A selective medium for recovering Penicillium from soil

Penicillium expansum causes a postharvest decay of apples and is common in orchard soils, but populations in eastern soils have not been quantified. We developed DG18-P agar, a modified DG18 agar, for recovering Penicillium species from soils. DG18-P contains 155 g glycerol, 10 g glucose, 5 g peptone, 1 g monobasic potassium phosphate, 0.5 g magnesium sulfate heptahydrate, 2 micrograms dichloran dissolved in 1 ml ethanol, 15 g agar, 0.1 g chloramphenicol, 1.0 ml tergitol NP-10 nonionic in 1000 ml distilled water. Dichloran is added after autoclaving. This medium suppresses fast growing fungi such as Trichoderma and Mucor species and eliminates most bacteria, yeasts, and actinomycetes. When plates of DG18-P and acidified potato dextrose agar were similarly inoculated with known numbers of P. expansum spores, colony counts were similar on both media. DG18-P does not eliminate other species of soil-inhabiting Penicillium, so quantification of P. expansum from soil dilution plates must be based on sub-sampling and colony morphology on Czapek yeast extract agar. DG18-P is being used to study Penicillium densities in soil, on wooden bins, and on apple fruit in the field.