Bacterial Canker is a serious disease of stone fruits and a major facilitator of Peach Tree Short Life (PTSL). It is caused by the bacterium Pseudomonas syringae pv syringae. Despite the devastating damages the bacterium can cause on peach trees, it is a rather weak pathogen that usually coexists with the tree not causing any harm. In fact, the bacterium is likely a member of the rainmaking P. syringae family, playing a crutial role for bio-precipitation in the earths atmosphere. A peach tree becomes predisposed to infection when attacked by ring nematodes (which reduces N availability in the canopy), not receiving adaequate nitrogen (lack of fall fertilization or pH too low), and early pruning (before Jan). A weakened tree with high C:N ratio (N deficit) cannot hinder the bacterium from producing syringomycin, a toxin that destroys plant cells. At high concentrations the toxin even kills the pathogen itself, which is why scientists have often been unsuccessful in isolating the pathogen from symptomatic bark tissue.
Trees often do not bloom or leaf out in the spring. If they do leaf out, death will likely happen within 1 to 6 weeks. The bark of affected branch or trunk areas is completely brown and difficult to separate from the wood (which is in contrast to freeze injury). Discoloration stops at the soil line. Another symptom of the disease is the death of dormant buds with associated brown areas under the bark around the bud. Spring cankers on twigs appear moist, affecting the bark more than the wood. In contrast, infections by Xanthomonas arboricola pv. pruni causes the bark to be black and wrinkled (dried). Cankers are usually only a few inches in length and occur on previous season's twig growth.
1. Apply lime before planting to adjust pH to 6.5 or above in the top 16 in of soil.
2. Subsoil before planting to break up hardpans and promote improved root development
3. In sandy soils where peach trees have been planted previously and in other soils where nematodes are a problem, fumigate the soil before planting (last opportunity to control nematodes chemically).
4. Plant trees which have been grown in fumigated soil or certified to be free of nematodes.
5. Plant trees propagated on Guardian rootstock where nematodes and PTSL occur.
6. Apply fertilizer and lime as needed based on soil tests, foliar analyses, and local recommendations. Especially fall fertilization has been shown to reduce Bacterial canker. Split your fertilizer application for the year into two and apply one application after harvest but no later than early August. Growers with late varieties should be extremely cautious not to apply excessive N, which could stimulate growth and make the trees susceptibele to winter injury. The late winter application (second application) of nitrogen provides for growth, flower bud development, and fruit size.
7. Prune as late as possible, never before Jan 1, and preferably after Feb 1. If earlier pruning is unavoidable, prune older trees first. Early pruning is especially hazardous on old peach land. Discontinue summer pruning (including topping and hedging) by September 15.
8. Use recommended herbicides for weed control. Cultivation, if needed, should be shallow to avoid root injury.
9. Remove and destroy promptly all dead or dying trees including as much of the root system as practical
10. Dip pruning tools between trees in a 9 parts water and 1 part Chlorox solution in infected orchards
11. Consider copper applications PRIOR to severe rain storms in the fall during leaf drop.
Photos by Dave Ritchie, NC State