Apple Blotch

​Are you seeing spots? Black or brown spots on apples and apple leaves? It may be apple blotch.

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

Apple blotch (Marssonina coronaria, M. mali, Diplocarpon mali) is a fungal disease that attacks apple trees around the world, damaging fruit, causing early defoliation, and weakening trees. Also known as Marssonina blotch, this disease was first seen in Japan in 1907. While it is not the problem it used to be, thanks to the use of Bordeaux mixture, it is still a good idea to know what to look for.

[Note: I was unable to find photos of this disease that I could use. Any volunteers?]

To muddy the waters just a bit, there is another apple blotch disease. This one is caused by Phyllosticta solitaria fungi. The symptoms and treatments are mostly the same, but there are a few differences. Also, when these two diseases occur, it is not uncommon for other diseases to appear. Sooty blotch and flyspeck is one of those diseases. Alternaria blotch of apple is another. We will get to those another day.

Apple blotch symptoms

Spotted fruit and early leaf drop are signs something is wrong. Closer inspection of a tree infected with apple blotch will reveal dark green circular areas on the tops of mature leaves. Tiny yellow spots develop within those areas. Those yellow spots get bigger and turn into grayish-brown round lesions (0.2”–0.4” diameter) with black pinhead-sized fruiting fungal bodies called acervuli. As the infection spreads, leaves turn more yellow than green with brown patches. These symptoms usually appear in mid-summer and severe defoliation usually begins two weeks after the first symptoms are seen. These symptoms look a lot like black rot and Alternaria blotch of apple.

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

Fruit is less commonly affected by Marssonina fungi, while fruit infected with Phyllosticta will often display brown spots that can coalesce into large, scabby areas. Fruit infected with apple blotch it’s still edible but I would give it a good wash first.

Apple blotch management

Since apple blotch fungi overwinter in leaf litter, you can help prevent this disease by removing fallen leaves from under the tree each autumn. If an infection is suspected, those leaves should be thrown in the garbage. Bordeaux mixture sprays are effective against apple blotch, as are several fungicides. These treatments are usually applied as soon as blossoms fall. Then you need to start counting the hours that tree leaves are wet from rain. After 175 hours, spray again and then spray every 10 to 14 days throughout the growing season.

You can help your apple tree healthy with good pruning, sanitation, and regular feeding. This helps your tree protect itself.