Effect of Exposure to Freezing Temperatures on Necrosis in Sweet Cherry Shoots Inoculated with Pseudomonas syringae pv. syringae or P. s. morsprunorum. P. Sobiczewski, Institute of Pomology and Floriculture, 96-100 Skierniewice, Poland. A. L. Jones, Department of Botany and Plant Pathology and the Pesticide Research Center, Michigan State University, East Lansing 48824. Plant Dis. 76:447-451. Accepted for publication 3 December 1991. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/PD-76-0447.

Dormant 1-yr-old shoots of two sweet cherry (Prunus avium) cultivars were wound-inoculated with Pseudomonas syringae pv. syringae or with P. s. morsprunorum and then incubated sequentially at 15, –10, and 15 C for 7, 1.5, and 10 days, respectively. Dark-stained necrotic tissue extended downward from the point of inoculation at the tip of the shoots. Inoculations with P. s. syringae resulted in significantly greater necrosis than inoculations with P. s. morsprunorum. The population and the distribution of bacteria in shoots just before exposure to freezing temperatures were greater for P. s. syringae than for P. s. morsprunorum. Ten days after exposure to freezing temperatures, populations of both pathovars were much higher in shoots subjected to –10 C for 1.5 days than in shoots maintained at 15 C through the experimental period. The extent of necrosis in wound-inoculated shoots of the cultivar Hedelfingen decreased, whereas necrosis in shoots of the cultivar Gold increased with the period of dormancy. When inoculations preceded exposure to freezing temperatures by 7 days, the extent of necrosis in dormant shoots of cultivars inoculated with P. s. syringae was as follows (in decreasing order): Napoleon, Emperor Francis, Gold, Nelson, Ulster, Sam, Vega, Windsor, Schmidt, Hedelfingen, Valera, Vic, and Viva. Cultivars Ulster, Vega, and Napoleon developed the most necrosis when inoculated with P. s. morsprunorum.

Keyword(s): bacterial canker.