Relative longevity of Macrophomina phaseolina and associated mycobiota on residual soybean roots in soil

Survival of the charcoal rot pathogen (Macrophomina phaseolina) in soybean (Glycine max) on residual root systems was studied over a 2-year period. Root segments colonized by M. phaseolina were placed into fiberglass-mesh bags and buried at depths of 0, 7.6, and 25.4 cm in a Marietta fine sandy loam soil in field microplots. Samples for year I and year 2 were buried in October 1999 and 2000, respectively, and sampled every 2 months over a 14-month period. Mean percent frequencies from both years for M. phaseolina showed a decreased linear trend over time from 91% before burial to 24.7% 2 months later, 10.3% at 6 months, and 0.3% after 14 months. Tissues were degraded in the June samplings of both years and this degradation was believed to be partially responsible for the reduced survival of M. phaseolina. Mean isolation frequencies were significantly (P less than or equal to 0.05) greater at the 0-cm depth than at 7.6 and 25.4 cm across all sampling dates. Other fungi which were present in the preliminary assay, including Fusarium spp. and those in the Phomopsis/Diaporthe complex, also declined by the end of the study. Trichoderma spp. was isolated at significantly (P less than or equal to 0.05) greater frequencies and a significant positive linear trend over time. It is possible that Trichoderma spp. were involved in direct tissue degradation and nutrient depletion of the root segments, or may have acted as a mycoparasite, reducing the survival of the pathogen M. phaseolina and other associated fungi. Results from this study indicate that farm practices which increase residue destruction immediately after harvest or those that enhance Trichoderma spp. populations may directly or indirectly lower the relative longevity of soilborne pathogens, including M. phaseolina.