Identification of pathogenic races of Phytophthora fragariae Hickman in Germany

Summary

The fungus Phytophthora fragariae Hickman is the causal agent of red stele disease in strawberry (Fragaria × ananassa) this being a major disease in many areas with cool and moist conditions. Success of resistance breeding can be nullified by the appearence of specific races of the fungus capable of overcoming the introduced resistance. In some countries (USA, UK, Japan and Canada) races were identified by using a differential set of strawberry cultivars. The absence of an international standard differential set and the use of different test methods make a comparison of the identified races difficult or even lead to contradictory results for one variety/race combination. The aim of this study was to obtain information about the spectrum of pathogenic races in Germany as a basic contribution to research on resistance breeding against the fungus. The susceptibility of different strawberry cultivars to German isolates of P. fragariae was evaluated. The inoculation was done by modifying a method described by Milholland et al. (1989). Rooted in vitro plants, four weeks after transferring them to the soil, were used for the investigation. Inoculation was done with a zoospore suspension of defined concentration. Up to now three German isolates, G-l, G-2, and G-3, can be separated by their ability to infect and produce oospores in the roots of the strawberry cultivars’senga Sengana’, either’saladin’,‘Redgauntle? or ‘Climax’ and Fragaria chiloensis clone‘YaquinaB’. These results confirm the existence of pathogenic races of P. fragariae in Germany and should be taken into consideration for resistance breeding against the fungus.