Monocyclic Components for Evaluating Disease Resistance to Cercospora arachidicola and Cercosporidium personatum in Peanut

Abstract

Cultivated peanut (Arachis hypogaea L.) is an economically important crop that is produced in the United States and throughout the world. However, there are two major fungal pathogens of cultivated peanuts, and they each contribute to substantial yield losses of 50% or greater. The pathogens of these diseases are Cercospora arachidicola which causes early leaf spot (ELS), and Cercosporidium personatum which causes late leaf spot (LLS). While fungicide treatments are fairly effective for leaf spot management, disease resistance is still the best strategy. Therefore, it is important to evaluate and compare different genotypes for their disease resistance levels. The overall goal of this study was to determine resistance levels of different peanut genotypes to ELS and LLS. The peanut genotypes (Chit P7, C1001, Exp27-1516, Flavor Runner 458, PI 268868, and GA-12Y) used in this study include two genetically modified lines (Chit P7 and C1001) that over-expresses a chitinase gene. This overall goal was addressed with three specific objectives: 1) determine suitable conditions for pathogen culture and spore production in vitro; 2) determine suitable conditions for establishing infection in the greenhouse; 3) compare ELS and LLS disease reactions of young plants to those of older plants. The suitable culture medium for C. arachidicola was found to be potato dextrose agar and peanut oatmeal broth for C. personatum. The suitable sporulating medium was found to be V8 agar for both C. arachidicola and C. personatum. Greenhouse trials indicated inoculation with 5.0×103 conidia/ml was the best option because it resulted in distinct single lesions. Six peanut genotypes that included two genetically modified lines, a parent line, and commercial standard lines at both vegitative and reproductive growth stages (VGS and RGS, respectively) were inoculated with 5.0×103 conidia/ml; both and C. personatum were separately evaluated. The monocyclic components evaluated were incubation period, number and size of lesions, and proportion of defoliation. Peanut plants inoculated with C. arachidicola at vegetative growth stage have significantly longer incubation period, fewer lesions, less defoliated leaflets at both 30 and 42 DAI than those inoculated at RGS. Peanut plants inoculated with C. personatum at vegetative growth stage has significantly longer incubation period, fewer lesions at both 30 and 42 DAI, smaller lesions, and fewer defoliation than those inoculated at RGS. Among all six genotypes evaluated, GA-12Y had the worst disease reaction to both ELS and LLS; in contrast Flavor Runner 458 has the greatest disease resistance. Genetically modified lines did not show different disease resistance levels compared to their parental line (Exp27-1516). Methods developed in this study could be used in other studies. These evaluations of monocyclic components as plant phenotype data could be used as a baseline for any disease resistance study.