), particularly during the bloom and early post-bloom periods when lack of control on young clusters can result in significant crop loss. Of more recent note is the documentation of downy mildew resistance to Group 40 fungicides, found in three vineyards in Virginia and one in North Carolina by Feng and Baudoin, 2018. This group of fungicides is represented by mandipropamid (Revus/Revus Top) and dimethomorph (one of the ingredients in *NYZampro). This is not too surprising given that resistance to this group of active ingredients was found in Europe before 2000, and has more recently been documented in Japan (2013) and now India and China (2017).

Downy mildew management programs should focus on (a) preventing early disease establishment on leaves and destructive cluster infections during the prebloom and early postbloom periods, and (b) limiting secondary spread on the foliage during the summer and early fall. The timing and intensity of these sprays should be determined by varietal susceptibility, weather conditions, and the availability of downy mildew inoculum. Because primary infections can first occur 2-3 weeks before bloom, protection may need to start at this time on vinifera cultivars and on highly susceptible hybrid and native American cultivars (e.g., Chancellor, Catawba, Niagara) if the weather is wet. This is particularly true if significant disease occurred the previous year, providing high levels of overwintering inoculum within the vineyard. Clusters should be protected on all but the most highly resistant cultivars from the immediate prebloom period through the first or second postbloom spray, with the intensity of the program depending on the weather and cultivar susceptibility. Continued protection against disease spread during the summer should be based on cultivar susceptibility, the extent of favorable weather conditions, and the amount of disease already present in the vineyard (secondary inoculum). Downy mildew has the potential for "explosive" spread if the disease is active and weather conditions favor its continued development. However, in many years, hot, drier weather causes the downy mildew organism to become inactive during mid-summer. Thus, it is worthwhile to scout vineyards during this time for the presence of active disease and to determine the need for protective sprays based on such findings. Also, recognize that fruit lose their susceptibility to infection by midsummer: repeated trials have shown that all control of cluster infections is provided by fungicides that are applied through 4 weeks after the start of bloom, even on highly susceptible varieties. However, protection against leaf infections and consequent defoliation may need to continue throughout the summer and even into early autumn, depending on weather conditions, cultivar, and disease levels in the vineyard. See Table 3.1.1 for varietal susceptibility to this disease.

34247

EUTYPA DIEBACK and BOTRYOSPHAERIA DIEBACK

EUTYPA DIEBACK and BOTRYOSPHAERIA DIEBACK are fungal diseases that appear as cankers on trunks and arms of infected grapevines. Over time, they can be responsible for considerable losses in productivity and reduced vineyard lifespans. With Eutypa dieback, new shoots above cankers often appear stunted, with shortened internodes, and bear small, cupped, greenish-yellow leaves in the spring. (Such symptoms on new shoots superficially resemble those caused by Roundup and similar herbicides.) Healthy shoots usually overgrow and obscure affected shoots by early- to midsummer. Although the severity of shoot and leaf symptoms may fluctuate from year to year on any given vine, they become progressively worse over time until, eventually, the entire portion of the trunk or cordon above the canker dies.

A similar disease of the trunk and cordons, called Botryosphaeria dieback, is now recognized as occurring commonly throughout NY and PA. Like Eutypa, it infects through pruning wounds and produces cankers that gradually expand and girdle the wood, killing new growth above them. However, Botryosphaeria dieback is caused by a different group of fungi than Eutypa, and infection does not result in the stunted, yellowed shoots characteristic of the latter disease. Rather, the most characteristic symptom of Botryosphaeria dieback is "blind" spur or cane positions above the sites of the cankers. With both diseases, the cankered, dead zone in the wood gradually expands from the original site of the infection (a large pruning wound) both laterally around the arm or trunk and downwards. Cutting crossways through this diseased wood reveals a wedge-shaped canker.

In winter or early spring, during rainfall or snowmelt, fungal spores responsible for one or both diseases are released from fruiting structures embedded within the dead, infected wood of the cankers. Spores are dispersed by the wind and infection occurs when they enter fresh pruning wounds, germinate, and begin to grow. Cankers and the foliage symptoms (Eutypa) usually are not evident until 2 to 4 years after infection; then, vine deterioration continues slowly until the trunk or arm is finally killed.

