Crop and Pest Management Guidelines

3.3 Diseases of General Importance or Occurrence

Because of the large number of crops covered in this document, and the wide variety of diseases that affect them, this section covers those dieases that have a broad host range and are most common in nurseries and landscapes.

3.3.1 Crown Gall

Crown gall, caused by Agrobacterium tumefaciens, occurs in nurseries and plantings throughout New York State. The list of woody plants susceptible to the disease includes plants in at least 77 genera and 32 families.

The disease becomes established in nursery crops when clean stock is planted in infected soil and when infected stock is planted in previously clean soil. Once in the soil, the bacteria can persist indefinitely in decomposing debris from galls on susceptible plants. The wounds necessary for entry of the pathogen occur during planting, cultivating, grafting, and pruning.

3.3.2 Verticillium Wilt

A fungus, Verticillium dahliae, causes this disease, which occurs in at least 46 genera of woody plants grown in New York State and adjacent areas. Maples comprise the most important group of highly susceptible woody plants. The pathogen is soilborne and can persist many years in soil without host plants. The disease is common in nurseries that have been established on land formerly devoted to fruit, bramble, vegetable, or field crops, many of which are also susceptible. If Verticillium wilt has occurred in previous crops on a particular site, susceptible plants should not be planted.

There is no guaranteed way to rid soil of V. dahliae once the soil has been contaminated. Soil fumigation may help but is probably a bad investment if done for the sole purpose of controlling Verticillium. Instead, prevent the disease from getting started by planting clean stock on sites where previous occurrence of the disease has not been noted. Seek guarantees from suppliers that stock you receive is Verticillium-free.

If a tree or shrub already established in a landscape becomes infected with Verticillium, it may still live and be a serviceable plant for many years. Trim symptomatic parts from the plant, disinfecting tools between cuts. Be sure the plant has adequate water and nutrition because Verticillium spreads more quickly in plants growing in less than optimal sites.

If an established tree or shrub must be removed because it is dying from Verticillium wilt and a replacement is desired, use a resistant plant. Yews and conifers are not affected by Verticillium wilt. Many broadleaf trees and shrubs commonly planted in New York also seem to be resistant.

Although maples are generally thought to be highly susceptible to Verticillium wilt, red and sugar maples appear to be much more resistant than Norway maples. 'Jade Glen' and 'Parkway' are reported to be more resistant than other cultivars of Norway maples. 'Charlotte,' 'Tryon,' and 'Union' are moderately susceptible, but have not been widely field tested for other performance characteristics.

3.3.3 Shoestring Root Rot

Several different species of fungi, once all lumped together under the name Armillaria mellea, cause this disease, which can be lethal to hundreds of kinds of woody plants. The fungus kills by girdling trees and shrubs at the butt or root collar. Two conditions must usually be satisfied before lethal attack occurs: (1) the pathogen must have access to a large energy source or food base such as a colonized tree stump or root and (2) the tree or shrub must be under physiological stress such as from water shortage, defoliation, or construction damage. Control depends on optimizing growing conditions and eliminating food bases. Keep trees and shrubs vigorous by protecting against defoliation and water stress.

Fertilize at regular intervals. Remove the stump and roots of a killed plant if others of value are nearby. If disease is detected in a valuable living tree or shrub while confined to one root or to one side of the butt, expose the entire root collar area and as much of the butt as possible to the air for several months. Remove all dead roots and dead bark. Fertilize and water as needed to promote vigorous growth.

3.3.4 Powdery Mildew

Many species of fungi that have certain morphological features in common are classified as powdery mildews. These fungi have similar life cycles, parasitize only the outermost cells of leaves and succulent stems, and can be controlled by the same methods. In general, wait until symptoms appear. Then, if necessary, apply an appropriate fungicide according to label directions. Powdery mildews on most woody plants intensify during the latter half of the growing season and have little effect on overall plant health. Thus, fungicide application is often unnecessary except to preserve the appearance of the foliage.

Thiophanate-methyl, triadimefon, and myclobutanil are effective in controlling powdery mildews on a wide array of trees and shrubs. Powdery mildew on rose is also controlled with applications of copper or propiconazole. Horticultural oil is registered for control of powdery mildew on rose.

3.3.5 Chlorosis Caused by Manganese or Iron Deficiency

In areas where soil pH is high (i.e., > 6.5), root uptake of essential nutrients, especially iron and manganese, may be

inhibited. Maples, oaks, and ericaceous plants (e.g., rhododendrons, azaleas) are highly intolerant of such conditions, but other species may also show chlorotic foliage if they lack sufficient amounts of these nutrients. Because deficiencies of either iron or manganese cause symptoms that look alike, it is advisable to have both soil and foliage tested before treatment.

It is usually not feasible to lower pH by adding chemicals to the soil. Thus, the needed nutrients must either be applied directly to the plant through injection into the vascular system or spraying onto the foliage, or to the soil in a chelated, more readily available form.

3.3.6 Cutting Rots Caused by Several Species of Fungi

To prevent rot in recently stuck cuttings, pasteurize or sterilize the rooting medium with steam and take precautions to ensure that the cutting does not come in contact with untreated soil. Use sharp tools and heat them briefly in a flame periodically to minimize chances for disease spread on dirty tools. Drench rooting medium with etridiazole per label directions.

3.2 Nursery Hygiene

3.4 Disease Control