Black plant disease, also know as black rot (Botryosphaeria obtusa), causes widespread damage to apple orchards and grape vineyards. The fungal infestation manifests itself in three forms, which can occur together or singularly on an infected plant or tree; a leaf spot known as frogeye leaf spot, a fruit rot and a limb canker.
Black plant rot first occurs in early spring. The infected tree or plant will have foliage that appears with small purple spots or streaks. As the foliage matures the lesions enlarge and take on a tan appearance with a margin of purple. As the fungus progresses a small area of black occurs inside of the tan spot that gives the appearance of a frog's eye, which is why the lesions are called, "frogeyes", according to the University of Florida.
Infected Fruit Identification
Fruit of infect plants and trees have reddish lesions. The lesions quickly take on a purple appearance as the fruit grows. The purple lesions are surrounded by a red ring. As the fruit mature the lesions quickly turn black with a red perimeter. The lesions grow in diameter on the infected fruit and often have a black and brown banding within the red perimeter. The fruit remains firm to the touch but the area of rot reaches deep within the fruit's flesh and towards its very core, rendering it inedible.
Trees or shrubs that have sustained the fungal infection and then suffer a wound in the bark can quickly be invaded by the fungus. Cankers develop near the site of the fungal entrance. The cankers appear reddish and sunken. If the tree is in good health it can often overcome the cankers and they disappear within a year but a weak tree will die from the fungal infection. The cankers will become larger and the wood will begin to crack. The tree often become so severely girdled by cankers that entire limbs or the tree itself dies.
Cold wintertime temperatures do not kill the fungus. It will over winter within cankers or even in mummified fruit that remains attached to the tree or falls beneath it. The fungus requires water to germinate and reproduce. During the wet spring weather spores are produced. During rainfall the spores attach themselves to a plant where they germinate within four hours if the temperature is between 61 to 90 degrees Fahrenheit, according to the West Virginia University. Spores spread by rain, wind and insects.
Any tree that suffers cankers should be closely monitored and diseased wood promptly removed. Old fruit needs to be promptly picked and disposed of. Pick up old leaves and fruit from beneath the tree or plant. Diligent applications of fungicides will also help combat the fungus. Capton and myclobutanil fungicide formulation works against the fungus.