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

Abstract

Black spot of rose is distributed throughout the world and is the most serious disease of roses (Rosa spp.) in the outdoor landscape. Resistance breeding has been frustrated by the occurrence of races of the causal pathogen Diplocarpon rosae. Races from Germany, North America and the UK have been characterized and maintained in a pathogenic state. However, these races were characterized using independent sets of host genotypes and are referenced using different nomenclatures. In the present study, a total of 15 D. rosae isolates from these locations, as well as Belgium and Italy, were inoculated to a common set of 15 rose cultivars in replicated, detached leaf trials. Baby Love TM (cv. Scrivluv) was resistant to all isolates except for one originating from the UK. The rose cultivars Mrs Doreen Pike (Ausdor) and Hansa were resistant to all isolates except for one originating from Minnesota, USA. No rose genotype was universally susceptible. A total of 11 pathogenic races were differentiated based on their unique host ranges and were assigned an international race nomenclature. Nine cultivars are proposed as the first standard set of differential genotypes for characterization of D. rosae races.

The gene diversity pattern of Diplocarpon rosae populations is shaped by the age, diversity and fungicide treatment of their host populations

“…However, the isolates GVH, IGWA and Rasse 6 were classified as single pathotypes 3, 9 and 7 by Whitaker et al . () and also as three different multilocus genotypes (MLG D, E and F) in the present study. The four single‐spore isolates D 002, IGWA, Rasse 6 and Kassel 05‐11 were not grouped with other isolates but had different MLGs.…”

“…The isolates Dort E4, B 005 and Br 2402 E1 were differentiated by Whitaker et al . () into the pathotypes 6, 5 and 2, respectively, but these isolates were all grouped together in MLG A in the present study. However, the isolates GVH, IGWA and Rasse 6 were classified as single pathotypes 3, 9 and 7 by Whitaker et al .…”

“…Because of the small overlap of the isolates analysed by Whitaker et al . () and even more by Debener et al . () with the strains used here, a correlation among groupings cannot be ruled out.…”

“…() and race 6 by Whitaker et al . (). The six isolates with MLG A were sampled from different geographic locations, such as Belgium (B 005), Italy (I 001) or Sweden (S 003 and S 004) but also from two other areas in Germany.…”

“…Nevertheless, more pathotypes are to be expected if one considers the relatively small number of isolates investigated to the number of 11 proposed races by Whitaker et al . (). This is supported by the data from the few molecular marker studies carried out on the genetic diversity of D. rosae from single‐spore isolates.…”