Apple scab

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  Apple fruits with scab
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  Apple scab on leaves
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  Apple scab under scanning electron microscope

Apple Scab

Apple scab is a fungal disease that affects apple trees and is caused by the pathogen Venturia inaequalis. It is one of the most common and economically significant diseases of apple trees worldwide. The disease primarily affects the leaves and fruit of the apple tree, leading to reduced fruit quality and yield.

Symptoms

The symptoms of apple scab are most noticeable on the leaves and fruit of the apple tree. On leaves, the disease manifests as olive-green to brown spots that are roughly circular and have a velvety texture. As the disease progresses, these spots may coalesce, causing the leaves to become distorted and eventually fall off the tree prematurely.

On the fruit, apple scab appears as dark, sunken lesions that can crack and deform the fruit. These lesions are often surrounded by a corky, raised margin. In severe cases, the fruit may become misshapen and unmarketable.

Life Cycle

The life cycle of Venturia inaequalis involves both sexual and asexual stages. The fungus overwinters in fallen leaves and fruit on the ground. In the spring, as temperatures rise and moisture increases, the fungus produces ascospores, which are released into the air and infect new leaves and fruit.

Once the initial infection occurs, the fungus produces conidia, which are asexual spores that can cause secondary infections throughout the growing season. These secondary infections can rapidly spread the disease, especially in wet and humid conditions.

Management

Managing apple scab involves a combination of cultural practices, resistant cultivars, and chemical controls. Cultural practices include removing and destroying fallen leaves and fruit to reduce the source of overwintering inoculum. Pruning to improve air circulation can also help reduce the humidity levels that favor the disease.

The use of fungicides is a common method for controlling apple scab, especially in commercial orchards. Fungicides are typically applied during the early stages of leaf and fruit development to prevent initial infections. It is important to follow a fungicide rotation program to prevent the development of fungicide-resistant strains of the pathogen.

Breeding and planting resistant apple cultivars is another effective strategy for managing apple scab. Some apple varieties have been developed with genetic resistance to Venturia inaequalis, reducing the need for chemical controls.

Related Pages

• Fungal plant diseases
• Apple tree
• Plant pathology
• Integrated pest management