Evaluation of plant growth-promoting rhizobacteria for control of Phytophthora blight on squash under greenhouse conditions

Phytophthora blight caused by Phytophthora capsici is a serious threat to vegetable production worldwide. Currently, no single method provides adequate control of P. capsici. Greenhouse studies were conducted to evaluate the potential of the use of bacilli plant growth-promoting rhizobacteria (PGPR) for control of Phytophthora blight on squash. PGPR strains were applied as a soil drench 1 and 2 weeks after planting (WAP), and P. capsici was applied to squash roots at 3 WAP. PGPR strains SE34 and SE49 applied at 1 × 108 CFU/ml significantly (P < 0.05) reduced disease severity in all three repeated greenhouse trials compared to the nontreated control. Treatments with PGPR strains SE52, SE76, INR7, IN937a, and IN937b demonstrated significantly lower disease in two of three trials when compared with the nontreated control. Certain PGPR strains applied as 2-, 3-, and 4- strain mixtures significantly reduced disease severity. Treatment with T4 + SE56 demonstrated significantly lower levels of disease than any individual PGPR strain, indicating either an additive or synergistic effect on disease reduction achieved by mixing PGPR strains. Others such as INR7 + T4 + SE56 and INR7 + IN937a + T4 + SE56 have a high potential to significantly improve the control efficacy. When applied as seed treatments, only PGPR strain1PC-11 at 1 × 105 CFU/seed resulted in significant reduction in Phytophthora blight disease in all trials, while PGPR strains SE56 at 1 × 105 and 1 × 106 CFU/seed, GB03 at 1 × 105 CFU/seed, 1PC-11 at 1 × 106 CFU/seed, and 1PN-19 at 1 × 104 CFU/seed significantly suppressed the disease in two of three trials. These results indicate that certain PGPR strains are effective against P. capsici on squash, and improved disease control can be achieved by multiplexing them.