Your fruit appears to have brown rot fungus (Monilinia fructicola), a fungal disease that affects stone fruit trees, such as the plum tree. The fruit and branches of infected trees become covered in a brownish fungus that turns them sunken and soft and the fruit inedible. The flowers turn brown and sometimes excrete a brown, sticky gum. Control brown rot fungus in your plum and other stone fruit trees through manual and chemical means.
Control brown rot fungus starting in the late summer or early fall. Remove all diseased fruit that has fallen to the ground as well as fruit mummies and any cankered pieces on the tree itself. It is also beneficial to remove diseased branches in the winter when the tree is dormant. Dispose of diseased pieces immediately in a garbage container.
Prune trees to create better air circulation, which will discourage the growth of brown rot fungus. When watering plum trees, use low sprinklers that will not hit tree flowers, fruit and foliage, as this can create optimal conditions for the fungus to flourish.
Apply a synthetic fungicide or one that contains copper if you are concerned about brown rot fungus affecting your plum trees in the springtime. Follow the manufacturer’s directions for exact application instructions. Spray fungicide while the trees are still in their pink bud stage for the fungicide to work best.
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