STATISTICAL PROCEDURES FOR ASSESSMENT OF RESISTANCE IN A MULTIPLE FOLIAR DISEASE COMPLEX OF PEANUT

Early leaf spot caused by Cercospora arachidicola, late leaf spot caused by Cercosporidium personatum, and rust caused by Puccinia arachidis are the three most important foliar fungal diseases of peanut (Arachis hypogaea) worldwide. A 10-parent diallel cross was performed using peanut lines susceptible and resistant to the three major foliar fungi. The objective was to investigate the interrelationship of the multiple disease complex with host genes for resistance. Parents and F1 hybrid progeny were planted in the field in Thailand in 1985. Leaves of randomly selected plants within plots were evaluated for resistance in the field and in the greenhouse using a detached leaf technique. A correlative structure was observed among traits measured, with no single parameter predominating the disease complex. Late leaf spot lesion number and early leaf spot lesion size were significantly correlated with defoliation in the field. Resistance to rust and late leaf spot was correlated (r = 0.48-0.60). Moderate correlation coefficients existed between the two leaf spot diseases. Lesion size and sporulation ratings were moderately correlated for the leaf spots, indicating a genetic and/or physiological relationship within the host involving lesion development for both leaf spots. Principal component analysis and biplots were used to illustrate the correlative nature of parameters of the multiple disease complex and the variation in response among peanut genotypes. Tree diagrams were used to visualize genetic relatedness of resistance among parents and hybrids for the disease complex. Genotypic means, biplots, and tree diagrams aided in relating disease parameters and determining hybrids that reacted similarly to the disease complex. Crosses PI 314817 X ICGS-4 and PI 314817 X (TG 3 X EC 76446[292]) were selected on the basis of parental general combining ability for resistance to all three diseases and biplots for further investigation and selection.