Greenhouse testing of rootstocks for resistance to phytophthora species in flatwoods soil

Kim D. Bowman, Joseph P. Albano, James H. Graham


New rootstocks are needed to replace stunted or declining trees on sour orange and Swingle citrumelo in many east coast flatwoods sites. Important factors that make existing rootstocks unsuitable for use in these areas include susceptibility to Phytophthora nicotianae or Phytophthora palmivora , and intolerance of common flatwoods soils. A technique is described to rapidly test rootstocks for response to these conditions. Winder soil from a field site in Indian River County was collected and used as a potting medium in the greenhouse, with and without inoculation by roots infested with P. nicotianae and P. palmivora. Seed germination, seedling growth, and survival were evaluated for 13 rootstocks. When compared with a peat/perlite potting mix, non-infested Winder soil supported good germination, good seedling growth, and did not cause significant seedling death for any of the rootstocks, including sour orange, Volkamer, Cleopatra, Carrizo citrange, and Swingle citrumelo. In contrast, there were highly significant levels of seedling death in Winder soil infested with Phytophthora spp. Rootstocks with the highest levels of mortality in infested Winder soil were Swingle citrumelo, Carrizo citrange, and Flying Dragon, while rootstocks with the lowest levels of mortality were sour orange, Sun Chu Sha, and US-897. The responses of different rootstocks to this rapid greenhouse test were similar to the relative field performance of these rootstocks on Winder soil in the Indian River area. This greenhouse assay appears to be valuable for rapidly screening and evaluating new rootstocks for potential adaptation to soil and pathogen conditions prior to the establishment of long-term field trials in flatwoods sites.


citrus aurantium; c. reticulata; poncirus trifoliata; swingle citrumelo; carrizo citrange; rootstock improvement

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Proc. Fla. State Hort. Soc.     ISSN 0886-7283