What is Apple Scab?
Apple Scab Overview
Many apple and crabapple trees become infected with a fungal pathogen, Venturia inaequalis, that causes unsightly scabs on leaves and fruit. Apple scab manifests as dull black, grey, and/or brown lesions on the surface of the leaves, buds, and/or fruits. The fruits and undersides of leaves are the most susceptible areas for crabapple scab. The disease rarely kills the tree, but the quality and yield of fruit will be reduced. Scabbing causes serious problems for apple orchards that sell to consumers who generally avoid blemishes on food. Crab apple trees are for ornamental purposes, so fruit quality and yield do not matter all that much. Scabbing on crabapples definitely detracts from the overall attractiveness of the tree, which can cause problems for horticulturalists, garden centers, and landscapers. Overall, apple scab is mainly an aesthetic problem.
Life Cycle of Venturia inaequalis
The fungus begins its life cycle in spring when temperatures and humidity levels are optimal. As the fungus reproduces, it releases spores into the air with the help of wind and rain. The spores are usually dispersed via leaf litter of the past year. The spores are transferred onto the surfaces of leaves, buds, and fruit. The infection persists until autumn when the leaves begin to drop. The infected leaves fall to the ground and the cycle repeats in the spring. The cycle can be repeated for the life of the tree if attempts are not made to control the infection.
Apple Scab Prevention
Some varieties are more resistant to apple scab compared to other varieties. Selecting a resistant variety is a major factor of apple scab control. Research different varieties before purchasing and planting apple and crabapple trees. Using plants that have a natural resistance to diseases will reduce the time and money spent on preventative and curative measures.
The best way method of cultural control is to remove and destroy dropped leaves and trimmings. This will prevent the future release of spores into the air. Removal of waste should occur before the leaves and trimmings dry out and become brittle. Brittle leaves make raking and collecting very difficult, and spores are easier dispersed when leaves crumble into bits.
Additional steps may be necessary to effectively control apple scab, especially in cool, moist coastal regions. Applying zinc and fertilizer-grade urea (or another nitrogen application) to leaves in autumn will hasten leaf fall. Limestone powder should be added to leaf litter piles beneath the tree as well.
Never water the foliage of a tree before night fall. The foliage should be avoided when watering entirely, but sometimes sprinklers cause water to land on the leaves. Leaves that are wet during cool, nighttime temperatures are disease prone.
Apple Scab Cures
There are products available to help control apple scab, but Venturia inaequalis can quickly develop resistance to chemical control. Applications work by coating the leaves and buds in a protective layer that cannot be penetrated by Ventura inaequalis. Contact fungicides are viable options, but the timing of applications are key. Applied too early or too late and the application will be wasted.
The recommended application time is just after the buds begin to open until a month after the flower petals fall. Fungicide sprays should only be applied if the weather is rainy and leaves are likely to remain wet for 8 hours or more. A second application may be needed if the weather remains humid and rainy. Pay close attention to any symptoms even after applications have been applied. Reapply application if symptoms are visible.
Myclobutanil (sold under 'Spectracide Immunox Multipurpose Fungicide') is a synthetic fungicide that is effective against apple scab. Copper-based spray applications are effective when applied at the proper times as well. Copper applications are often considered organic since there are no synthetic agents. Both can be applied after the buds open until flower petals fall.
Economic Feasibility of Curing Apple Scab
Curing ornamental trees may not be economically feasible or desired as long as the tree remains relatively healthy. Many homeowners simply prefer to ignore apple scab on crabapples, instead of spending time and money to rid the leaves and fruit of spots. Although, orchards and garden centers want to eliminate apple scab since customers do not want ugly spots on their trees and fruit.