Resistance Evaluation of G, CG, or M Series Apple Rootstocks to Soil-borne Diseases (Phytophthora Root Rot, White Root Rot, and Southern Blight) and Woolly Apple Aphid

In this study, the resistance of the industry-standard M26 and M9 rootstocks and the promising elite G or CG rootstocks was evaluated not only for soil-borne diseases, such as Phytophthora root rot (Phytophthora cactorum), white root rot (Rosellinia necatrix), and southern blight (Athelia rolfsii), but also for woolly apple aphid [Eriosoma lanigerum (Huasm.)]. On February 27, 2019, each unworked rootstock plant was harvested from stool-beds, then planted in 2-L pots and grown in a greenhouse. For P. cactorum, M9 was the most resistant, and CG5087 and G935 were not. There was little difference in susceptibility to R. necatrix among rootstocks, but G or CG rootstocks were generally less susceptible than the M series. The susceptibility to A. rolfsii was higher (p < 0.05) in M series rootstocks than in G or CG series rootstocks, and the mortality reached 80% and 53% for M9 and M26, respectively, and 7% for G935. As a result, M9 and G11 showed resistance to P. cactorum, whereas G or CG rootstocks showed relative resistance to R. necatrix and A. rolfsii. For woolly apple aphid, G11, G202, G214, and CG5087 were immune, but G935, CG4814, M26, and M9 were not. Additional key words: colony, Geneva apple rootstock, mycelium, rootstock infection, soil infestation