EFFECTS OF ANTAGONIST CELL CONCENTRATION AND 2-STRAIN MIXTURES ON BIOLOGICAL-CONTROL OF FUSARIUM DRY ROT OF POTATOES

Eighteen bacterial strains were individually assayed against Gibberell a pulicaris (5 x 10(5) conidia per ml) by coinoculating antagonist and pathogen in wounds in cv. Russet Burbank potatoes. All antagonist con centrations (10(6), 10(7), and 10(8) CFU/ml) decreased disease (38 to 76% versus control, P < 0.05). When four strains were assayed at 11 co ncentrations (range 10(5) to 10(8) CFU/ml) against G. pulicaris, linea r regression of the log-dose, log-response data was significant for al l four strains (P < 0.001 to 0.01, R-2 = 0.50 to 0.74). Challenging G. pulicaris with all possible antagonist pairings within 2 sets of 10 a ntagonist strains (5 x 10(5) CFU of each strain per ml) resulted in 16 of 90 pairs controlling disease better than predicted based on averag ing the performance of the individual strains making up the pair (P < 0.10). Successful pairs reduced disease by similar to 70% versus contr ols, a level of control comparable to that obtained with 100 times the inoculum dose of a single antagonist strain. Neither strain genus nor soil of origin were useful in predicting successful antagonist pairs. Factors potentially influencing dose-response relationships and the e ffectiveness of antagonist pairs in controlling disease are discussed.