Stone Fruit

Bacterial spot occurs in most countries where stone fruits are grown. Common hosts include peach, nectarine, prune, plum, and apricot. Other hosts are sweet and tart cherry, almond and wild peach.

Black knot of plum, caused by the fungus Apiosporina mobosa, is well-named because of the characteristic black, warty knots it forms on the branches of infected trees. Infected trees grow poorly and gradually become stunted; occasionally, their limbs are girdled. The disease is most important on plum, prune, and sour and wild cherry trees.

Brown rot, caused by the fungus Monilinia fructicola, is one of the major stone fruit diseases in Pennsylvania. The disease affects peaches, apricots, nectarines, plums, cherries, and most commercially grown Prunus species. The fungus can cause a blossom and twig blight, a canker, a leaf infection, and a fruit rot. Infected fruit will rot on the tree and after being harvested.

Cherry leaf spot, caused by the fungus Blumeriella jaapii, attacks the leaves, leaf stems, fruit, and fruit stems of tart, sweet, and English Morello cherries. The disease is most severe on leaves and may cause them to drop prematurely.

Cytospora canker is one of the most destructive diseases of peaches, nectarines, apricots, sweet cherries, and plums in Pennsylvania. Also known as perennial canker, peach canker, Valsa canker, and Leucostoma canker, the disease may cause trees in young orchards to die. Infected trees in older orchards gradually lose productivity and slowly decline.

Peach leaf curl, caused by the fungus Taphrina deformans, is a common disease of peach and nectarine. This fungus destroys young peach leaves. Although new leaves develop, their growth reduces established food reserves, weakens the tree, and can reduce yield. Defoliation by peach leaf curl in successive seasons can kill the tree.

A number of diseases of stone fruit are caused by fungi similar to the leaf curl fungus. In the northeastern United States, the most important disease affecting the American-type plum is known as plum pockets, or bladder plum. It is caused by the fungus, Taphrina pruni. This fungus occurs wild or on abandoned plum trees and its occurrence is rare.

Powdery mildew, sometimes called rose mildew because it affects some woody ornamentals, is not often serious and occurs only sporadically. The causal fungus, Sphaerotheca pannosa, is usually rare in peach orchards. The fungus can attack leaves, twigs, and fruit, however, fruit infections cause the greatest economic loss.

Rhizopus rot, caused by Rhizopus stolonifer, can be very destructive to harvested fruit. While it can develop in hail-injured or cracked fruit on the tree, it most commonly affects fruit in storage, during transit, and at the marketplace. Ripe fruit of peaches, nectarines, sweet cherries, and plums are most susceptible. Rhizopus fruit rot is usually of minor importance in the field but can cause important postharvest losses.

Leaf spot of plums and prune-type plums is caused by the fungus Coccomyces prunophorae. The fungus, its life cycle, and the disease it causes are very similar to those of cherry leaf spot. On plum leaves, the spots tend to be smaller, and severely infected leaves often have a tattered appearance. Unlike cherry infection, severe plum leaf infection is often followed by a heavy fruit drop.