Rating System for Phytophthora Root Rot Influences Quantitative Trait Locus Detection and Reveals Incomplete Dominance and Duplicative Recessive Epistatic Gene Action in Capsicum annuum

Phytophthora capsici is one of the most devastating pathogens facing pepper (Capsicum annuum) producers worldwide. Numerous factors, such as the race of the pathogen, the growing environment, and the source of resistance, have resulted in an overall lack of widely applicable molecular markers associated with resistance. Our objective was to determine the effect of the rating system on quantitative trait locus (QTL) detection and understand inheritance patterns of host resistance that can influence selection and molecular marker accuracy. We evaluated an F-2:11 recombinant inbred line population screened against the highly virulent strain (Pc134) and scored using two widely used methods, developed by Bosland and Lindsey and by Black. The rating system developed by Bosland and Lindsey resulted in slightly higher logarithm of odds for the QTL on chromosome 5, and we detected a QTL on chromosome 12 uniquely using this rating system. A QTL on chromosome 10 was detected using both rating systems, but Black resulted in considerably higher logarithm of odds for this QTL compared with the Bosland and Lindsey system. Molecular markers developed were nominally better at accurately predicting the phenotype than previously published molecular markers but did not completely explain resistance in our validation populations. The inheritance pattern of resistance in one of our F-2 populations did not significantly deviate from a 7:9 segregation ratio, indicating duplicative recessive epistasis. However, these results could be confounded by the presence of incomplete gene action, which was found through the improved selection accuracy when the phenotypes of heterozygous individuals were grouped with those with susceptible alleles.