Competitive Parasitic Abilities ofPhytophthora parasiticaandP. palmivoraon Fibrous Roots of Citrus

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 esperiment, or in sterilized or nonsterilized soil