A comparison of the resistance against Phytophthora fragariae var. rubi, the causal agent of a raspberry root rot

Zoospore encystment, sporulation, oospore formation and detection of Phytophthora fragariae var. rubi, the causal agent of raspberry root rot, on three Rubus genotypes with different levels of resistance were investigated. There were no qualitative or quantitative differences in encystment as determined by scanning electron and fluorescence microscopy. Susceptibility was associated with an extended period of sporulation on roots with a proportion of roots containing oospores increasing continuously until the whole root system was colonised and the fungus could be isolated from the stem bases. Quantitative resistance as found in some raspberry (Rubus idaeus) cultivars was associated with a shorter sporulation period and decreasing proportion of the roots containing oospores due to the strong regeneration of healthy roots and limited spread of the fungus in the root system. Stem bases were consequently not affected. Sporulation was very limited on a blackberry × raspberry hybrid that was almost totally resistant to root rot and sustained infections could not be found.