VIEW ARTICLE

Ecology and Epidemiology

Competitive Parasitic Abilities of Phytophthora parasitica and P. palmivora on Fibrous Roots of Citrus. S. E. Zitko, Biological scientist, University of Florida, IFAS, Citrus Research and Education Center, 700 Experiment Station Road, Lake Alfred, FL 33850; L. W. Timmer, professor, University of Florida, IFAS, Citrus Research and Education Center, 700 Experiment Station Road, Lake Alfred, FL 33850. Phytopathology 84:1000-1004. Accepted for publication 19 June 1994. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/Phyto-84-1000.

The competitive abilities of Phytophthora parasitica and P. palmivora to infect and multiply on fibrous roots of citrus were compared using the replacement series technique. Greenhouse-grown citrus seedlings were transplanted to infested soil containing inoculum ratios of P. parasitica; P. palmivora of 4:0, 3:1, 2:2, 1:3, and 0:4 propagules per cubic centimeter. At the end of the experiments, percentage of root infection and densities of soil propagules were measured by plating on a selective medium. Yields of P. palmivora were usually near expected proportions for final propagule densities and for root infection, but yields of P. parasitica were nearly always depressed in mixed inoculations. Under screenhouse conditions, P. palmivora outcompeted P. parasitica whether measured in a short-term experiment, periodically in a multiple-harvest experiment, in a long-term experiment, or in sterilized or nonsterilized soil. P. parasitica is widespread in Florida citrus soils, whereas P. palmivora is very localized. The local distribution of P. palmivora could not be explained by its inability to compete with P. parasitica.