Pseudomonas lachrymans inoculum on infected cucumber leaves subjected to dew- and rain-type wetting.
The amount of readily available P. lachrymans inoculum present on cucumber leaf surfaces with lesions was determined by vacuuming dry leaves and by dilution-plating of water on leaves subjected to various moisture conditions. No inoculum was lifted by vacuum from dry infected leaves unless the lesions were abraded. With no wetting period (except for a standard 10 min wash prior to dilution plating), 2.8 X 104 colony-forming units (CFU) were obtained from leaves with one lesion. Increasing lesion numbers/leaf did not yield a comparable increase in CFU. On leaf surfaces wet with dew, both the pathogen and other bacteria increased for the first 12 h, but the pathogen increased much more rapidly. Following this rise there was a precipitous decline in the population of P. lachrymans and a very large increase in other bacteria on the leaf surface. Similarly infected leaves subjected to continuous rain had increasing populations of both pathogen and nonpathogens for 8-16 h until a relatively steady state was reached. Inoculum production during discontinuous wetting was not additive; wet periods of 4 or 8 h/day on successive days yielded pathogen and nonpathogen levels similar to those found with single 4 or 8 h periods of wetting on one day. Also, the post-12 h decline in pathogen population and increase in other bacteria found with continuous dew was not reproduced by several 4, 8 or 12 h wetting periods on successive days. Considering the P. lachrymans CFU available for dissemination from wet infected leaves, it seems likely that angular leaf spot epidemics are not limited by low inoculum levels but by factors that control the ingress of the pathogen to the intercellular leaf spaces.