Baseline and non-baseline sensitivity of Magnaporthe oryzae isolates from perennial ryegrass to azoxystrobin in the northeastern United States

Abstract

Gray leaf spot, caused by Magnaporthe oryzae, is chiefly managed by the application of fungicides. Among several systemic fungicides, azoxystrobin has been effectively used to control the disease; however, the development of resistance in M. oryzae to azoxystrobin has been reported in some regions of the United States. The sensitivity level to azoxystrobin of pathogen isolates from perennial ryegrass turf has not been documented in a comprehensive manner in the northeastern United States. This survey was conducted to evaluate the sensitivity to azoxystrobin of 135 isolates of M. oryzae collected between 1995 and 2004 from golf courses primarily in the northeastern United States. Media components, incubation time, and temperature were optimized for a modified in vitro spore germination assay to determine the sensitivity of the isolates. The EC50 value of 111 baseline isolates ranged from 0.001 to 0.083 µg/mL (mean 0.039 µg/mL) and 21 non-baseline isolates from Pennsylvania (PA) ranged from 0.007 to 0.066 µg/mL (mean 0.029 µg/mL). No significant difference in sensitivity (P ≤ 0.05) was observed between baseline and PA non-baseline isolates. DNA sequence analysis of the cytochrome b gene (CYTB) showed that no mutation had occurred among PA non-baseline isolates. The study established azoxystrobin baseline sensitivity for M. oryzae in the northeastern United States and indicated that because of limited use of QoI (quinone outside inhibitor) fungicides and the practice of using fungicide rotations and mixtures for gray leaf spot control, a shift in sensitivity to azoxystrobin has not yet occurred in the PA non-baseline isolates.