Conidial quality of the biocontrol agent Coniothyrium minitans produced by solid-state cultivation in a packed-bed reactor

Conidial germination and novel sand- and soil-based sclerotial parasitism assays were used to test the quality of conidial inoculum of the biocontrol agent Coniothyrium minitans IVT1 produced in different packed bed reactor runs. The fermenter airflow rate was either kept constant, resulting in a transient increase in temperature at the top of the fermenter above 30 °C (above the maximum for conidial production), or varied to maintain a constant lower temperature (27 °C) at the top of the fermenter. Conidia were harvested from the top or bottom of the fermenter (where the temperature remained near the constant inlet temperature of 18 °C) and were used non-dried or following spray-drying. In addition, the different conidial preparations were stored for 6 months at 5 °C and germination tests and sclerotial parasitism assays repeated. In general, percentage germination of spray−dried conidia harvested from the top of the fermenter and tested immediately was consistently lower than of non-dried conidia collected from the top of the fermenter, or any conidia collected from the bottom of the fermenter. However, low percentage germination of either non-dried or spray-dried conidia from any position in the fermenter, in any fermenter run, was not necessarily associated with a decrease in ability to infect and reduce viability of sclerotia in either a sand or a soil sclerotial parasitism assay. After storage, very low germination was observed for the non-dried conidial suspension inocula irrespective of position in the fermenter or fermenter run, and this inoculum gave a very low infection of sclerotia in both sclerotial infection bioassays. Storage of spray-dried conidial inocula from the fermenter, resulted in a significant decrease in percentage germination on PDA compared with conidia harvested from PDA. However, in the sand bioassay, percentage infection of sclerotia was reduced for spray−dried conidia harvested from the bottom of the fermenter but this was not found in the soil-based bioassay. Thus, there was no obvious correlation between percentage conidial germination and ability of the conidial inoculum to infect and reduce viability of sclerotia suggesting that conidial germination may not always reflect the quality of the inoculum of this biocontrol agent.