Detection and estimation of conidial abundance of Penicillium verrucosum in soil by dilution plating on a selective and diagnostic agar medium (DYSG)

Penicillium verrucosum is one of the main producers of ochratoxin A (OA) in agricultural commodities. To forecast the risk of OA contamination, there is a need to improve our knowledge on the ecology of P. verrucosum in the field. Dilution plating on 'dichloran yeast extract sucrose 18% glycerol agar' (DYSG) offers a simple and very sensitive method of detecting P. verrucosum propagules in soil. The properties of DYSG are illustrated in a suspension mixture experiment and confirmed in a soil mixture experiment. In the latter, P. verrucosum could be detected in conidial concentrations below 200 colony forming units (cfu) g(-1) soil even when it constituted no more than 0.3% of the cfu. Furthermore, the DYSG method can be used to estimate the abundance of P. verrucosum propagules in soil with good precision. In some of the analysed cases, however, it was necessary to use appropriate mathematical models to treat results with high numbers of cfu on the Petri dishes.