Relative Humidity and the Survival of Epiphytic Bacteria with Buds and Leaves of Cucumber Plants

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Abstract
Terminal buds of cucumber seedlings were inoculated with ice-nucleating-active isolates of Pseudomonas syringae pv. lachrymans (pathogenic to cucumber) or P. s. pv. syringae (pathogenic to bean), and the plants kept in a screened greenhouse at different levels of relative humidity (RH)-alternating (50-60% during the day and more at 90% at night), low (30-50%), or high (80 to more than 90%). The foliage was not wetted. Later, bacteria were detected with a selective medium. At low RH, no pathogens were detected by agar prints of healthy leaves; nor were they in terminal or axillary buds. At high RH, they were on nearly all of both bud types; at alternating RH, values were intermediate. Pathogen loci were distributed uniformly on the youngest, erratically on the intermediate, and sparsely on the oldest healthy leaves at high or alternating RH. Microscope observations of leaves showed that most bacteria were erratically distributed, even within 1-2 mm, on or near veins, parts that wetted best. Favored short-term survival sites were both types of buds and near the junction of the main veins and the petiole on the leaf lower surface. Field tests indicated that the terminal bud was colonized continuously by P. s. pv. lachrymans. Conclusions are that high RH favors or continuous colonization of terminal buds, distribution of bacterial cells on unfolding plant parts, and survival of bacteria on exposed parts; low RH favors none of these events, which can be sequential.