Currently, both diseases are managed primarily by (i) pruning out and destroying cankered portions of the vine, to help limit spread by fungus spores that are disseminated from the infected wood; and (ii) subsequently training new shoots to become structural wood to replace that which was removed. Ideally, infected cordons or trunks should be removed in late spring when foliar symptoms of Eutypa are noticeable and the resultant wounds remain susceptible for a more limited period of time than if made earlier. If this is not possible, they also can be marked with flagging tape for removal during the following dormant pruning operation. Pruning should be a minimum of 6 to 8 inches below any dead or discolored wood associated with the canker. All infected wood or stumps should be removed from within the vineyard at the very least, and burned or buried if practical. Topsin-M fungicide has a NY State "special local needs" (SLN) registration for application to fresh pruning wounds as a concentrated paste or directed spray for protection against these diseases. Although often impractical for routine pruning cuts, this treatment is likely to be worth the time and expense on a limited number of major pruning wounds that could each cause the loss of an entire cordon or trunk (hence, vine) if they became infected. Note that multiple-trunk training systems with regular trunk renewal, as typically practiced on V. vinifera vines in regions such as upstate NY, often manages these diseases at a commercial level by removing cankered portions of the vine before they have time to become seriously girdled.

34248

GRAPEVINE LEAFROLL DISEASE

GRAPEVINE LEAFROLL DISEASE or GLRD is associated with the presence of phloem inhabiting plant viruses of the family Closteroviridae that cause a degeneration of the primary phloem in shoots, leaves, and cluster stems. There are currently five species of grapevine leafroll associated viruses; GLRaV-1, 2, 3, 4, and 7. These viruses have been spread across long distances (worldwide) through the sale and distribution of infected nursery material. Short distance spread of GLRaV-1, 3, and 4, within the vineyard or between adjacent vineyards, can occur by phloem feeding insect vectors, specifically species of mealybugs and scales. No vectors have yet been discovered for GLRaV-2 and 7.

The most obvious symptoms of the disease are cupping and loss of chlorophyll in the leaves in late summer and fall, during the ripening period. On red-fruited varieties, like Vitis vinifera 'Cabernet Franc', leaves of infected vines can display red coloration of the interveinal tissue, while veins remain green. On white-fruited varieties like Chardonnay, symptoms are less noticeable and leaves tend to look yellowish and cupped. These symptoms are not necessarily diagnostic of the disease, and may be confused with symptoms of nutrient deficiencies, water stress, and even crown gall. Therefore confirmation of infection by GLRaVs can only be made in the laboratory through serological or molecular analysis of phloem tissues in leaf petiole or dormant cane samples. More significant, and perhaps less recognized effects of GLRD are reduced yield and vegetative growth, and even lower cold hardiness--a factor of critical importance for varieties grown in the northeastern U.S. GLRD can also lead to a delay in fruit maturity with negative effects on fruit chemistry at harvest (lower soluble solids, higher titratable acidity), and reduced color development in red grapes of V. vinifera grapevines; all factors that might adversely impact perceived wine quality. Vineyards can be scouted annually for GLRD during the ripening period, and tissue samples from symptomatic vines can be sent to a laboratory for confirmation.

There is no curative treatment for GLRD as infection by GLRaVs is permanent, and the disease is best managed through roguing of infected vines and replanting with certified virus-free material. Research has shown that local spread of GLRaV-1, 3, and 4 can be minimized by targeting crawler stages of the vectors (mealybug and soft scale crawlers) with well timed insecticide applications. There are no known sources of resistance to GLRaVs among Vitis species and these viruses have been found in V. labrusca, to Vitis interspecific hybrids, and V. vinifera. Infections of V. labrusca appear to remain latent or dormant and have not been shown to result in visual symptoms of the disease or economic impact, though research on native varieties has been minimal. On the other hand, V. vinifera is severely affected, and GLRD has been shown to result in substantial economic losses among those cultivars.

