EFFECTS OF A PROTECTANT VERSUS A SYSTEMIC FUNGICIDE ON DISEASE COMPONENTS OF PEANUT LATE LEAF-SPOT

The epidemiological effects of a protectant versus a systemic, sterol-inhibiting fungicide on disease components of Cercosporidium personatum were quantified by linear regression and/or probit analysis. The effects of chlorothalonil (a protectant fungicide) and tebuconazole (a systemic, sterol-inhibiting fungicide) were quantified for the following disease components: 1) infection frequency, 2) incubation period, 3) lesion size, and 4) sporulation. An inoculation technique to quantify the protectant versus systemic effects of fungicides on disease components was developed. Both fungicides greatly reduced spore germination when applied to peanut leaf surfaces prior to inoculation. However, in addition to its action as a protectant, the systemic activity of tebuconazole also reduced infection frequency on nontreated leaves. Chlorothalonil had no effect on infection frequency after C. personatum germ tubes entered the peanut leaves. Tebuconazole also increased the incubation period and reduced the size of C. personatum lesions, while chlorothalonil had no effect on these components. In field experiments, chlorothalonil and tebuconazole reduced sporulation by up to 60 and 97%, respectively. Because of its systemic activity, tebuconazole used alone, or in combination with chlorothalonil, may provide more flexibility and less risk compared to the exclusive use of protectant fungicides when used in conjunction with late leaf spot spray advisories and/or chemigation systems.