Isolation of Pseudomonas syringae pv. syringae from Cankers and Effect of Free Moisture on Its Epiphytic Populations on Sweet Cherry Trees. B. A. Latorre, Adjunct Professor, Departmento de Ciencias Vegetales, Facultad de Agronomía, Pontificia Universidad Católica de Chile, Casilla 114-D, Santiago, Chile. J. A. González, J. E. Cox, and F. Vial, Former Students, Departmento de Ciencias Vegetales, Facultad de Agronomía, Pontificia Universidad Católica de Chile, Casilla 114-D, Santiago, Chile. Plant Dis. 69:409-412. Accepted for publication 30 October 1984. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/PD-69-409.

Frequency of isolation of Pseudomonas syringae pv. syringae (Pss) from bacterial cankers on 3- to 4-yr-old sweet cherry trees (cultivar Napoleon) was highest between late winter (August) and early spring (September). Large, abrupt changes in epiphytic populations of Pss were found on dormant buds of Napoleon cherry shoots under field conditions. These changes closely followed the presence or absence of free moisture resulting from rainfall or dew. Populations of Pss greater than 10⁴ colony-forming units per gram of buds were only recorded during wet periods; otherwise, populations remained low or undetectable. Field observations were corroborated by greenhouse experiments on detached, dormant cherry shoots where 10-fold or higher fluctuations in bacterial populations occurred in response to 24 hr of wetness or dryness.