Biocontrol potential of Microsphaeropsis ochracea on microsclerotia of Verticillium longisporum in environments differing in microbial complexity

The potential of the fungal antagonist Microsphaeropsis ochracea to control the soilborne pathogen Verticillium longisporum was investigated in environments with varying microbial complexity (in vitro vs. in vivo, sterile vs. unsterile, controlled conditions vs. field). A semi-quantitative PCR assay was developed for the detection of M. ochracea on unsterile plant debris. In vitro, M. ochracea caused high levels of mortality to V. longisporum microsclerotia (51–100 %) from 4 to 24 °C, with a broad optimum between 16 and 24 °C. In controlled conditions, M. ochracea significantly reduced the viability of V. longisporum microsclerotia grown on dead rapeseed stems in autoclaved sand, but not in unsterile soil. Likewise, in two experimental years, no significant reduction of V. longisporum inoculum was detectable on rapeseed straw buried in small plots in the field in any of the treatments (soil depths, exposure duration, doses of M. ochracea). Germination of M. ochracea pycnidiospores was inhibited by general soil fungistasis in unsterile soil from a field, botanical garden and grassland. Accordingly, V. longisporum infection of rapeseed plants in the greenhouse was reduced only at artificially high doses of M. ochracea inoculum and no biocontrol efficacy in disease control was recorded in field experiments conducted with winter oilseed rape during two subsequent seasons in an experimental field near Göttingen, with a soil homogenously infested with V. longisporum. The results demonstrate that M. ochracea, although having shown promising potential in controlling pathogens with melanised resting structures on leaf litter, evidently lacks microbial competitiveness to effectively control pathogens in the soil such as V. longisporum, even though the latter is effectively inhibited in vitro.