The influence of Piriformospora indica on the resistance of two rhododendron cultivars to Phytophthora cinnamomi and P. plurivora

Experiments were carried out on the use of Piriformospora indica, an endomycorrhizal-like fungus, to increase the resistance of two rhododendron cultivars ‘Nova Zembla’ and ‘Alfred’ to the pathogens Phytophthora cinnamomi and P. plurivora. The interaction between P. indica and the pathogens was evaluated in dual cultures on KM and PDA media. The growth of P. indica colonies was restricted by pathogens during the 7–13 days of culture due to their quicker growth. Neither hyphae lysis nor atypical morphology was observed. No inhibition zones were observed between colonies of pathogens and P. indica but they were present in monocultures of P. indica. The ability to release zoospores by Phytophthora was not affected in the cultures with P. indica. In an experiment performed in vitro, P. indica colonised the roots of microplants, but after pathogen inoculation, its protective role was limited only to delaying dieback. In pot greenhouse experiments, all the young plants grown in virgin peat substrate, and in the substrate inoculated with P. indica survived, whereas all the plants grown in the substrate infected with the pathogens died. From 40 to 60% of plants grown in the substrate inoculated with P. indica, to which pathogen infected substrate was added after 21 days, survived one year without symptoms. The rate of success depended on the pathogen species and rhododendron cultivar. Our results show a practical possibility of biological protection of rhododendron plants against Phytophthora with the help of P. indica.