Azoxystrobin sensitivity and resistance management strategies of Magnaporthe grisea causing gray leaf spot on perennial ryegrass (Lolium perenne) turf

Gray leaf spot, caused by Magnaporthe grisea, is a serious disease of perennial ryegrass (Lolium perenne L.) turf. Control of the disease is primarily achieved by the application of fungicides. The QoI fungicide, azoxystrobin, has been proven to be one of the most effective chemicals used in fungicide spray programs on golf courses in the United States. In recent years, resistance to azoxystrobin in M. grisea has been detected in several regions of the U.S. However, the sensitivity level of the pathogen population to azoxystrobin has not been documented in the Northeast. In addition, the information on alternative chemical approaches to resistance management and the dynamics of the resistance in the pathogen population are currently unavailable.;The objective of the first study is to evaluate azoxystrobin sensitivity of 135 isolates of M. grisea, collected between 1995 and 2004 from golf courses in the northeastern U.S. The EC50 value of 111 baseline isolates ranged from 0.001 to 0.083 mug/ml (mean 0.039 mug/ml) and 21 non-baseline isolates ranged from 0.007 to 0.066 mug/ml (mean 0.029 microg/ml). No significant difference in sensitivity was observed between baseline isolates and non-baseline isolates (P≤0.05). DNA sequence analysis of the cytochrome b gene showed that no resistance mutation had occurred among non-baseline isolates. The results of this survey indicate that a shift in sensitivity to azoxystrobin might not have occurred in the M. grisea populations from the golf courses in the Northeast.;The objective of the second study is to develop alternative chemical approaches to the resistance management by testing efficacies of azoxystrobin, chlorothalonil, mancozeb, propiconazole, thiophanate-methyl, and half-rate mixtures of the three systemic fungicides with contact fungicides, chlorothalonil or mancozeb for control of the disease caused by pathogen populations that contain the resistant (G143A mutant) pathogen population. The results suggest that gray leaf spot, caused by the azoxystrobin-resistant G143A mutant, may be effectively managed by a preventive fungicide program that includes at least one of the fungicides and mixtures tested except azoxystrobin but include mixtures of azoxystrobin with either chlorothalonil or mancozeb treatments. Also a curative program that includes propiconazole or thiophanate-methyl and half-rate mixtures of these two systemic fungicides with chlorothalonil or mancozeb treatments are effective in controlling the resistant mutant.;The third study was conducted to research dynamics of azoxystrobin resistance in the pathogen population. An azoxystrobin-resistant G143A mutant of M. grisea was compared with a sensitive wild type strain regarding fitness and competitive ability. Inoculation of ryegrass plants with both strains revealed that the G143A mutant exhibited significantly greater sporulation, however, the sensitive strain was significantly more virulent than the G143A mutant based on disease severity assessment. In absence of fungicide, when detached leaves were inoculated with mixed populations of resistant and sensitive strains, the proportion of conidia produced by the sensitive strain increased, and that of the mutant decreased after symptom development. In the presence of azoxystrobin, propiconazole, thiophanate-methyl, chlorothalonil, mancozeb, and half-rate mixtures of the three systemics with chlorothalonil or mancozeb, azoxystrobin treatment replaced 95% of the sensitive strain with 5% of the mutant in the initial inocula. The 5% mutant was eliminated by the rest of fungicide treatments, including half-rate mixtures of azoxystrobin with chlorothalonil or mancozeb. The results of this study demonstrate that the sensitive strain of M. grisea has a competitive advantage over the mutant in absence of fungicides. Mixtures and alternations of fungicides with different modes of action may prevent the rapid build-up of resistance in the gray leaf spot pathosystem.