Differentiation, prediction and management of early and late leaf spot of peanut in the southeastern united states and haiti
Fulmer, Abraham Michael
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Early (ELS) and late (LLS) leaf spot, caused by the fungi Cercospora arachidicola (Ca) and Cercosporidium personatum (Cp), respectively, are distinct diseases of peanut. Although similar in appearance, disease cycle and threat to the crop, the predominance and onset of the two diseases can differ greatly, and the underlying mechanisms are still not clear. The primary objective was to discern the epidemics of ELS and LLS, to identify factors responsible for their differential development, and to determine whether risk predicted via Peanut Rx and management with reduced input fungicide programs were equally applicable. Field trials were conducted in Georgia and Florida from 2010 to 2016 to evaluate the effect of fungicide treatment, inoculum source (infested peanut residue), planting date (April, May or June), and peanut cultivar. Components of resistance and disease associations were evaluated in greenhouse trials via separate or co-inoculations. Prediction via Peanut Rx was evaluated based on 168 unique combinations of risk factors. An inoculum source of each pathogen was identified as the primary factor regulating the onset and prevalence of ELS and LLS. Planting date and cultivar, dependent upon inoculum level, also had significant additive effects on disease development. The development of LLS tended to be more suppressed than the development of ELS when plots or leaflets were co-inoculated with Ca + Cp. Peanut Rx risk points had a strong relationship with leaf spot onset, and were a better predictor for ELS than LLS. Delayed fungicide applications in prescription programs provided statistically better LLS control than normal programs, and did not result in a significant yield loss. Overall results from these studies demonstrate that predominance and onset of ELS and LLS can be predicted, and that fungicide treatments can be enhanced by timing applications based on expected disease development. Fungicide trials were conducted from 2015 to 2017 in Haiti. Leaf spot and rust (Puccinia arachidis) severity and yield loss generally increased with decreasing fungicide applications. Absolute yield loss was 64 and 43%, for the local Haitian runner and Georgia-06G, respectively, and ranged from 28 to 34% for the local Haitian Valencia and New Mexico Valencia A.