VIEW ARTICLE

Relationship of Density of Chlamydospores and Zoospores of Phytophthora palmivora in Soil to Infection of Papaya. B. N. Ramirez, Research Assistant, Department of Plant Pathology, University of Florida, Gainesville 32611; D. J. Mitchell, Assistant Professor, Department of Plant Pathology, University of Florida, Gainesville 32611. Phytopathology 65:780-785. Accepted for publication 20 February 1975. DOI: 10.1094/Phyto-65-780.

Autoclaved or nontreated soil was infested separately with suspensions of cultures of Phytophthora palmivora that had been sonicated so that chlamydospores were the only viable propagules which remained. After 8 days of incubation in moist paper towels, and an additional 7 days of exposure to infested soil in plastic beakers at 30 C in growth chambers, 50% of the 15-day-old seedlings were infected at 0.5 chlamydospores/g of soil, and 95% were infected at 10 chlamydospores/g of soil. Under the same conditions, the same results were obtained with nontreated, infested soil as with autoclaved, infested soil. Fifty and 95% of plants in pots in the greenhouse were infected at 0.1 and 10 chlamydospores/g of soil, respectively, after 45 days of growth in autoclaved soil and an additional 30 days exposure to infested soil. Although percentages of infection were greater with increasing age of papaya seedlings after 30 days of exposure to infested soil, the percentages of mortality decreased with increasing age of the seedlings. Synergism between chlamydospores of P. palmivora was suggested when inoculum density-disease incidence transformations were analyzed. Percentages of infection of 45-day-old seedlings exposed for an additional 30 days to zoospore-infested sand in the greenhouse increased with increasing densities of zoospores, and 75% of the seedlings were infected at 10⁵ zoospores per container. Mortality was first observed at 10⁴ zoospores per plant and increased to 40% at 10⁵ zoospores per plant. Populations of P. palmivora after 1 week in infested soil without plants were 70-100 percent of the original chlamydospore densities. Populations in infested soil with papaya seedlings were four to five times greater after 1 week than the original chlamydospore density.