Partial resistance to black spot disease in diploid and tetraploid roses: general combining ability and implications for breeding and selection

Abstract

Black spot disease, incited by the fungus Diplocarpon rosae Wolf, is the most important disease of roses (Rosa hybrida L.) in the outdoor landscape. Though partial resistance exists in cultivated germplasm, the genetic basis of this trait has not yet been elucidated. Six diploid and six tetraploid rose cultivars were crossed in two factorial combining ability arrays. Whole plant and detached leaf inoculation methods were used to assess partial resistance under two different disease pressures using a characterized single-spore isolate. Parents from both arrays had significant general combining ability effects across multiple inoculation methods and environments. Specific combining ability was not significant for either array. Parent per se performance was highly correlated with progeny performance on a family mean basis. High positive correlations among whole plant and detached leaf inoculation methods indicate that detached leaf assays can substitute for whole plant assays. Based on these results, a breeding strategy including parental selection and early, among-family selection is proposed. To our knowledge, this is the first investigation of combining ability for disease resistance in rose.

A standard set of host differentials and unified nomenclature for an international collection of Diplocarpon rosae races

“…Sporulation capacity has been previously documented as a component of partial resistance in rose (Xue & Davidson, 1998). A previous inoculation experiment with D. rosae race 3 (isolate GVH) showed that the same detached leaf assay technique used in the present study could serve as a proxy for whole plant inoculations when assessing partial resistance (Whitaker & Hokanson, 2009). In that study, some susceptible genotypes with high partial resistance produced small and/or few spore masses under high disease pressure but did not exhibit sporulation under low disease pressure.…”

Evaluation of Roses from the Earth-Kind® Trials: Black Spot (Diplocarpon rosae Wolf) Resistance and Ploidy

“…Conidial concentrations of the inoculum varied with availability on each date and ranged between 30,000 and 100,000 conidia/ mL. These concentrations were within the range of inoculum concentrations used in other studies (Whitaker et al, 2007b(Whitaker et al, , 2010bWhitaker and Hokanson, 2009a). At each of the five inoculation dates one susceptible control cultivar was used.…”

“…Such race-specific resistance can only be characterized with controlled inoculations of roses with individual races as opposed to a field setting where the presence and prevalence of specific races are not known. Using characterized races for inoculations under controlled conditions is also advantageous when surveying for partial resistance (Whitaker and Hokanson, 2009a;Xue and Davidson, 1998).…”

“…Detached leaf assays have been an efficient tool to characterize the resistance of rose seedling populations, and they have been found to be strongly correlated with whole plant inoculations (Hattendorf et al, 2004;Von Malek and Debener, 1998;Whitaker and Hokanson, 2009a). Detached leaf assays are preferable as a result of greater ease in controlling humidity and inoculum levels (Whitaker and Hokanson, 2009b).…”

“…However, the number of races represented by these isolates is not known. Recently, Whitaker and Hokanson (2009a) reported using detached leaf assays to characterize the partial resistance to black spot of segregating populations using the measurement of LL, defined as the diameter of the lesion at its widest point. LL was chosen by the authors because of the ease of measurement and its significant correlations with three other measures of partial resistance (Whitaker et al, 2007b;Xue and Davidson, 1998).…”