Combined Use of PGPR and Reduced Rates of Azoxystrobin to Improve Management of Sheath Blight of Rice

Farmers heavily rely on the use of strobilurin fungicides to manage sheath blight (ShB) caused by Rhizoctonia solani AG1-IA, the most important disease in rice in the southern United States. Greenhouse and field studies were conducted to evaluate the potential use of plant growth-promoting rhizobacteria (PGPR) in combination with a reduced rate of azoxystrobin as a strategy to improve the current fungicide-reliant management. Of the nine antagonistic PGPR strains screened in the greenhouse, Bacillus subtilis strain MBI600 provided the most significant and consistent suppression of ShB. Efficacy of strain MBI600 was further evaluated at the concentrations of zero, 103, 106, 109, and 1011 CFU/ml alone or in combinations with zero, 17, 33, 50, 67, 83, and 100% of the recommended rate (0.16 kg a.i./ha) of azoxystrobin. Strain MBI600 applied at 106,109, and 1011 CFU/ml alone was effective in reducing ShB severity. Combinations of this strain at these rates with 33% or more of the recommended rate of azoxystrobin further reduced ShB severity. A dose response model defining the relationships between strain MBI600, azoxystrobin, and ShB severity was established. Estimates of the effective concentrations, EC50 and EC90, of strain MBI600 when applied in combination with 50% of the recommended rate of azoxystrobin were 104 and 109 CFU/ml, respectively. A field trial was conducted over 4 years to verify the efficacy of their combinations. Strain MBI600 alone, when applied at 109 CFU/ml at the boot stage, reduced ShB severity but did not significantly increase grain yields each year. Combination of strain MBI600 with azoxystrobin at the half-recommended rate improved efficacy of strain MBI600, reducing ShB severity to a level comparable to that of azoxystrobin applied at the full rate in all four years. The combined treatment also increased grain yield by 14 to 19% comparable to the fungicide applied at the full rate in 3 of 4 years. Combined use of PGPR strain MBI600 with a reduced rate of azoxystrobin can be a viable management option for control of ShB while allowing producers to use less fungicide on rice.