Evaluation of phosphonate fungicides for management of leaf spot diseases of peanut in Georgia

Efficacy of phosphonate fungicides (FRAC Group P 07) and micronized elemental sulfur (FRAC Group M 02) for control of early leaf spot (Passalora arachidicola) and late leaf spot (Nothopassalora personata) of peanut (Arachis hypogaea) was evaluated in five field experiments in Tifton, GA between 2021 and 2025. Treatments included phosphonate (2 kg a.i. ha-1) and sulfur (2.7 kg a.i. ha-1) fungicides applied individually and in tank-mix combinations, as well as chlorothalonil (1.26 kg a.i. ha-1) (FRAC Group M 05), a standard protectant fungicide, and a nontreated control. Final leaf spot severity and standardized area under the disease progress curve (sAUDPC) were lower for both the phosphonate alone and sulfur alone treatments than for the nontreated control but were higher than for the chlorothalonil standard. Final ratings for leaf spot severity and sAUDPC were lower for the phosphonate + sulfur mixtures than for any other treatment. For effects on final defoliation, effects of the phosphonate + sulfur mixture were greater than additive. In vitro experiments indicated that the phosphonate fungicide had no effect on germination of conidia of either pathogen but reduced biomass development of both Passalora arachidicola and N. personata by 60% after conidial germination. These results indicate that the combination of a phosphonate fungicide with micronized elemental sulfur can be an effective treatment for management of leaf spot and could be used as replacement for chlorothalonil for leaf spot control.