Two antagonists, the bacterium Pseudomonas syringae and the pink yeast Sporobolomyces roseus, against blue mold (caused by Penicillium expansum) on apple controlled this disease more effectively when combined at approximately equal biomass (50:50 of the same turbidity) than in individual applications. Addition of L-asparagine enhanced the biocontrol effectiveness of P. syringae but decreased that of S. roseus and had no significant effect when the antagonists were combined. Populations of both antagonists increased in apple wounds and were further stimulated by the addition of L-asparagine. The carrying capacity of wounds for P. syringae was not affected by S. roseus. Populations of P. syringae in wounds inoculated individually or in a 50:50 mixture with S. roseus reached the same level after 3 days at 22 degrees C. However, populations of S. roseus recovered after applications of the mixture were consistently lower than those recovered after individual applications. Similar effects were observed in in vitro tests in which populations of S. roseus grown in mixtures with P. syringae were consistently lower than those grown alone, while the populations of P. syringae were not affected by the presence of S. roseus. A total of 36 carbon and 35 nitrogen compounds were tested for utilization by both antagonists. Fourteen nitrogenous compounds were utilized by both P. syringae and S. roseus, and an additional nine compounds were utilized by P. syringae. S. roseus and P. syringae utilized 17 and 13 carbon sources, respectively; 9 sources were common to both antagonists. Populations of these antagonists in apple wounds appear to form a relatively stable community dominated by P. syringae.(ABSTRACT TRUNCATED AT 250 WORDS)