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Apple Scab Apple scab, caused by the fungus Venturia inaequalis, is generally considered a serious disease of apples in California, causing loss or severe surface blemishing of fruit. Apple scab is most severe in coastal areas where spring and early summer weather is cool and moist; however, it can be a problem wherever apples are grown when conditions are favorable for its development.
Scab infections are usually noticed first on leaves. Affected leaves become twisted or puckered and have black, circular spots on their upper surface. On the underside of leaves, the spots are velvety and may coalesce to cover the whole leaf surface. Severely affected leaves may turn yellow and drop.
Scab can also infect flower stems and cause flowers to drop. Later in the season, scabby spots may be found on fruit. They begin as sooty, gray-black lesions and may have a red halo. The lesions later become sunken and tan and may have spores around their margins. Infected fruit become distorted and may crack, allowing entry of secondary organisms. Severely affected young fruit may drop.
Fruit is susceptible to infection by the scab fungus anytime during its development. Early infections can result in dropping of the blossoms and young fruit. Lesions on the young fruit resemble those on leaves. As the fruit matures, the affected areas become brown, corky and often cracked. Late summer and fall fruit infection show up as small black specks called pin-point scab.
Disease Cycle : Although research in New York has shown that the scab fungus can overwinter in trees as conidia on bud scales, the pathogen generally overwinters in leaves and fruit on the orchard floor. Ascospores are the major source of primary inoculum and are produced within pseudothecia that develop in leaves during the winter months. In a typical year in most locations, the first mature ascospores are capable of causing infections at about the time of bud break or soon thereafter.
Ascospores continue to mature and are discharged over a period of five to nine weeks, with peak discharge during the pink to petal fall phenological stages. The length of time required for infection to occur depends on the number of hours of continuous wetness and the temperature during the wet period. Young leaves remain susceptible for five to eight days, but their lower surfaces may become infected in late summer.
For fruit, the duration of the wet period required for infection increases with the age of the fruit, which remains susceptible until harvest. Once the fungus is established in the leaf or fruit, conidia form on the surface of the lesion and become the source of secondary inoculum for the remainder of the season. Conidia are disseminated to developing leaves and fruit by splashing rain and wind. Several secondary cycles of conidial infection may occur during the growing season depending upon the frequency of infection periods and the susceptibility of host tissue.
Causal Organism Apple scab is caused by the fungus, Venturia inaequalis. It survives the winter in the previous yearâ€™s diseased leaves that have fallen under the tree. In the spring, the fungus in old diseased leaves produces millions of spores.These spores are released into the air during rain periods in April, May and June. They are then carried by the wind to young leaves, flower parts and fruits. Once in contact with susceptible tissue, the spore germinates in a film of water and the fungus penetrates into the plant.
Depending upon weather conditions, symptoms will show up in 9 to 17 days. The fungus produces a different kind of spore in these newly developed lesions. These spores are carried and spread by splashing rain to other leaves and fruits where new infections occur. The disease may continue to develop and spread throughout the summer. Because a film of water on leaves and fruit is required for infection to occur, apple scab is most severe during years with frequent spring rains.
Control 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, Williamâ€™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 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 smallĂśthe further into the season that chemicals are applied, the more residue is likely to be on edible fruit.
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