November 12, 2015 – Kenosha News
Question: All of my peaches had black spots and looked like they were oozing sap. They were loaded with peaches and most had to be thrown away. What can I do next season to prevent this? T.N.
Answer: Oozing on the fruits along with black spots indicates your peaches have “Bacterial Spot” disease. This disease is caused by the bacteria Xanthomonas campestris pv. pruni. Bacterial Spot is most common in peach and nectarine, but may also infect apricot and plum. The bacteria will infect leaves and small twigs as well as fruits. On leaves the bacteria causes small dark spots that eventually drop out leaving a shot hole appearance. The bacteria survive winter in small purplish cankers on twigs of infected trees. Dark spots followed by cracking and oozing are typical of fruit infections.
You can lessen bacterial spot disease by keeping your tree actively growing with proper pruning, fertilization and watering. Do not over fertilize with nitrogen which may make the disease worse. “Growing Apricots, Cherries, Peaches and Plums in Wisconsin” available at http://learningstore.uwex.edu will give you more specific cultural information. If you decide to replace your peach tree, some peach varieties are less susceptible to bacterial spot but I couldn’t find any cold hardy enough for our Zone 5b in Kenosha County. I suggest checking with your favorite fruit nursery.
Bacterial spot can be challenging to control in the home orchard because it may be difficult to find bactericides. According to 2015 Midwest Fruit Tree Spray Guide, “Bacterial spot of peach can be a serious problem in certain varieties, areas, and years. The disease is favored by stormy, rainy weather during June and July. Planting cultivars that are resistant to bacterial spot provides the best control. An antibiotic, oxytetracycline (Mycoshield or FireLine) provides good control when properly applied. For best results, oxytetracycline must be used at 12 oz per 100 gallons of dilute spray. Use dilute or 2x; higher concentrates are not effective and may be phytotoxic. Once per week spraying of the entire tree is essential. Begin sprays at shuck-split and continue at 7-day intervals until 3 weeks before harvest. Copper sprays, applied at autumn leaf drop, also may aid in control of bacterial spot.”
Copper sprays should be used very carefully because peaches are prone to copper toxicity. Antibiotics and copper bactericides must be labelled for use on peaches and directions for spraying should be followed exactly.
Barb Larson is horticulture educator for the Kenosha County University of Wisconsin Extension. She holds a master’s of science in horticulture from the University of Wisconsin-Madison. If you have a plant or gardening question, email her at firstname.lastname@example.org or call 262-857-1942.