How can I organically control plum pox and bacterial spot in peaches?
Answer: Sharka virus was confirmed in Adams County, Pennsylvania—the first outbreak in North America. The disease has since been found in Cumberland County, Pennsylvania, and in Canada. This has caused much concern among producers because plum pox is an exceptionally destructive disease of stonefruit. Infected fruit are unmarketable because of spots and ring blemishes, and fruit may also drop prematurely.
Plum pox is transmitted either by aphids or by grafting. The disease was not found in nursery stock, and an ongoing quarantine has apparently contained the outbreak of plum pox to a limited area, although testing is ongoing.
In 2007, USDA plant breeder Dr. Ralph Scorza released the ‘HoneySweet’ plum, which is functionally immune to plum pox (Kaplan, 2007). The pox-resistant genes from this plum can be incorporated into peach and other stonefruit so that, eventually, pox-resistant peach cultivars will probably be available to growers. (Note: ‘HoneySweet’ is the product of genetic engineering, and, as such, is not allowed in certified organic systems.)
The telltale symptom of bacterial spot (caused by the bacterium Xanthomonas pruni) is small light brown lesions on leaves. Eventually the affected tissue falls out, leaving a characteristic shotgun-hole appearance. Severe bacterial spot infections may cause premature defoliation and subsequent re-sprouting, similar to peach leaf curl. Bacterial spot on fruit occurs as sunken, dry lesions that eventually crack, opening the fruit to secondary infections and reducing fruit quality.
Selecting disease-resistant cultivars is the principal means of controlling bacterial spot. The ATTRA publication Peaches: Organic and Low-Spray Production includes a table from the Kearneysville Tree Fruit Research and Extension Center at West Virginia University on peach cultivar susceptibility to bacterial spot.
Fortunately for organic growers, copper fungicides—unique in that they also function as bactericides—are allowed for control of bacterial spot. The first spray should be applied before the tree leafs out in the spring; this timing often allows copper-based peach-leaf-curl sprays to double for bacterial-spot treatment. The next period when infection pressure is heavy is petal fall and for three weeks thereafter. Additional spray coverage may be necessary depending on varietal susceptibility and humid weather conditions.
Since the occurrence and severity of bacterial spot depend on moisture, this disease is rarely a problem west of the Rocky Mountains, and in the East, growers are able to rely on resistant varieties as the best line of defense. Contact the Cooperative Extension Service for resistant varieties suited to your region.
You’ll benefit from reading 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. It profiles a successful organic peach grower in California, discusses new-generation synthetic and organic pesticides, and describes organic peach research for the East, as well as a model reduced-spray program for the East. A “postscript” examines the dilemma of the environmentally conscious consumer in regard to peaches.
Kaplan, Kim. 2007. Plum pox resistant trees move forward. Agricultural Research Service News and Events. July 25.