CTC medium: A novel dodine-free selective medium for isolating entomopathogenic fungi, especially Metarhizium acridum, from soil

The selective media most commonly used for isolating hyphomycetous species of entomopathogenic fungi from non-sterile substrates rely on N-dodecylguanidine monoacetate (dodine) as the selective fungicide. Although these media are effective for isolating many species of Metarhizium and Beauveria from soil, they are inefficient media for isolation of an important Metarhizium species, Metarhiziumacridum, from non-sterile soil. Our current study was directed to formulating a dodine-free selective medium that is efficient for isolating naturally occurring Beauveria spp. and Metarhizium spp., especially M. acridum, from soil. The selective medium (designated CTC medium) consists of potato dextrose agar plus yeast extract (PDAY) supplemented with chloramphenicol, thiabendazole and cycloheximide. In comparisons with selective media previously reported in the literature, the CTC medium afforded colonies that were larger and had both earlier and more abundant conidiation of entomopathogenic fungi, features which greatly facilitated identification of the emerging entomopathogenic fungi. In addition to efficient re-isolation of M. acridum, this medium also is an effective tool for selective isolation of Metarhizium brunneum, Metarhizium robertsii, Beauveria bassiana and Beauveria brongniartii from non-sterile field-collected soil samples inoculated (spiked) with fresh conidia in the laboratory.