Tolerance of apple rootstocks against Phytophthora root rot.

Phytophthora root- and crown rot, caused by Phytophthora cactorum, leads to economic losses in apples. In South Africa, the tolerance of the Geneva rootstocks remains untested. Some have been reported to show promise for disease management. This project assessed their tolerance to P. cactorum compared to currently grown Malling and Malling-Merton rootstocks under glasshouse conditions.

Six trials were conducted evaluating the susceptibility of eight to 12 apple rootstocks (Geneva, Malling and Merton-Malling) to P. cactorum through artificial inoculation under glasshouse conditions. A pathogenicity and aggresiveness trial was also conducted evaluating 11 P. cactorum isolates using Golden Delicious-derived seedlings. The most aggressive isolates were used to conduct the two last two rootstock susceptibility trials (2023 and 2024 trials). Rootstock susceptibility was assessed using increases in plant weight and -length.

Only the last two of six trials yieded reliable data due to contamination problems in earlier ones. Susceptibility to P. cactorum varied, only M.7 EMLA was consistently susceptible. Relative percentage length (RPL) data were more reproducible than total plant weight. Based on RPL, G.202, G.757, G.890, and G.778 were more tolerant than M.7 EMLA, whereas G.213 and G.228 yielded variable results. Most Geneva rootstocks matched M.9, except G.890 (only 2024 trial), which was more tolerant based on an increase in length.

Due to variability between trials, no final conclusions can be made regarding the suceptibility of apple rootstocks to Phytophthora diseases. Protocols for conducting the trials were improved during the course of the study. Conducting trials for at least four months using high irrigation levels, and also assessing shoot weight and P. cactorum root colonisation may improve future trial results.

Based on the trials conducted, assumming that the Geneva rootstocks may be more tolerant to Phytophthora diseases than M.9 ELMA is premature. Due to the high susceptibility of M.7 ELMA to Phytophthora, the Geneva rootstocks G.202, G.757, G.890 and G.778 may be more tolerant. However, more trials will have to be conducted, due to the variable results obtained between trials in the current study, to confirm observations and hypotheses made in the current study.