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BP-1-W

Fruit Diseases

Purdue University

Cooperative Extension Service

West Lafayette, IN 47907

The most common and damaging disease of apple trees in Indiana is apple scab. Most apple varieties are susceptible to this disease, including flowering crab apples (see BP-39). Apple trees severely infected with scab can be completely defoliated and the fruits reduced to distorted, blemished "nubbins." Through the proper use of resistant varieties, cultural and/ or chemical control, apple scab can be prevented, assuring you a healthy, vigorous tree with fresh, bountiful fruit

BP-39

Symptoms

Leaves: Brown to olive-green; roughly circular, fuzzy spots often originate along the veins of the leaves. In time, spots become black and velvety and develop fringed margins. Finally, leaves yellow and drop.

Fruit: Black, scabby spots of various sizes appear on fruits and often cause them to be dwarfed, cracked, and/ or mishapen.

Cause

Apple scab is caused by the fungus Venturia inaequalis. The fungus lives through the winter within infected leaves. In early spring, spores (ascospores) of the fungus are shot into the alr when leaves become wet, spores are then carried by wind to the newly developing apple leaves and cause leaf and/or fruit infection. Once infection has occurred, a different kind of spore is produced; these "summer" spores are capable of causing further infections throughout summer and early fall. This cycle repeats itself annually.

Control

1. Resistance: Planting apple scab-resistant varieties is the best and easiest means for homeowners to avoid apple scab problems. Recommended disease resistant varieties include: Jonafree, Prima, Liberty, Freedom, Redfree, Enterprise, and Gold Rush.

2.Cultural Control: Since fallen leaves harbor the scab fungus, rake and destroy them before they become brittle and break into tiny fragments that are difficult, if not impossible, to rake. Prune apple trees in late winter to maintain an open' tree. A well-pruned tree allows better air circulation, faster drying conditions, and provides for better penetration of spray materials.

3.Chemical Control: The most critical time to apply fungicides for scab control is spring (April to early June). Apple trees should be sprayed on a regular schedule starting shortly after bud break, when 1/2 inch of green leaf tissue is visible. Continue spraying on a 7- to 10-day schedule (7 days during wet weather, 10 days if dry) until petal fall. After petal fall, if dry weather persists, a 10- to 14-day spray schedule is adequate for control of scab. NOTE: fungicides act as a protective coat of "paint" on the leaf surface; where possible, apply fungicides just before a prolonged wet period occurs, not after.

Two readily available fungicides for control of apple scab are Captan and Immunox. Refer to the pesticide label for directions on rate of use, method of application, and safety warnings. General purpose fruit sprays sold as "Home Orchard Spray," "All Purpose Fruit Spray," "One Package Fruit Spray," etc. may be used in place of the specific fungicides recommended above. However, do not spray with general purpose sprays during bloom; they are toxic to honeybees.

Apple Scab on fruit and leaf

Reference to products in this publication is not intended to be an endorsement to the exclusion of others which may be similar. Persons using such products assume responsibility for their use in accordance with current label directions of the manufacturer.