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Evaluation of Tolerance of Citrus Rootstocks to Phytophthora Root Rot in Chlamydospore-Infested Soil. J. H. Graham, Associate Professor, University of Florida, Institute of Food and Agricultural Sciences, Citrus Research and Education Center, Lake Alfred 33850. . Plant Dis. 74:743-746. Accepted for publication 26 February 1990. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/PD-74-0743.

Seven citrus rootstocks of commercial importance in Florida were evaluated for their tolerance to Phytophthora root rot. Six-month-old seedlings were planted in soil artificially infested with chlamydospores at 0, 0.5, 1, 5, and 10 propagules of Phytophthora parasitica per cm3 of soil. For all rootstocks, at 6 wk after inoculation there was little increase in disease rating with an increase in inoculum density above 1 propagule/cm3. Disease severity at the higher inoculum densities was significantly greater for the susceptible rootstocks (sour orange, Ridge Pineapple sweet orange, Cleopatra mandarin, and Carrizo citrange) than for the tolerant rootstocks (trifoliate orange and Swingle citrumelo). Volkamer lemon was intermediate between the two groups. Within the susceptible and tolerant groups, no significant differences among rootstocks were detected. Tolerance was related to the ability of trifoliate orange and Swingle citrumelo to regenerate roots in the presence of P. parasitica. The same grouping of rootstocks was observed when 3-mo-old seedlings were inoculated with high inoculum densities (≥ 10 propagules/cm3) and evaluated 3 wk later but not when older seedlings were used. Thus, a rapid method for primary screening of citrus rootstocks for tolerance to root rot was established through the use of chlamydospore-infested soil and very young seedlings.