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Resistance to Q_oI (Strobilurin-like) Fungicides in Isolates of Pyricularia grisea from Perennial Ryegrass

In August 2000, azoxystrobin was ineffective in controlling gray leaf spot of perennial ryegrass at a golf course in Lexington, KY and at two golf courses in Illinois. Isolates suspected of being fungicide-resistant (“suspect isolates”) were compared to “baseline” isolates obtained from sites with no known use of quinol-oxidizing inhibitor (Q_oI) fungicides. Conidial germination of Pyricularia grisea was tested in vitro with 100 μg of salicylhydroxamic acid per ml. For baseline isolates, 50% effective concentration (EC₅₀) values for azoxystrobin and trifloxystrobin were 0.015 to 0.064 and 0.013 to 0.078 μg/ml, respectively; EC₅₀ values for suspect isolates were 2.39 to 44.8 and 0.31 to 111, respectively. All suspect isolates exhibited significantly (P = 0.05) lower sensitivity to Q_oI fungicides than all baseline isolates. The mean EC₅₀ values for suspect isolates for azoxystrobin and trifloxystrobin were 690 and 827 times higher, respectively, than the means for baseline isolates. In the laboratory, azoxystrobin and trifloxystrobin provided essentially complete control of disease induced by nine baseline isolates in vivo. Azoxystrobin and trifloxystrobin provided poor to no control of disease induced by six of eight suspect isolates; control of disease induced by the remaining two isolates was partial for azoxystrobin and complete for trifloxystrobin. We conclude that one or more biotypes of perennial ryegrass-infecting strains of P. grisea with resistance to Q_oI fungicides have emerged. This is the first report of resistance to Q_oI fungicides in P. grisea. Furthermore, this is one of two Q_oIresistant fungal pathogens collected in the United States during the 2000 growing season, the first instances reported for North America.