EVALUATION OF YEASTS FOR BIOLOGICAL-CONTROL OF FUSARIUM DRY ROT OF POTATOES

Thiabendazole-resistant strains of Fusarium sambucinum and F. solaniva r. coeruleum threaten to negate chemical control options for post harv est treatment of Fusarium dry rot. Biological control of dry rot of st orage tubers is feasible using bacterial antagonists (25). The impact of yeasts on dry rot has not been investigated. Initial biological con trol tests employed strains of twenty species of yeasts from the ARS C ulture Collection (NRRL). Strain selection was based on strain isolati on from plant matter or from environments that would indicate a high l ikelihood of strain survival in soil. The control potential of these a nd 29 additional strains isolated from soil adhering to recently harve sted tubers was evaluated using a whole Russet Burbank tuber bioassay. At 2x10(6) cells/ml, only two unidentified strains and Cryptococcus l aurentii strain NRRL Y-2536 reduced disease (P=0.05, P=0.10, respectiv ely) while bacterial strain Pseudomonas fluorescens 2-79 (NRRL B-15132 ) was more effective (P=0.01). Conversely, Debaryomyces robertsiae inc reased disease (P=0.05). No yeast strains significantly controlled dis ease in a subsequent trial. One of six additional strains of C. lauren tii (P=0.10), none of five strains of Pichia farinosa and neither unid entified strain controlled disease at 5 x 10(7) cells/ml whereas P. fl uorescens again reduced disease (P=0.01). After 6 h, four yeast strain s decreased and three increased conidial germination of F. sambucinum R-6380 though there were no differences after 18 h. Five yeast strains , including two strains of C. laurentii (NRRL Y-2536, NRRL Y-7139) wer e marginally effective in controlling disease incited by F. solani var . coeruleum S-1257. Though additional testing may identify yeast strai ns with considerable promise as biological control agents active again st Fusarium dry rot, evidence to date indicates bacterial agents have a greater potential for commercial development.