INFLUENCE OF AN ANTAGONISTIC STRAIN OF PSEUDOMONAS-FLUORESCENS ON GROWTH AND ABILITY OF TRICHODERMA-HARZIANUM TO COLONIZE SCLEROTIA OF SCLEROTINIA-SCLEROTIORUM IN SOIL

Pseudomonas fluorescens strain 2-79RN10 (nalidixic acid and rifampicin-resistant mutant of wild type strain 2-79) was used to study potential effects of bacterial antagonism in soil on growth and biocontrol efficacy of the biocontrol fungus Trichoderma harzianum isolate ThzID1, which was formulated into alginate pellets. In steamed soil (25 C, -100 or -500 kPa matric potential), strain 2-79RN10 maintained its initial high populations (approximately 3 x 10(4) or 3 x 10(7) cfu/g of soil) over a 14-day period, and significantly reduced hyphal radius, hyphal density, and recoverable numbers of propagules of ThzID1. In raw soil under similar environmental conditions (22-25 C, -10 to -1,000 kPa), populations of 2-79RN10 decreased by approximately four log10 units over a 3-wk period, and did not affect the ability of Trichoderma spp. to colonize sclerotia of Sclerotinia sclerotiorum. In two years of field experiments using raw or steamed soil in microplots, populations of 2-79RN10 decreased gradually after 1-2 wk and did not reduce the ability of Trichoderma spp. to colonize sclerotia of S. sclerotiorum. Colonization of sclerotia by Trichoderma spp. after 9 wk was significantly higher in steamed soil (mean = 65%) than in raw soil (mean = 30%) when ThzID1 was added, suggesting possible inhibition of ThzID1 by indigenous soil microbes, or utilization by ThzID1 of nutrients released by steaming of soil. In treatments where ThzID1 was not added, low levels of colonization of sclerotia were observed, apparently due to indigenous Trichoderma spp., and these levels were higher in raw soil (mean = 18%) than in steamed soil (mean = 5%). These results suggest that under certain restrictive conditions, high population levels of antagonistic bacteria in bulk soil suppressed a fungal biocontrol agent, but that this suppressive effect was reduced or eliminated when a high bacterial population was not present.