Answer: The two related fungi (Leucocytospora cincta and L. leucostoma) that incite cytospora canker are opportunists, invading sites where damage has occurred due to mechanical injury, cold, poor pruning techniques, improper pruning time, borers, or other causes. The first visible symptom is the oozing of gummy sap near the wound, beginning when temperatures warm in the spring. Since peaches exude this gummy sap in response to almost any wound (e.g., borer attack), it can be difficult to diagnose this disorder correctly. One diagnostic clue is that cytospora cankers usually have an elongated or elliptical shape because the fungus advances more rapidly up and down the branch than around the branch. The bark dries out and dies but usually remains intact the first year. In succeeding years the bark becomes broken, disfigured, and covered with a black fungus overgrowth. The disease progresses slowly, and a tree with cytospora can survive for many years past the initial infection.
While cytospora can be found in California (more commonly on European plums), it is a much more serious disease on peaches in the eastern half of the country where winter temperatures— especially fluctuating winter temperatures—often lead to tissue damage. In fact, it is probably the leading cause of peach tree death in much of the eastern United States.
Control is limited to cultural techniques. Management begins by choosing planting sites away from older peach and plum trees and eliminating wild or untended plums and peaches near the orchard. Because a cold-damage area is often the primary infection site, painting trunks with white latex paint to refl ect the winter sun can be helpful, although this practice is not allowed in organic production. Avoid planting on a southor southwest-facing slope because such a site can induce the trees to warm up too soon in the early spring or late winter, resulting in cold damage if temperatures fall.
Other management techniques likewise center around minimizing damage to the trees, thus denying infection sites to the pathogens. Such techniques include pruning only in the early spring when temperatures have warmed, avoiding leaving pruning stubs, removing dead and diseased branches, and controlling borers.
You can find much more information in the ATTRA publication Peaches: Organic and Low-Spray Production. This publication describes the major diseases and insect pests of peaches and discusses organic or least-toxic control options for each. It emphasizes the considerable climatic differences between the arid West, which is relatively amenable to organic peach production, and the humid East, where it is more difficult to grow peaches without synthetic fungicides and insecticides.
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