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

Farmers rely heavily 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 (PGPRs) in combination with a reduced rate of azoxystrobin application 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 0, 10³, 10⁶, 10⁹, and 10¹¹ CFU/ml alone or in combinations with 0, 17, 33, 50, 67, 83, and 100% of the recommended application 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% of the recommended application 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 application 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 half of the recommended application rate improved efficacy of strain MBI600, reducing ShB severity to a level comparable to that of azoxystrobin applied at the full rate in all 4 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 application can be a viable management option for control of ShB while allowing producers to use less fungicide on rice.