34249

PHOMOPSIS CANE AND LEAF SPOT AND FRUIT ROT

PHOMOPSIS CANE AND LEAF SPOT AND FRUIT ROT is most likely to become problematic when the Phomopsis fungus is allowed to build up on dead canes or pruning stubs in the vines and/or when early-season sprays for this disease are omitted. Thus, hedged vineyards are at a particular risk of incurring economic losses from Phomopsis, although serious problems occur regularly in hand-pruned vineyards when conditions are favorable and sprays are not properly timed or thoroughly applied. Economic losses have been especially severe on Niagara, and to a lesser extent, Concord, although many other native, hybrid and V. vinifera cultivars also are susceptible (see Table 3.1.1) and can incur significant loss in wet years if the disease is not managed. Infected rachises and shoots develop black lesions that may split the green tissue (shoots) or appear sunken (rachises). Numerous lesions on the lower portion of the shoot can give its surface a blackened, scabby appearance and may coalesce on the rachises to girdle them. Severe infection weakens the tissues at these spots and can cause infected shoots to break off during high winds before they become supported by tendrils; or, infected clusters may break before and during harvest, causing fruit to fall to the ground. Small, pinprick-sized lesions, with brown or black centers surrounded by a thin and somewhat yellow margin, can be numerous on infected leaves early in the season. These infections cause little harm themselves, but provide a good indication that the fungus is present and capable of causing more serious losses on clusters, should they be left unprotected during favorable weather periods. Infected berries remain symptomless until late summer or preharvest, when they turn brown, commonly beginning at the point of attachment to the pedicel (berry stem), and eventually become sparsely covered with black, pimple-like fruiting bodies of the fungus. If symptoms develop soon enough before harvest, such berries may eventually shrivel up into raisin-like "mummies", at which time they look very similar to berries with black rot. On fruit, these two diseases are best distinguished by the initial location, timing, and development of symptoms, in addition to the strength of the attachment of individual fruit to the berry stem. On fruit, Phomopsis lesions typically (but not always) start where the berry is attached to its stem, whereas black rot lesions start at random locations on the berries. Also, Phomopsis lesions do not appear until late summer or early fall, often just before harvest; in contrast, most black rot symptoms have appeared by late July or early August, and all diseased berries should be evident by veraison. Finally, berries infected with Phomopsis are usually quite easy to detach from their stem by lightly touching, giving them a gentle pull, or by modestly shaking the vine trunk; in contrast, most of those with black rot remain attached firmly to the berry stem.

Black fruiting bodies of the Phomopsis fungus overwinter in infected wood (diseased canes, spurs, or pruning stubs) and rachises that are retained in the vine after pruning. During wet periods from bud break through early summer, spores ooze from the fruiting bodies and are distributed by raindrops onto nearby susceptible tissues. For this reason, young shoots and clusters directly beneath old canes and pruning stubs are at greater risk than those that are trained to grow above these sources. Multiple, extended periods of wet weather during the early part of the growing season are particularly favorable for disease development. Shoot and leaf infections can occur anytime between bud break and early summer, although they are most common during the first few weeks of growth. Shoot and leaf lesions appear within 3 to 4 weeks after infection, but they do not serve as a source of disease spread during the current season. Rachises can be infected any time after the young clusters first emerge until fungal spores are depleted in early summer, although infections that occur soon after cluster emergence in the early growing season (typically, from about 3 to 5 inches of shoot growth until 2 or 3 weeks later) are the most damaging. Infections that occur on the pedicels (berry stems) during this period can eventually move into the fruit, causing them to rot before harvest. Although berry stem infections, which can occur several weeks to a month before bloom, appear to be where most fruit rot begins, berries can also be infected directly by spores of the fungus. These can occur from bloom until pea-sized, after which few spores are available to cause new infections. Fruit infection occurs sporadically, since it appears to require extended periods of rain and wetness; however, serious losses can result if the early growing season is especially wet and protection with an effective Phomopsis fungicide is not maintained during this time, particularly in vineyards with a history of the disease (high levels of resident inoculum). The practice of omitting early season sprays in some susceptible juice grape blocks has led to significant fruit and rachis infections in several wet years.

Diseased canes should be removed during pruning to reduce inoculum. Recent research has shown that dead canes and pruning stubs can produce extremely high levels of Phomopsis spores, and these sources should be specifically targeted for removal as part of a Phomopsis management program. The need for fungicidal protection programs on susceptible cultivars is dependent on the level of inoculum within the vineyard and the frequency and duration of wetness periods. Research from Ohio suggests that when inoculum is present, moderately-severe infection can develop after about 26 hr. of wetness at an average temperature of 48°F, 16 hr. at 54°F, and 12 hr. at 60-68°F (shorter or longer periods of wetness at any given temperature should reduce or increase disease severity, respectively). Captan, mancozeb, and ziram have been the most effective fungicides against Phomopsis in several local trials. Early-season applications of the strobilurin fungicides have provided only fair control in research trials, although they have provided good additional protection of rachises and fruit when used from bloom onward to supplement at least one earlier application of a traditional protectant fungicide. Copper and sulfur are only weakly effective; thus, organic growers or others seeking to minimize the use of conventional fungicides should pay strict attention to the removal of infected wood from within the canopy. Little to no control is provided by the DMI fungicides, several newer materials released specifically for control of powdery mildew or downy mildew, or the phosphorous acid products. Protective sprays of mancozeb, captan, or ziram may need to begin as early as 1-inch shoot growth in vineyards with heavy inoculum pressure if rain is anticipated soon, and should generally not be delayed beyond the 3- to 5-inch shoot stage on susceptible cultivars. Clusters may benefit from protection from the time of their emergence through the early fruit set period, although the earliest part of this period is the most critical time to protect against both rachis and fruit infections if conditions are favorable for the disease. The first couple of weeks after bloom can sometimes be an important time to maintain protection against fruit infection on highly susceptible cultivars such as Niagara if the block has a history of Phomopsis, particularly under wet conditions. See Table 3.1.1 for varietal susceptibility to this disease.

