The prunus plant group includes several stone fruit trees or drupes, such as peaches, plums, apricots and cherries. These trees have several ugly fungal infections in common; if you keep drupes in your yard, it's important to know what to look for so that you can help prevent infections. Some fungal diseases usually kill the plant, while others may merely weaken it. None, of course, are desirable.
Black knot, Dibotryon morbosum, attacks prunus species; some, such as sour or tart cherries, are less susceptible. When first noted by late 19th-century growers, they considered black knot to be one of the most destructive prunus diseases around. Today, it is still common, but somewhat easier to control. The fungus causes the tree's cells to reproduce too rapidly and swell, forming the signature black knot. These knots, up to a foot long and about 20 inches wide, weaken the tree. In a bad infestation, the tree may die. You may be able to prune out the affected parts in dry weather and use drip irrigation at ground level to avoid dampening the tree. Also, protect the tree from deer, since their chewing gives fungal spores a place to enter your tree.
Shothole and Leaf Spots
Another common fungal issue in prunus is shothole (Wilsonomyces carpophilus) and leaf spots. The latter are caused by several types of fungi, including those in the Cercospora family. Tree leaves develop dead spots with margins; the holes may drop out, leaving a ragged appearance. To avoid these problems, stick to drip irrigation and remove all fallen leaves. As they may be infected, destroy rather than compost them. It may be possible to spray your tree, but you must first determine for sure what kind of infection this is -- the same hole-creating symptoms can be caused by bacteria. This kind of determination requires expert advice.
As the name suggests, rots are diseases that cause various types of rot in plants. Several rots affect almost every part of prunus species. Brown rot (Monilinia fructicola) affects the blossoms and fruit. Rhizopus rot (Rhizopus stolonifer) causes black whiskers to appear on the fruit. Phytophthora root rot is caused by various types of Phytophthora fungus and kills off the roots. Practicing good tree husbandry, keeping moisture off the trunk and branches, and spraying with fungicides may help.
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- Montana State University Department of Plant Sciences & Plant Pathology: Pests and Environmental Problems of High Altitude Landscapes
- Oregon State University Extension: Leaf Spots and Shothole
- Texas AgriLife Extension Plant Handbook: Peach, Apricot and Nectarine
- Mildred E. Mathias Botanical Garden: Cherries, Peaches and Related Drupes