INDUCTION OF SYSTEMIC RESISTANCE IN CUCUMBER AGAINST BACTERIAL ANGULAR LEAF-SPOT BY PLANT GROWTH-PROMOTING RHIZOBACTERIA

Two strains of plant growth-promoting rhizobacteria (PGPR) Pseudomonas putida 89B-27 and Serratia marcescens 90-166, that previously induced systemic resistance (ISR) in cucumber against cucumber anthracnose in duced resistance against bacterial angular leaf spot caused by Pseudom onas syringae pv. lachrymans. Treatment of seeds or cotyledons with bo th strains resulted in significant decreases in lesion number and size compared to those of a noninduced disease control. Pathogen populatio ns (log CFU per cm(2) of leaf) in inoculated leaves also were signific antly decreased with PGPR treatments, declining from log 8.0 CFU per c m(2) in noninduced controls to log 5.5 CFU per cm(2) with strain 89B-2 7 and log 6.2 CFU per cm2 with strain 90-166 when cotyledon injections were used. There was no significant difference in the level of induce d resistance between seed inoculation and cotyledon injection with bot h PGPR strains. Classic ISR treatments (either induced by introducing Colletotrichum orbiculare or P. syringae pv. lachrymans on the first t rue leaves) also significantly decreased lesion number and size and po pulation densities of the pathogen in inoculated leaves compared to th ose of the noninduced disease control. Neither lesion size nor pathoge n population densities was significantly different between PGPR-mediat ed ISR and classic ISR. Local necrotic lesions on cotyledons were obse rved 5 days after inoculation with the pathogen, and necrosis was requ ired in the classic ISR system. However, no visible lesions developed on cotyledons after injections of PGPR. Populations of strains 89B-27 and 90-166 increased rapidly in the cotyledons after injection and wer e maintained above log 9 CFU per cotyledon up to 14 days after inocula tion. The results indicate that PGPR-mediated ISR is similar to classi c ISR in that multiple pathogens may be controlled. However, unlike cl assic inducing agents, PGPR do not cause visible localized necrosis.