Assessment of the resistance to Phytophthora fragariae var. fragariae of the USA and Canadian differential series of strawberry genotypes

The resistance of the USA and Canadian differential series of strawberry (Fragaria sp.) genotypes was assessed against nine USA isolates (A1-A4, A6-A10) and three Canadian isolates (NS2-NS4) of Phytophthora fragariae var. fragariae. The resistance of part of the UK series was also examined. Of the 157 host genotype-isolate combinations tested, 13 were classified differently from earlier reports. These differences can be explained by the existence of different clones under the same cultivar name (Aberdeen and Perle de Prague), the substitution of Aberdeen by Sparkle, which differ in their resistance, and by the application of different criteria for resistance. Here, incomplete resistance was considered as resistance. The discernment of the US differential series can be improved by including the UK differential Perle de Prague and the Canadian differential Sparkle. The Canadian series can be improved by including Perle de Prague and the US differential Aberdeen. This leads to one unified differential set for North America.The results also allow the proposition of a formal gene-for-gene model which, in turn, provides for a universal differential series for this pathosystem.