Green Speak

Apple scab

Apple scab has been particularly active this year. Many who grow apples or crabapples have likely seen this disease before and may or may not know it. In fact apple scab is so common that plant pathologists expect to see it every year. With our wet and cool spring, apple scab has been particularly severe this summer.

Apple scab (Venturia inaequalis) is a fungus that infects apples and crabapples. Symptoms typically start on the underside of leaves, forming light brown to olive green lesions that are irregularly shaped. As the infection progresses the lesions become more circular with the spots eventually turning black or brown. Infected tissue will thicken creating a bulge on the upper leaf surface. Leaves may curl and scorch at the margins. Apple scab can progress to the leaf petioles and fruit pedicels, causing early leaf and fruit drop.

While not generally fatal to an infected tree, there are some options in dealing with apple scab.

1. Plant resistant varieties. This is the most desirable means of managing apple scab in the landscape. With excessively wet years (like what we have experienced) even resistant varieties are showing signs of this disease.
2. Keep the area beneath the tree clear of infected leaf residue. Removing fallen leaves can be beneficial, just note that apple scab is so common and disperses so easily infection is likely to reoccur.
3. Promote a healthy tree. Mulch with an inch layer of compost and water the tree in times of drought.
4. Use preventative fungicide sprays. Fungicides labeled for apple scab control include: mancozeb, chlorothalanil, calcium polysulfide, copper sulfate or potassium bicarbonate. Read and follow all label directions. Most of these sprays are preventatives, not curatives. So you will need to spray before the fungus occurs.

Once the leaves start to yellow and drop it is too late to protect the tree. So if a defoliated tree in August is an eyesore in your landscape, you may need to follow my colleague, Sandra Mason's advice and "prune horizontally at the soil line", thusly removing the tree and replanting with a resistant variety. Sandra recommends 'Snowdrift' and 'Prairiefire'. Morton arboretum has a listing of desirable crab apples HERE. Or read more about this disease in a report created by U of I Extension HERE.