Production of the fungal biocontrol agent Ulocladium atrum by submerged fermentation: accumulation of endogenous reserves and shelf-life studies.

A method was developed for the induction of submerged conidiation of Ulocladium atrum Preuss (isolate 385) for the first time, using an oatmeal extract broth. Two inoculum types were produced by this process: spores and mycelial fragments. Spore production was stimulated by reducing the broth water potential (psi) to -2.1 MPa and adding 20 mM calcium chloride. In contrast, mycelial fragments were dominant at -7.0 MPa psi. Maximum total inoculum (mycelial fragments and conidia) yields were approximately 2 x 10(7) ml(-1) after 9 days incubation at 25 degrees C at 100 rpm. Biomass from liquid cultures responded to water-stress by accumulating increased concentrations of endogenous sugar alcohols (polyols), particularly glycerol. Long-term shelf-life studies showed that submerged inoculum from cultures subjected to an intermediate water-stress (-2.1 MPa psi) and containing enhanced levels of glycerol (> 300 mg g(-1) freeze-dried material) retained viability significantly better (P < 0.05) than that from unstressed cultures, when assessed on agar with fully available water. This level of viability was comparable to that of aerial U. atrum spores from a 4-week solid-substrate fermentation on oat grains. However, in contrast to aerial spores, the ability of submerged biomass to germinate in drier conditions declined significantly after 6 months.