Quantitative trait loci mapping in a diploid rose population for pathotype specific powdery mildew resistance

Disease resistance is highly desired in plant breeding programs. Involving multiple genes withstanding more pathotypes can make resistance more durable. Among diseases on roses, one of the most important is powdery mildew, Podosphaera pannosa. Recent studies show pathotypes of powdery mildew and different types of resistances in roses. In this study 2 rose genotypes, Rosa wichurana and Rosa ‘Yesterday’, were found to react differently towards 2 powdery mildew pathotypes (R-E and R-P). Although susceptible to R-P, ‘Yesterday’ showed immunity to R-E by arresting fungal development after conidium germination on its leaves. R. wichurana shows partial resistance to pathotype R-P and was more resistant to pathotype R-E by means of different resistance mechanisms. Hybridization of ‘Yesterday’ x R. wichurana resulted in a diploid F1 population (90 genotypes). This population was screened for resistance mechanism specific segregation to both fungal pathotypes.
We construct a genetic linkage map encompassing 20 AFLP primer combinations, 43 SSR and two morphological markers. By applying the F1 pseudo test cross population strategy, two parental linkage maps were constructed (parent ‘Yesterday’ 536 cM; parent R. wichurana 526 cM). Both parental maps comprised seven linkage groups with an average length of 70 cM (Kosambi) corresponding to the seven haploid rose chromosomes. These new maps were used to identify QTLs controlling disease resistance. QTLs controlling pathotype specific disease resistance were mapped by applying Kruskal-Wallis rank-sum tests and simple interval mapping.
In total nine QTL loci were detected on linkage groups 2, 3, 5 and 6 for the two pathotypes analyzed, explaining 15-73% of the phenotypic variance for pathotype specific disease response. The genetic maps developed here are useful for future rose breeding, pathotype specific resistance research and development of a consensus map for roses.