Quantitative Comparison of the Resistance to Phytophthora Root Rot in Three Avocado Rootstocks

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Abstract
The resistance of 3 avocado (P. americana var. drymifolia) rootstocks to Phytophthora root rot was compared quantitatively in greenhouse experiments. Susceptible seedlings of rootstocks Topa Topa and resistant cuttings of rootstocks Duke-7 and G6 were planted in avocado field soil naturally infested with Phytophthora cinnamomi at 0.3-3.1 propagules per gram (ppg) of dry soil. Soil populations of P. cinnamomi, percent of roots infected per plant, and shoot and root weights were determined after 8, 15 and 20 wk. At 20 wk, root infection was 55, 27 and 11% in ''Topa Topa'', ''Duke 7'' and ''G6'', respectively. Soil populations after 20 wk were 55, 42 and 14 ppg for ''Topa Topa'', ''Duke 7'' and ''G6'', respectively. Compared to uninfected controls, significant reductions in root weight of infected plants occurred with each rootstock, but the percent reduction was greater with ''Topa Topa'' than with ''Duke 7'' of ''G6''. Generally, more propagules were recovered on dilution plates from detached roots infected in culture with zoospores than from attached roots infected on the greenhouse while growing in infested soil. In the absence of P. cinnamomi, ''Duke 7'' and ''G6'' had a significantly greater capacity for root regeneration than the susceptible rootstocks ''Walter Hole'' and ''Topa Topa''.