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What is it?
Apple tree black rot is a plant disease caused by the fungus of the physalospora species. Black rot is also known as frog-eye leaf spot, and this plant disease attacks pears as well as apples. Fungi of the physalospora species over winter in rotted apples and cakerous lesions on apple trees and emerge at temperatures as low as 55 degrees to spread again. Black rot is an especially dangerous disease to apple trees and the apple harvest if left uncontrolled.
What does it look like?
Black rot appears a light brown spots which are surrounded by darker brown lines on the actual fruit of the apple. After appearing, the brown spots quickly turn black and form a hard, concentric circle on the apple itself. The small concentric spot of rings widens quickly and rots the fruit in no time, often rotting a large portion of the apple or the entire fruit in less than a couple of days. Once the fruit has dried, it remains attached to the apple stems and the physalospora species fungi spreads into the leaves which may show brownish or reddish spots on the foliage as the plant disease infects the apple leaf tissue. Once infected with physalospora the leaves will usually wilt and may discolor to yellow or brown as they too dry and die off. The apple tree bark including twigs, branches and trunk may also be affected and show black rot damage by the presence of elongated lesions. Usually these cankerous lesions will be sunken and a deep reddish or black color. Often these black rot lesions can grow up to several feet in length and may cover the entire length of the branch of an apple tree.
How does it manifest?
Apple black rot occurs when spores infect new foliage and fruit in the spring. Often the fungi over winters on the cakerous lesions on the bark of the tree or in fallen plant debris or mummified fruits. Physalospora species fungi can also survive the colder temperatures especially since it emerges to be active at temperatures as low as 55 degrees. Once these slightly warm and moist spring conditions occur, the spores then infect new twigs, branches, leaves and the developing fruit which is especially susceptible to black rot. The most severe damage from black rot occurs when the fruit is approximately one-half to two-thirds grown, however damage from physalosporacan occur at any stage of the apple fruits development and to any area of the apple tree. Infected leaves, bark and fruit create more spores which can then spread future infection through splashing water, rain or wind. The main conditions which affect the intensity and severity of apple tree black rot infection are the amount of diseased material which has managed to over winter, as well as the conditions of moisture and warmth which occur in the spring.
What can you do about it?
Black rot infected regions of the apple tree should be pruned out, being sure to destroy all infected fruit, twigs and branches. Do not just prune the areas and let them drop as that can release more spores to otherwise unaffecte regions of your apple tree. Be sure to carefully prune out any cankerous lesions no matter how small they seem and going at least 8 inches around the visible cankers, because the lesions will grow if left unattended and do extend beyond the visible lesion. Also be sure to clear and destroy all infected plant debris. Unfortunately, once the fruit has begun to rot and die fungicidal treatments are no longer helpful. However, you can be prepared next year when the spores may reinfect new apple tree development. Your local cooperative extension service should be able to provide you with the name of the fungicide most effective for black rot infections for apple trees in your geographic region and the best time to begin treatment. Typically once you begin fungicide treatments, continue at ten day intervals until the fruit is full-sized.
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