34250

POWDERY MILDEW

POWDERY MILDEW is a fungal disease that affects all green tissues. Diseased tissues appear to be covered with a white to grayish-white powder. Severe leaf infection can result in cupping, drying, and premature drop. Infected berries may fail to ripen properly; remain covered with a dusty mass of the fungus; turn dark brown; and/or shrivel and split, depending on the time and severity of infection. Fruit infection may promote growth of spoilage microorganisms and reduce wine quality on grapes intended for that use, even when symptoms are relatively mild. The powdery mildew fungus overwinters on the bark of the vine as tiny black fruiting bodies ("chasmothecia", formerly called "cleistothecia"). Spores ("ascospores") contained in the chasmothecia are released during rains of approximately 0.1-inch or more, from bud break until shortly after bloom. They are wind-dispersed to young leaves and clusters, and can infect wet or dry tissue at temperatures of 50°F or higher. These first mildew colonies that result from ascospore infection then produce masses of white, powdery secondary spores ("conidia"). Conidia are wind-dispersed throughout the vineyard and do not require rain for release or infection, although humid conditions particularly favor disease development. New colonies that result from these secondary infections produce additional conidia, which can continue to spread the disease. This repeating cycle of infection, spore production, spore dispersal, and re-infection continues throughout the season if susceptible tissue is present, at a rate that is driven by temperature (Table 3.1.3). Thus, at optimum temperatures in the mid-60s to mid-80s (°F), a new generation of the fungus can multiply every 5-7 days with or without rain, resulting in an epidemic of powdery mildew unless it is managed efficiently.

The powdery mildew fungus, which grows primarily on the surface of infected tissues, is harmed significantly by direct exposure to sunlight and its ultraviolet radiation. Thus, in addition to moderately warm temperatures, disease development is strongly favored by prolonged periods of cloudy and rainy weather, which provide both filtered sunlight and high humidities that are optimal for the fungus. Therefore, management programs may need to be intensified (e.g., shorter spray intervals, higher fungicide rates, more efficacious materials) during periods when such conditions occur. For the same reason, powdery mildew can be particularly severe in the center of dense canopies, near wooded edges of the vineyard that receive regular periods of shading, etc. Conversely, the harmful impact of sunlight on the powdery mildew fungus can be exploited by adopting pruning and training practices that promote good light exposure throughout the canopy, thereby utilizing this natural "fungicide" to help manage the disease.

34122

Table 3.1.3 Approximate generation period for powdery mildew (time from infection by a spore until production of a colony with new spores) at different constant temperaturesa

Temp (°F)

Days

44

32

48

25

52

16

54

18

59

11

63

7

74

6

79

5

86

6

90

-b

a. Data of C.J. Delp (University of California, Davis; 1954)

b. Little or no disease development while temperatures remain above 90 degrees.

Berries are highly susceptible to infection from the immediate prebloom stage until about 2 to 3 weeks after fruit set (berries of cv. Concord become resistant earlier than do those of V. vinifera cultivars). Severe fruit damage observed later in the season almost always is the result of infections that occurred during this peak period of susceptibility, although they may not be visible immediately. On berries of V. vinifera and certain hybrid cultivars, infections can continue to occur until bunch closure or slightly thereafter. Such midsummer infection usually results in the development of sparse, inconspicuous fungus colonies that can be important as entry points for Botrytis and other decay organisms that produce symptoms near harvest, or for spoilage microorganisms (e.g., Brettanomyces) that reduce wine quality. Rachises appear to remain susceptible until harvest, although the economic importance of mid- or late-summer rachis infections on processing fruit is questionable).

