Apples are synonymous with autumn, as their fragrance and flavor have become mainstays of the season which many call their favorite. Connecticut is no stranger to apple production—our state harvests over half a million bushels of apples annually. However, plant pathogens and pests do not harbor the same love for apples as many of us do. These agents wreak havoc year-round, impacting the health of apple trees even before fruit has begun production. One of these agents is Venturia inaequalis, the fungal pathogen responsible for apple scab.
Apple scab is one of the most common and economically impactful diseases of apple crops worldwide. Members of the genus Malus—most notably including apples, crabapples, and their cultivated and wild varieties—can be susceptible to infection.
Venturia inaequalis overwinters in the form of fungal fruiting bodies on apple leaf debris. The pathogen has two phases of its disease cycle. The primary phase begins when saprophytic growth, meaning fungal growth unrelated to disease, occurs for about a month after leaves fall. In the spring, rainfall prompts the release of ascospores (reproductive fungal structures) at the same time apple leaf buds begin to emerge and unfurl. Under the proper environmental conditions, these spores travel to new hosts with wet leaves which they can enter via leaf cuticles. The secondary phase of the disease cycle occurs as the pathogen obtains nutrients from the leaves, prompting its conidia (another form of fungal spore) to emerge and spread to other hosts. In what is known as secondary infection, conidia germinate on new apple leaves, allowing thread-like fungal structures called hyphae to inoculate the host. Numerous secondary infection cycles of Venturia inaequalis can occur during a single growing season depending on the amount of rainfall and humidity present in the environment.
Symptoms of apple scab are distributed across multiple regions of the host plant, with leaves and fruits being the most visibly impacted. In the primary stage of infection, small, raised dark green or olive lesions appear on either surface of young leaves. These lesions increase in size, sometimes covering the entire leaf in what is known as sheet scab, and change color to yellow or black. In severe cases, leaves become scaly, shriveled, and eventually fall prematurely. Severity of infection is dependent on host age—younger hosts are more susceptible to the disease. Secondary lesions develop during the growing season, appearing as brown, irregular, raised leaf spots. Fruit tissue becomes gray or black in patches, which turn corky and crack as disease progression advances. Apples infected by Venturia inaequalis near the end of the growing season or in storage may develop pinpoint scab—small, dark spots on the tissue surface.
The ability of apple scab to manifest at several points in the host’s life cycle and on a multitude of tissue surfaces makes the disease one of great economic impact for commercial growers. Cultural and chemical control methods are available, with proper sanitation and cleanup practices being essential to remove potential surfaces for the pathogen to overwinter on. Additionally, apple cultivars such as Liberty, Akane, and Chehalis have resistance to apple scab. Samples suspected of having apple scab may be submitted to the UConn Plant Diagnostic Lab for confirmation.