Herbicide and Nutrient Effects on the Development of Gray Leaf Spot Caused by Pyricularia grisea on Tall Fescue

Abstract

Gray leaf spot, induced by Pyricularia grisea, is a disease of increasing importance in tall fescue in the southeastern United States. Previous research has shown that several herbicides may predispose turfgrasses to some diseases and that certain essential nutrients may have antagonistic effects on fungal plant pathogens. The objectives of this research were to evaluate the effects of herbicide and nutrient treatments on gray leaf spot development in tall fescue. Inoculation techniques were also evaluated for establishing gray leaf spot on tall fescue in controlled environments. Field studies revealed that 2,4-D applications resulted in significantly higher quality turf and lower gray leaf spot incidence than the untreated control. Turf treated with 2,4-D amine + mecoprop + dicamba also exhibited significantly less foliar blight symptoms than the untreated control. In vitro experiments revealed the growth-inhibiting effects of 2,4-D on P. grisea implicated in the field, as mycelial growth was completely inhibited at concentrations of 500 and 1000 mg L⁻¹. Colony growth was not affected at 2,4-D concentrations up to 100 mg L⁻¹. Phosphorous acid treatments resulted in a reduction in turf quality compared to an untreated control, as did manganese and zinc treatments. Foliar blight caused by P. grisea was substantially increased in H₃PO₃-treated plots in 2003, where a 40% difference in blighted turf was observed between plots that received H₃PO₃ treatments every 14 days and the untreated control. Area under the disease progress curve (AUDPC) analysis also revealed the significant detrimental effects of the phosphorous acid treatments. No significant differences in disease incidence or leaf spot size among nutrient treatments were observed in greenhouse treatments. Isolate selection was a significant factor for disease development and leaf spot size following spray inoculation under optimal environmental conditions. In general, disease incidence increased as inoculum density increased. Placing plants in covered containers or plastic bags immediately following inoculation for a 24-h period also appeared to promote disease development. Seeding rate did not have a significant effect on gray leaf spot development. We conclude that herbicide applications do not predispose tall fescue to gray leaf spot development and that applications of nutrients alone do not suppress development of gray leaf spot in tall fescue. Adjusting cultural practices as additional control measures for gray leaf spot does not appear to be a successful approach to managing this increasingly important disease.