Effect of azoxystrobin and kresoxim-methyl on rice blast and rice grain yield in China.

Sensitivity to azoxystrobin and kresoxim-methyl of 80 single-spore isolates of Magnaporthe oryzae was determined. The EC₅₀ values for azoxystrobin and kresoxim-methyl in inhibiting mycelial growth of the 80 M. oryzae isolates were 0.006-0.056 and 0.024-0.287 µg mL⁻¹, respectively. The EC₅₀ values for azoxystrobin and kresoxim-methyl in inhibiting conidial germination of the M. oryzae populations were 0.004-0.051 and 0.012-0.105 µg mL⁻¹, respectively. There was significant difference in sensitivity to azoxystrobin or kresoxim-methyl between the tested isolates representing differential sensitivity to carbendazim ( MBC) and kitazin P ( IBP); however, there was no correlation between this difference in sensitivity to azoxystrobin or kresoxim-methyl and sensitivity to MBC or IBP, indicating that there was no cross-resistance between azoxystrobin or kresoxim-methyl and MBC or IBP. In the protective and curative experiments, kresoxim-methyl exhibited higher protective and curative activity than azoxystrobin when applied at 150 and 250 µg mL⁻¹ accordingly, while azoxystrobin exhibited stronger inhibitory activity against M. oryzae isolates than that of kresoxim-methyl in the in vitro test. The results of field experiments also suggested that both azoxystrobin and kresoxim-methyl at 187.5 g.a.i. ha⁻¹ gave over 73% control efficacy in both sites, exhibiting excellent activity against rice blast. Taken together, azoxystrobin and kresoxim-methyl could be a good substitute for MBC or IBP for controlling rice blast in China, but should be carefully used as they were both at-risk. [ABSTRACT FROM AUTHOR]