Apple scab (Venturia ineaqualis) is the most destructive fungal disease affecting the home apple planting. It is most severe in regions that are cool, humid and rainy. If left untreated, it will cause lesions on the leaves and fruit. It can defoliate susceptible varieties in mid-season, which will weaken trees, reduce yields, and blemish fruit so that they crack and split, making them fit only for cider.

Symptoms and Life Cycle:

Early in the spring, infected leaves from the previous season which have been lying on the ground all winter produce sexual spores called ascospores. Primary infections on new leaves develop when rain causes the ascospores to be forcibly ejected. In mid-May to early June, infections begin to develop on the underside of leaves, or fruit spur leaves, as these are the sides first exposed when the fruit buds open. The young spot or lesion has rather indefinite margins, but within two to three weeks the boundaries become very distinct and the entire lesion becomes velvet-green in color, indicating spore production. These asexual spores, called conidia, are spread by rain and wind and will infect the newly developing fruit and other leaves throughout the rest of the summer and into the fall. If infection occurs in late summer or early fall, rough black, circular lesions may develop on the fruit in storage. These lesions are usually very small, varying in size from tiny specks to 1/4 inch diameter, and are know as "pin point" scab.

Management:

Control of apple scab is possible through resistance, sanitation and/or fungicide treatments.

Resistance: There are many scab-resistant varieties available through nurseries and local garden centers. Choice of these varieties can reduce or eliminate the need to apply fungicide sprays to control apple scab. We have over 20 resistant varieties planted at our University research farm, but after 10 years of evaluation the only varieties we recommend for Rhode Island are Pristine, William's Pride, Redfree, Liberty, Nova Easygro, Freedom and Enterprise. See GreenShare Factsheet on Apple Scab Resistant Varieties for more information.

Cultural: Maintaining a healthy and vigorous orchard through proper watering, fertilization, and pruning should be an integral part of any pest management program. Prune trees well in the dormant season and also in late July to early August to thin out the canopy.

Sanitation: Sanitation should be used to enhance any chemical spray program. Providing good air circulation will help reduce disease development while allowing chemical applications to penetrate more effectively. Raking and burning (or bagging and hauling to landfill) all fallen leaves will help decrease the amount of early spring spores. Remember to include any scab-susceptible crabapples in your sanitation program. Do not compost infected leaves, as most homeowner compost heaps do not reach temperatures high enough to kill pathogenic fungi.

Fungicides: Managing primary scab is the key to winning the battle against this fungus. If controlled early, secondary infections will be far less severe. Susceptible trees such as McIntosh, Cortland, Red Delicious or Rome should be sprayed every seven days, from the time buds begin turning green in the spring until mid-June. If a fungicide must be applied, it is far better to spray early in the season before apples are formed or when they are smallÜthe further into the season that chemicals are applied, the more residue is likely to be on edible fruit.

For a detailed spray schedule, call the URI Cooperative Extension Hotline at 1-800-448-1011. See the "Home Tree and Small Fruit Pest Management Guide" for an overview of home apple tree management and sources of disease resistant fruit trees. For the latest information on infection periods or pest outbreaks during the growing season, visit the URI Apple IPM website at http://www.uri.edu/research/IPM

By David B. Wallace, Plant Protection Specialist, and Heather H. Faubert, IPM Specialist