Genetic association of agronomic traits with partial resistance to gray leaf spot in elite maize germplasm.

Abstract

Studies on gray leaf spot (GLS) of maize have reported inconsistencies in the relationship between partial disease resistance and agronomic traits. Understanding this variation could facilitate the use of agronomic traits as a basis for selection to improve partial resistance to GLS. Coinheritance of nine agronomic traits with partial resistance to GLS was examined among 48 maize (Zea mays L.) inbred lines artificially infected with Cercospora zeina in field evaluations across nine environments in western Kenya in 2013 and 2014. Five measures of disease severity and two disease resistance components were evaluated for their association with agronomic traits. Standardized area under disease progress curve (SAUDPC) was the most efficient in delineating differences in GLS severity between genotypes, whereas latent period (LP) was the least effective. Among maize genotypes, values of SAUDPC ranged from 29.3 to 97.9 across environments. Genotypic and phenotypic correlations were strongest between SAUDPC and the absolute rate of disease increase (ρ; r = .71), final percent diseased leaf area (r = .66) and International Maize and Wheat Improvement Center (CIMMYT) disease severity grade (r = .60), but weakest between SAUDPC and LP (r = −.19). Correlations of SAUDPC were significant (P = .05) with eight of the 11 agronomic traits examined, with the strongest being between SAUDPC and the stay-green characteristic (SGR; r = −.87), days to maturity (DTM; r = −.60) and ear/plant height ratio (r = −.52). Genotypic and phenotypic coefficients of correlation of SAUDPC with agronomic traits were all negative. Overall, absolute genotypic correlations were numerically larger than the corresponding coefficients of phenotypic correlation with the magnitude and direction of coheritability estimates mimicking trends in genotypic and phenotypic correlations. Causal mediation analysis indicated that covariation of GLS resistance with agronomic traits was mainly due to direct effects of days to anthesis and DTM and indirect effects of SGR and silking–maturity interval.