Leaf infections that occur beyond the fruit set period are much less serious on Concord and similar cultivars than on V. vinifera and susceptible hybrids. On Concord, such infections appear to have relatively minor effects on yield and Brix levels when vines carry low to moderate crop levels, but they can suppress both Brix and yield on more heavily cropped vines, particularly in years with poor ripening conditions. Thus, on this variety, the need for fungicide sprays after fruit set should be heavily influenced by both crop size and weather factors. On V. vinifera and highly susceptible hybrid cultivars, continued suppression of foliar mildew is required throughout the summer to avoid poor ripening, premature defoliation, and reduced winter hardiness. Also, maintaining low levels of leaf disease throughout one season significantly reduces disease pressure the following year, by limiting the number of chasmothecia (fungal fruiting structures) that form, overwinter, and initiate infection the next spring. Limiting the level of overwintering inoculum has been shown to have a particularly positive impact on the control of cluster infections the following season.

For effective management of powdery mildew, sprays may be required as early as 3- to 5-inch shoot growth on V. vinifera and highly susceptible hybrid cultivars, depending on rainfall and temperature in the early season. Note that cold nights near 40°F and below can significantly retard disease development, so spring seasons with overcast skies that minimize such episodes are at greater risk than those with a number of clear, cool nights.

Sprays may need to begin by the 10-inch shoot stage on Concord or other moderately susceptible cultivars, and should not be delayed beyond the immediate prebloom stage on any but the most highly resistant cultivars; sprays should be initiated immediately if the disease is observed before then. To protect against powdery mildew developing on the fruit, management programs should be at their peak intensity from just before bloom through fruit set or shortly thereafter, emphasizing the use of highly effective fungicides, full rates, appropriate spray intervals, and superior spray coverage (every row, proper speed, sufficient water volume). Spray programs may need to be especially "tight" when days and nights remain warm (see Table 3.1.3) and/or weather is cloudy and damp during this period. Protection of Concord berries is not required after fruit reach 0.25-inch in diameter, although continued foliar protection is likely to be beneficial under high-crop or poor-ripening conditions. For V. vinifera and susceptible hybrids, it is important to maintain excellent protection of the clusters through the bunch closure period, since powdery mildew infections at this time can promote the later development of bunch rots and/or wine spoilage microorganisms. Maintenance programs to protect foliage throughout the summer are necessary for attaining maximum fruit and vine quality on V. vinifera and susceptible hybrid cultivars. Good control of foliar infections through Labor Day will contribute significantly to the control of fruit infections the following year, since it will limit the overwintering inoculum needed to produce them.

34251

RIPE ROT

RIPE ROT (see "Bitter rot")

34252

SOUR ROT

SOUR ROT develops on injured berries when the weather is wet during the pre-harvest period, particularly if it is relatively warm (above 60°F, with 70s being optimum for disease development) for a significant length of time then. The characteristic vinegar smell from which the disease gets its name results from the activity of a specific group of bacteria, which oxidize the ethanol that wild yeasts produce from the juice of the injured berries. Such berries may also show signs of various "secondary" fungi that colonize decaying fruit, and may further be colonized by certain other yeasts that produce a compound (ethyl acetate) that smells like nail polish remover, which detracts from the quality of finished wines. Numerous fruit flies (sometimes called vinegar flies) - i.e., Drosophila spp. - also swarm around decaying berries and multiply by laying their eggs within them. These insects play a central role in the development of the disease and furthermore spread the decay organisms throughout the vineyard once some clusters begin to break down. The sour rot organisms also are spread as diseased berries drip contaminated juice onto nearby healthy berries, which in turn become infected through any wounds that might be available. Rain cracks, bird and insect damage, Botrytis or powdery mildew infections, and mechanical injuries caused as swelling berries pull away from their stems in tightly compacted bunches are among the many injury sites in which sour rot can become established.

Sour rot can increase very rapidly during the pre-harvest period, and it long has been thought to be almost impossible to stop the disease once it has become established if warm and wet conditions persist. However, recent research suggests that disease development can be reduced through a combination of insecticide sprays to control Drosophila fly populations and registered broad-spectrum sterilants (e.g., Oxidate) to limit microbial populations on the berries. This program is most effective if started around 15°Brix, as berries become susceptible and before symptoms develop, but it also can reduce further spread if initiated immediately after the disease is noticed through scouting activities. Limiting various causes of injury will also greatly reduce sour rot development. Open canopies and leaf removal around clusters, which allow them to dry once rains have stopped, can lower disease severity significantly. As with Botrytis, practices that loosen cluster compaction can have a major effect on limiting sour rot development; however, most of these practices remain experimental or difficult to employ.

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