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The Role of Ice Formation in the Infection of Sour Cherry Leaves by Pseudomonas syringae pv. syringae. S. Süle, Research Institute for Plant Protection of Hungarian Academy of Sciences, 1525 Budapest, P. O. Box 102, Hungary; E. Seemüller, Biologische Bundesanstalt für Land- und Forstwirtschaft, Research Institute for Plant Protection in Fruit Crops, 6901 Dossenheim, Federal Republic of Germany. Phytopathology 77:173-177. Accepted for publication 18 February 1986. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/Phyto-77-173.

Infection of sour cherry (Prunus cerasus) leaves by ice nucleation-active (INA+) and inactive (INA) strains of Pseudomonas syringae pv. syringae and P. s. pv. morsprunorum after leaf freezing was investigated. Leaves were infected only when ice previously had been formed in their intercellular spaces. This occurred between –2 and –3 C. INA+ strains induced freezing 0.5–0.8 C above the temperature required for INA strains. Leaves that had been sprayed and frozen weighed 2–4% more than leaves that had not been sprayed and frozen. After freezing, 103–104 bacteria per gram of leaf tissue could be isolated from the leaf interior. This ingress of bacteria appeared to be passive. P. s. pv. tabaci cells were also absorbed by sour cherry leaves as well as P. s. pv. syringae cells. Investigations on symptom development and leaf freezing showed that the infection is not related to low temperature, but to ice formation in the presence of P. s. pv. syringae. The infection by this bacterium did not result from freezing injuries of the leaf tissue. Freezing predisposed the plants to infection if inoculation occurred within 20 min of thawing. Inoculations made 30 min after thawing did not result in any lesions.