Abstract
Leaves of lettuce (Lactuca sativa L. cultivar Success) were dipped in a solution of Pseudomonas cichorii. Population of P. cichorii on leaves incubated at 30°C was 1.3×108cfu/g fresh weight 7 days after inoculation and disease incidence of bacterial rot of lettuce was severe. The bacterial population on leaves, which were incubated at 20°C, was less than 105cfu/g fresh weight, and disease incidence was slight. Lettuce plants were cultivated in a field, in which many lettuce plants had been severely infected with P. cichorii at any year, in Ichinohe-machi, Iwate Prefecture. P. cichorii had been isolated at the low density from soil since 37 days before transplanting. P. cichorii was isolated at the low density from entire plants before head formation. The bacterial population on leaves during head formation is lognormally distributed, and bacterial rot was detected on the bulked leaves, from which P. cichorii was isolated at the density of above 105cfu/g fresh weight. The bacterial populations increased on outerleaves and head-leaves in the early stage and the middle stage of head formation, respectively. The bacterial population on head-leaves positively correlated with that on outer-leaves. The percentage of lettuce plants with diseased head-leaves positively correlated with that with diseased outer-leaves. Therefore. P. cichorii exists on leaves of lettuce during head formation as the epiphytic bacterium and disease symptom is detected on the leaf in which population of P. cichorii is above 105cfu/g fresh weight. P. cichorii, with which outer-leaves are infected, is an important infection source of the disease on head-leaves.
