Evaluation of Phytophthora parasitica var. nicotianae for Biocontrol of Phytophthora parasitica on Catharanthus roseus . K. A. HOLMES, Graduate Research Assistant, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616. D. M. BENSON, Professor, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616. Plant Dis. 78:193-199. Accepted for publication 2 November 1993. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/PD-78-0193.

Isolates of Phytophthora parasitica var. nicotianae were selected for biocontrol of Phytophthora parasitica, which causes preemergence damping-off of Catharanthus roseus. Isolates of P. p. nicotianae pathogenic to tobacco were weakly pathogenic on C. roseus and variable in their biocontrol of damping-off. Rice grain cultures of P. parasitica (14-21 days old) at 0.05 g per 25.4 cm2 plug tray were optimal for screening potential biocontrol isolates in the greenhouse. After screening 41 isolates in preliminary experiments, 11 isolates were tested in three repeated experiments. Three isolates of P. p. nicotianae (402, 602, and 723) that represented the range of effective control of P. parasitica were chosen for further study. The isolates selected as potential biocontrol agents, however, stunted (P = 0.05) main-root extension in C. roseus. Above-ground growth of C. roseus also was stunted initially by P. p. nicotianae, as first and second leaves were shorter (P = 0.05) on plants grown with each isolate compared to leaves of plants in the uninfested control. Flowering was not affected. P. p. nicotianae was recovered from roots, crowns, and stems of C. roseus seedlings, but not to the same extent as was P. parasitica. Severity of preemergence damping-off caused by isolates of P. p. nicotianae was related directly to the amount of plant tissue colonized by each isolate. Isolate 723 caused the least amount of stunting, colonized the least amount of host tissue, and was the least pathogenic on C. roseus while giving 52% control of isolate 336 of P. parasitica. In addition, isolate 723 was effective (range 13-73%) in protecting C. roseus from several additional isolates of P. parasitica. Metalaxyl-insensitive isolates of P. parasitica were used in population studies with isolate 723 in peat-vermiculite medium seeded to C. roseus. Incorporation of metalaxyl into one-half of the assay plates permitted enumeration of both antagonist and pathogen in the growth medium. Populations of P. parasitica were lowered when P. p. nicotianae was present. Apparently, one possible mechanism of biocontrol is direct suppression of the pathogen population through competition for nutrients. However, use of P. p. nicotianae for biocontrol of preemergence damping-off of C. roseus caused by P. parasitica does not seem promising unless more effective nonpathogenic isolates can be found.