Was your apple or crab apple tree defoliated last year, with nothing but the apples left on the naked tree? Chances are, it was apple scab. I have seen many, many apple trees infected with scab this year as well, and I’m predicting we’ll see many more naked trees this fall.

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Initial apple scab lesion. Photo by A. Beissinger

Caused by the fungus Venturia inequalis, you’ll see apple scab infections start in late May/early June in Connecticut. Other species of Venturia fungi infect pear and willow as well (Venturia pirina and Venturia saliciperda, respectively), and cause similar symptoms. It is important to note that though Venturia fungi cause similar symptoms, each species is very host specific and will not infect if their host is not present.

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Apple scab lesions spreading on the leaf surface. Photo by A. Beissinger.

On apple and crab apple, infection first occurs at bud break, but is not usually detectable at this stage. Trees flower well. Spores are spread by rain-splash and wind. The first signs of infection are olive green to black lesions on susceptible leaves, and these lesions are actually the fuzzy spores of the fungus. As the fungus develops, the lesions grow in size and the infected leaves begin to yellow. Leaves will prematurely drop from the trees, and the colonized leaves will still be able to emit spores that continuously infect the tree throughout the season. When the trees set fruit, black, scabby raised spots will appear on the skin.

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Magnified lesion on the leaf. Note the velvety appearance of the spores. Photo by A. Beissinger

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Spores of Venturia pirina. Photo by A. Beissinger

Usually scab is more of a concern to commercial apple and pear growers because it reduces fruit yield and fruit quality. As a result, most apple orchards have either a regular spray program or other means of preventative apple scab management. This is especially important because apple scab is a polycyclic disease, meaning that there are multiple infection periods per season. If an orchard treated just once for apple scab, the infection could easily reemerge only a few days later, wasting time, money, and apples! Over several years, repeated defoliation can eventually lead to death of the tree if apple scab goes unmanaged.

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Apple scab infected leaves beginning to yellow. Photo by A. Beissinger

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Leaves will often completely yellow before dropping. Photo by A. Beissinger.

In a home setting, one of the most important things you can do is rake up your fallen leaves. The fungi overwinter in leaves and fallen debris. A fall application of urea around the base of the tree can be helpful as well. If your tree is already infected this year, a fungicide application will not help. Only consider fungicides in the spring of your 3rd year that the tree has been completely defoliated.

-Abby Beissinger