Answer: A bacterial disease, blossom blast (causal organism: Pseudomonas syringae), may afflict pears, usually as a blossom blight resulting in reduced fruit set. It can also cause twig dieback and bark cankers and may lead to severe wood damage of Asian-pear cultivars in particular. Because the presence of blossom-blast bacteria allows ice crystals to form at higher-than-normal temperatures, the disease increases the incidence of freeze damage during cold, wet weather. Asian pears are especially affected because their early bloom makes them more susceptible to frost injury. Among Asian-pear cultivars, Shinko and Ya Li are moderately resistant to P. syringae.
Controlling this disease is difficult because its occurrence is widespread on many plant species and not easily predicted; once symptoms appear, control efforts are too late. Protecting orchards from frost damage can limit injury. An early application of BlightBan A506 can help reduce frost-damage potential by excluding the ice-nucleating bacteria. In California, the application of fixed copper at the green-tip stage followed by streptomycin at early bloom has provided reasonable control. This treatment has also been used in Oregon and Washington, where cool, wet weather makes blossom blast a particular problem in pear production. Streptomycin or terramycin applied at early bloom to control fire blight also helps to control blossom blast, but sprays timed for fire blight control cannot be assumed to control blossom blast because P. syringae is favored by cool, wet conditions while E. amylovora proliferates in warm, wet conditions. In other words, predictive models for fire blight could easily miss an infection period for blossom blast. There are no predictive models for blossom blast, so the prudent grower with a history of blossom blast in the orchard will apply copper at green tip and follow up with BlightBan or streptomycin at early bloom regardless of what fire blight predictive models might indicate.
For more information, consult the ATTRA publication Pears: Organic Production. It covers pear diseases, disease-resistant cultivars, rootstocks, insect and mite pests, and their treatment, Asian pears, and marketing. Two profiles of organic pear growers are included. Electronic and print resources are provided for further research.
In addition, ATTRA's Pear Diseases Identification Sheet is a handy guide that helps identify pear diseases and offers low-spray and organic solutions.
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