Evaluation of yeasts for biological control of Fusarium dry rot of potatoes

Thiabendazole-resistant strains of Fusarium sambucinum and F. solani var. coeruleum threaten to negate chemical control options for post harvest treatment of Fusarium dry rot. Biological control of dry rot of storage tubers is feasible using bacterial antagonists (25). The impact of yeasts on dry rot has not been investigated. Initial biological control tests employed strains of twenty species of yeasts from the ARS Culture Collection (NRRL). Strain selection was based on strain isolation from plant matter or from environments that would indicate a high likelihood of strain survival in soil. The control potential of these and 29 additional strains isolated from soil adhering to recently harvested tubers was evaluated using a whole Russet Burbank tuber bioassay. At 2 times 10-6 cells/ml, only two unidentified strains and Cryptococcus laurentii strain NRRL Y-2536 reduced disease (P=0.05, P=0.10, respectively) while bacterial strain Pseudomonas fluorescens 2.79 (NRRL B-15132) was more effective (P=0.01). Conversely, Debaryomyces robertsiae increased disease (P=0.05). No yeast strains significantly controlled disease in a subsequent trial. One of six additional strains of C. laurentii (P=0.10), none of five strains of Pichia farinosa and neither unidentified strain controlled disease at 5 times 10-7 cells/ml whereas P. fluorescens again reduced disease (P=0.01). After 6 h, four yeast strains decreased and three increased conidial germination of E. 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) were marginally effective in controlling disease incited by F. solani var. coeruleum S-1257. Though additional testing may identify yeast strains with considerable promise as biological control agents active against Fusarium dry rot, evidence to date indicates bacterial agents have a greater potential for commercial development.