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Ecology and Epidemiology

Involvement of pH in the Competition between Cytospora cincta and Pseudomonas syringae pv. syringae. Elke Endert- Kirkpatrick, Former graduate research assistant, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616; David F. Ritchie, Associate professor, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616. Phytopathology 78:619-624. Accepted for publication 1 December 1987. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/Phyto-78-619.

Isolations from cankered and freeze-injured peach trees from 1982 to 1985 indicated that Cytospora cincta and various yeasts predominated, whereas Pseudomonas syringae pv. syringae was isolated infrequently. Less than 50% of fluorescent bacteria isolated were pathogenic. Growth and survival of C. cincta and P. s. pv. syringae were monitored during separate and mixed incubation in a liquid medium and after inoculation into bark of dormant 8- and 9-yr-old peach trees. In a liquid medium based on aqueous peach twig extracts, populations of P. s. pv. syringae declined rapidly in mixed cultures with C. cincta. In some replications, viable bacteria were no longer detected after 35 days of incubation. Growth of C. cincta was not significantly affected by the presence of P. s. pv. syringae. The pH of the medium declined rapidly during the first 2 days of incubation of C. cincta by itself or with P. s. pv. syringae. Mixed inoculation of C. cincta and P. s. pv. syringae into scaffold limbs of peach trees resulted in slightly larger cankers than did inoculation of C. cincta alone; however, P. s. pv. syringae could not be reisolated from these cankers. P. s. pv. syringae also failed to survive in sites inoculated with bacteria alone, but naturally occurring C. cincta was isolated from most of these sites. Colonization of bark by C. cincta reduced the pH of inner bark from pH 5.2 to as low as pH 3.8. We conclude that P. s. pv. syringae is incapable of long-term survival in a shared environment with C. cincta.