In vitro screening of apple rootstock MM106 somaclones with Phytophthora cactorum culture filtrate

The most widely used apple rootstock Malling-Merton 106 is highly susceptible to collar rot disease caused by Phytophthora cactorum. In the present study, in vitro selection method was employed for early screening of collar rot tolerance among regenerants of MM106. Toxic filtrates produced by three virulent isolates of P. cactorum were mixed and tested as a selection agent. Adventitious shoots were regenerated from leaf explants of MM106, cloned and exposed to proliferation MS medium enriched with 20–80% (v/v) of pathogen culture filtrate to isolate tolerant regenerants. Varying concentrations of culture filtrate resulted in browning, necrosis and reduced growth which led to the identification of tolerant shoots.The survived tolerant regenerants were subjected to three continuous and one discontinuous selection cycle. The critical concentration of culture filtrate was found to be 74%, resulting in 37.8% survival of regenerated shoots after becoming necrotic. Regenerants selected on 70%, 72% and 74% culture filtrate enriched medium showed 52%, 50% and 33.3%, survival respectively after 2nd selection cycle, while reduced to 87%, 81% and 0 respectively after 3rd selection cycle. In discontinuous cycle, 100% tolerant regenerants survived, though multiplication ability was decreased. After in vitro inoculation of the pathogen, seven out of eight somaclones exhibited tolerance to P. cactorum. Thus, it is suggested that selection approach using Phytophthora culture filtrate has potential in identifying the disease tolerant somaclones in apple.