EVALUATION OF PHYTOPHTHORA-PARASITICA VAR NICOTIANAE FOR BIOCONTROL OF PHYTOPHTHORA-PARASITICA ON CATHARANTHUS-ROSEUS

Isolates of Phytophthora parasitica var. nicotianae were selected for biocontrol of Phytophthora parasitica, which causes preemergence dampi ng-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 isolate, in the greenhouse, After screening 41 i solates in preliminary experiments, 11 isolates were tested in three r epeated 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 bio control 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 r ecovered from roots, crowns, and stems of C. roseus seedlings, but not to the same extent as was P. parasitica, Severity of preemergence dam ping-off caused by isolates of P. p. nicotianae was related directly t o the amount of plant tissue colonized by each isolate. Isolate 723 ca used the least amount of stunting, colonized the least amount of host tissue, and was the least pathogenic on C. roseus while giving 52% con trol of isolate 336 of P. parasitica. In addition, isolate 723 was eff ective (range 13-73%) in protecting C. roseus from several additional isolates of P. parasitica. Metalaxyl-insensitive isolates of P. parasi tica were used in population studies with isolate 723 in peat-vermicul ite medium seeded to C roseus. Incorporation of metalaxyl into one-hal f of the assay plates permitted enumeration of both antagonist and pat hogen 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 bioco ntrol of preemergence damping-off of C. roseus caused by P. parasitica does not seem promising unless more effective nonpathogenic isolates can be found.