Induced systemic resistance to cucumber diseases and increased plant growth by plant growth-promoting rhizobacteria under field conditions

Three field trials were conducted over a 2-year period to determine the capacity of plant growth-promoting rhizobacteria (PGPR) to induce systemic resistance against cucumber diseases, which was previously observed in the greenhouse. PGPR were applied as seed treatments alone or as seed treatments plus a soil drench at transplanting. Induced systemic resistance (ISR) activity, measured by significant reductions in the severity of angular leaf spot challenge-inoculated with Pseudomonas syringae pv. lachrymans on cucumber, occurred with three of three PGPR strains in 1992, with three of four strains in the first trial in 1993, and with four of four strains in the second trial in 1993. Most PGPR treatments also resulted in significant protection from naturally occur ring anthracnose (Colletotrichum orbiculare), while ISR controls (plants previously inoculated with C, orbiculare) had a significantly greater incidence of anthracnose compared with nontreated controls. In all three trials, most PGPR strains (with the exception of one strain) significantly promoted early-season plant growth, as measured by length of the main runner and number of leaves per plant. Yield (cumulative fruit weight) was significantly increased by two of three PGPR strains in 1992, two of four strains in the first 1993 trial, and three of four strains in the second 1993 trial compared with nontreated controls. In contrast, the ISR control had no significant effect on promotion of early-season plant growth or yield compared with nontreated controls. These results demonstrated that PGPR-mediated ISR was operative under field conditions with consistent effects against challenge-inoculated angular leaf spot and naturally occurring anthracnose, along with significant effects on early-season plant growth promotion and yield enhancement.