Gray leaf spot and thiophanate-methyl

Thiophanate-methyl (TM) continues to be an effective fungicde for control of gray leaf spot (Pyricularia grisea). Why is this surprising? Because TM has been used widely throughout the country where gray leaf spot occurs and resistance has not been detected anywhere. This is doubly confusing because TM belongs to one of the single-site fungicide groups (B1) that plant pathogens can normally figure a work-around that results in resistance to the active ingredient. For example, anthracnose and brown ring patch are…