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Black Rot or Blossom End Rot or Frogeye Leaf Spot

Black rot, AKA "Blossom End Rot" and "Frogeye Leaf Spot," is a disease primarily affecting apple and pear trees that are stressed or weakened by other infections, injury, or difficult growing conditions. It is caused by the fungus Diplodia seriata (AKA Botryosphaeria obtusa). The fungus overwinters in cankers and mummified (shriveled) fruits, and when it is active, it infects all parts of the tree.

The first symptom of infection will be leaf spotting in early spring. Initially small purple specks, the spots will grow and develop a brown center while the margins remain purple. In their final stage, these spots have what is called a "frogeye" appearance, with a pale center and a dark edge. A heavy infection may cause defoliation (leaf loss), further weakening the tree. A more dramatic symptom that you will not miss is rotted fruit. When fruit is infected, lesions, usually at the calyx end (hence "blossom end"), begin as reddish spots that will darken to purple with a red border. Finally, the lesion will turn brown-black, often with concentric rings of discoloration, and black spores may be visible on the surface. The end result is similar to brown rot on stone fruit, and equally unpleasant to encounter. A third symptom is sunken, red-brown cankers on branches. These are easy to miss, but if you find fruit symptoms, you should inspect your tree for cankers. They most often appear on the southwest side of the trunk, where trees tend to sustain winter injury, and at crotches, where branches join the trunk.

Black rot can be considered a secondary infection; the first step to avoiding it is to ensure that your trees are otherwise healthy. Avoid poorly drained soils, overwatering, and underwatering. Other diseases, especially fireblight, can result in dead wood that is easily infected by black rot. If your tree is suffering from black rot, prune out dead or diseased branches and remove mummies. Fallen leaves are not infectious, but dead leaves should be removed anyway, as they harbor many other diseases. If the tree is beyond repair, be careful to remove the stump, as this can infect your young and vulnerable replacement tree. McIntosh, Rome, and Paula Red are thought to be the most susceptible varieties.

For more information, see the Michigan State University IPM Factsheet on Black Rot and A Grower's Guide to Organic Apples from Cornell.

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