Alternaria Blotch of Apple

Do your apple leaves have big orange freckles? It may be apple measles. Or it may be Alternaria blotch of apple.

Alternaria blotch of apple (Dalphy O.C. Harteveld) CC BY 2.0

This leaf spot disease is also known as Alternaria leaf blotch, cork spot, and storage rot. This isn’t the same thing as apple blotch, though both are fungal diseases of apple trees.

Alternaria blotch of apple symptoms

Apples infected with Alternaria mali have raised black or brown lesions that are commonly found near the blossom end, though they may appear anywhere on the fruit. Infected fruit does not store well and may turn dry and corky.

Alternaria leaf blotch on apple (Dalphy O.C. Harteveld) CC BY 2.0

Most symptoms of this disease are seen in the leaves. Leaves may turn unusual colors, dead areas develop, and yellowing is common. Most often, you will see rust-colored circular spots in late spring or early summer. These spots can merge into large, irregularly-shaped areas. Those spots have light brown centers and purple halos. Eventually, infected leaves die and fall off, reducing photosynthesis and increasing sunburn damage. Green, woody tissue, such as petioles, can also become infected, but this is rare.

These symptoms are easily confused with chemical overspray, frog-eye leaf spot, cedar apple rust, and magnesium deficiencies. You can take a sample to your local County Extension Office to find out for sure.

Alternaria leaf blotch on apple tree (Dalphy O.C. Harteveld) CC BY 2.0

Alternaria blotch of apple management

There are already bacteria in the soil (Bacillus cereus) that attack Alternaria mali. The problem pest is red mites. While the connection isn’t fully understood yet, research has shown that high red mite populations correspond to serious damage by Alternaria blotch of apple. European red mites are controlled using delayed dormant horticultural oil. Miticides are not effective against red mites.

As always, remove mummies, prune for good airflow, and remove dead leaves from under the tree. Healthy leaves can be composted, while infected leaves belong in the garbage can.

Take a closer look at your apple tree. Are there spots on the leaves? What’s your favorite apple?