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Comparative Use of Soil Infested with Chlamydospores to Screen for Relative Susceptibility to Phytophthora Foot Rot in Citrus Cultivars. G. S. Smith, Former Assistant Professor, Texas A&I University Citrus Center, P.O. Box 1150, Weslaco 78596. D. J. Hutchison, and C. T. Henderson. Research Geneticist, and Biological Technician, Agricultural Research Service, U.S. Department of Agriculture, Horticultural Research Laboratory, 2120 Camden Road, Orlando, FL 32803. Plant Dis. 75:402-405. Accepted for publication 18 October 1990. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1991. DOI: 10.1094/PD-75-0402.

Soil-agar blocks infested with chlamydospores of Phytophthora parasitica were an effective inoculum source for screening citrus rootstocks for relative susceptibility to foot rot. Mean percentage of stem girdling, lesion area, and relative lesion area were similar on stems inoculated with soil-agar blocks containing seven to 100 chlamydospores or inoculated with 1,000 zoospores. Foot rot severity ratings were significantly higher when inoculum consisted of 0.5-cm-diameter mycelial agar disks compared with chlamydospores or zoospores. Commercial rootstocks known to be moderately resistant to or with field tolerance to foot rot were rated as susceptible when mycelial agar disks were used as an inoculum source. The use of quantifiable inoculum sources such as chlamydospores or zoospores may allow evaluation of rootstocks with intermediate levels of resistance to foot rot.