|James Kohut, Staff Writer|
The genus 'Malus' contains some of the most ornamental of all the landscape trees that we northerners can grow in our yards and gardens, not to mention some of the tastiest too. But plants in this group, which include the eating apples as well as all flowering crab apples, are also rather susceptible to a number of bothersome diseases. And among these diseases, none is more prevalent across our region than apple scab.
Apple scab is a fungal infection that affects the foliage and fruit of most apples, crab apples and flowering crabs. It usually appears first as raised circular brown spots on the leaves in late spring into summer, followed by round or irregular brownish or gray patches on the fruit which form rough cracks and fissures as the fruit matures.
Although it is rarely fatal to the tree, it is a serious problem both in terms of landscape value and fruit production. In a serious infestation, the leaves may turn yellow and the entire tree may prematurely defoliate, rendering it a stark skeleton in an otherwise attractive summer landscape. Mild cases don't affect the edibility of the fruit but certainly render the fruit unsightly, while severe cases may cause splitting of the fruit, which subsequently rots and falls from the tree. Clearly, in a genus valued for its ornamental attributes and fruit quality, this disease is a serious limitation. Besides ornamental effects, an infection can stress the plant and predispose it to winter damage.
The surest way to protect against apple scab is to plant resistant cultivars, although there aren't that many available. The key to managing apple scab in non-resistant varieties is in understanding its life cycle and how it is transmitted. The fungus overwinters in the fallen leaves and fruit, releasing spores in spring which are carried by air currents up into the tree. These develop into lesions on the leaves and flowers, which release additional spores which then infect the fruit. The cycle thus repeats throughout the season, although the leaves and fruit become less susceptible to infection as they mature. Being a fungus, moisture exacerbates the problem, with scab infections particularly bad in unusually wet or humid years and in stale air.
Cultural control is therefore achieved by minimizing moisture in the foliage, maximizing air movement, and keeping the area underneath apples and ornamental crabs clean and tidy. First and foremost, remove all fallen leaves from around the trees in fall. When planting, leave adequate space between trees for air movement when they reach their mature spread, and undertake a program of regular pruning for openness and air movement within the canopy. And water only at the base of the tree and only when necessary; apples typically do not require supplemental moisture except during periods of prolonged drought.
In the worst cases, chemical control is an option for apple scab using registered fungicides, but for the average home gardener, cultural control of this disease should prove effective in most cases.