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 induced resistance against bacterial angular leaf spot caused by Pseudomonas syringae pv. lachrymans. Treatment of seeds or cotyledons with both strains resulted in significant decreases in lesion number and size compared to those of a noninduced disease control. Pathogen populations (log CFU per cm2 of leaf) in inoculated leaves also were significantly decreased with PGPR treatments, declining from log 8.0 CFU per cm2 in noninduced controls to log 5.5 CFU per cm2 with strain 89B-27 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 induced resistance between seed inoculation and cotyledon injection with both PGPR strains. Classic ISR treatments (either induced by introducing Colletotrichum orbiculare or P. syringae pv. lachrymans on the first true leaves) also significantly decreased lesion number and size and population densities of the pathogen in inoculated leaves compared to those of the noninduced disease control. Neither lesion size nor pathogen population densities was significantly different between MPR-mediated ISR and classic ISR. Local necrotic lesions on cotyledons were observed 5 days after inoculation with the pathogen, and necrosis was required 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 were maintained above log 9 CFU per cotyledon up to 14 days after inoculation. The results indicate that PGPR-mediated ISR is similar to classic ISR in that multiple pathogens may be controlled. However, unlike classic inducing agents, PGPR do not cause visible localized necrosis.