Isolation ofPseudomonas syringaepv.syringaefrom Cankers and Effect of Free Moisture on Its Epiphytic Populations on Sweet Cherry Trees

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Abstract
Frequency of isolation of P. syringae pathovar syringae (Pss) from bacterial cankers on 3- to 4-yr-old sweet cherry trees [Prunus avium] (cultivar Napoleon) was highest between late winter (Aug.) and early spring (Sept.). 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/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 h of wetness or dryness.