Application Rate and Treatment Interval Influence the Efficacy of Heritage Fungicide for the Control of Alternaria Leaf Spot on Marigold

Abstract Efficacy of Heritage 50WG (azoxystrobin) over a range of application rates and treatment intervals was compared with Daconil Weather Stik 6F (chlorothalonil) and Eagle 40W (myclobutanil) for the control of Alternaria leaf spot (Alternaria tagetica) in a simulated landscape planting of marigold in southwest Alabama. When applied at 2-week intervals, all rates of Heritage 50WG gave better disease control in all four years than Daconil Weather Stik 6F or Eagle 40W, applied on the same schedule. In two of three years, weekly applications of Daconil Weather Stik 6F decreased Alternaria leaf spot severity compared with the non-fungicide treated control, but when applied at 2-week intervals, disease levels were similar to the non-fungicide treated control in two of four years. Typically, Alternaria leaf spot intensified when Heritage 50WG application rates declined from 0.16 to 0.04 g ai·liter−1 and treatment intervals increased from 2-to 4-weeks. At the 0.04 g ai·liter−1 rate of Heritage 50WG, disease ...