INFLUENCE OF FUSARIUM-SOLANI ON CITRUS ROOT-GROWTH AND POPULATION-DYNAMICS OF PHYTOPHTHORA-PARASITICA AND PHYTOPHTHORA-CITROPHTHORA

To determine the influence of Fusarium solani on citrus root growth and subsequent development of rhizosphere populations of Phytophthora parasitica or Phytophthora citrophthora, Troyer citrange was grown for 30 days in soil infested with F. solani prior to infestation with A parasitica or P. citrophthora. Root weights, root lengths, development of root tips, percentage of roots colonized by Fusarium, and rhizosphere densities of P. parasitica or P. citrophthora were measured over time. Rhizosphere densities of P. parasitica were greater at the end of the experiment than at the beginning. Infestation of soil with F. solani suppressed P. parasitica densities in the rhizosphere of citrus by 53% but did not alter rhizosphere density fluctuations. Root lengths of plants grown in soil infested with F. solani were 25% less than those of plants grown in noninfested soil. The average percentage (over 28 days) of root tips that were new was lower in soil drenched with suspensions of P. parasitica zoospores than in nondrenched soil. However, root tip development of plants grown in soil infested with F. solani was no different from that of plants grown in noninfested soil. Rhizosphere density of P. citrophthora was not different whether soil was infested with F. solani or not. Rhizosphere densities of P. citrophthora fluctuated over 24 days but were not affected by soil infestation with F. solani. When citrus was inoculated with P. citrophthora, soil infestation with F. solani did not affect root length but resulted in 43% fewer new root tips.