Ecological diversity, niche overlap, and coexistence of antagonists used in developing mixtures for biocontrol of postharvest diseases of apples

A method was developed to select antagonists to be combined in mixtures that controlled blue mold of apple better than individual antagonists alone. Microorganisms selected for these mixtures were isolated from exposed apple tissue at weekly intervals over 5 weeks before harvest. The isolated microorganisms were classified and grouped into various nutritional clusters on the basis of their utilization of 95 carbon sources in Biolog plates using MicroLog and MLCLUST programs, respectively. Yeasts dominated isolations on nutrient yeast-dextrose agar at all isolation times. Bacteria were isolated only occasionally. Isolates were screened for their ability to control blue mold caused by Penicillium expansum on Golden Delicious apple fruit. The most promising antagonists from different clusters were paired in subsequent tests to control blue mold, with preference given to antagonists colonizing the same fruit, secondly to those colonizing different fruit, but isolated at the same time, and finally to those colonizing different fruit at different times of isolation. Among 21 yeast antagonists tested in 13 combinations, four combinations were superior to individual antagonists. In a more extensive test, control of blue mold by a combination of antagonist isolates T5-D3 and T5-E2 was consistently superior to the individual isolates. Nutritional profiles of these antagonists, based on utilization of 35 carbon and 33 nitrogen sources, revealed significant differences in carbon catabolism. These differences caused niche differentiation and allowed populations of both antagonists to flourish in the same wound. De Wit replacement series revealed a high level of coexistence between the two antagonists. This was further confirmed by the relative yield that was close to unity at all antagonist proportions tested. Combining antagonists on the basis of niche differentiation was an effective method of improving control of postharvest blue mold, and probably also for other pre- and postharvest diseases.