Morphology of Verticillium dahliae and V. tricorpus on semi-selective media used for the detection of V. dahliae in soil.

The morphology of two soil-borne Verticillium species, V. dahliae and V. tricorpus, was studied on two semi-selective agar media, in the absence and presence of soil. Morphology of the fungi differed considerably between the media, with respect to presence and shape of microsclerotia, dark hyphae (i.e. short melanised hyphae attached to the microsclerotia) and dark mycelium (i.e. melanised mycelium throughout the colony). On modified soil extract agar (MSEA), a pectate based agar, V. dahliae always had globose to elongate microsclerotia, without dark hyphae or dark mycelium, whereas V. tricorpus always had dark hyphae or dark mycelium, and microsclerotia, whenever present, were globose to irregular in shape. On ethanol agar (EA), V. dahliae had large microsclerotia and abundant dark hyphae, whereas V. tricorpus did not form microsclerotia, but always abundant dark mycelium. For the first time we observed the formation of dark hyphae by V. dahliae to a great extent. In the presence of soil, most characteristics were less pronounced, and V. dahliae microsclerotia were smaller, but V. tricorpus produced large microsclerotia, even when they were absent in pure culture. Morphological characteristics suitable for discrimination between the two species on MSEA plates in the presence of soil were selected and tested with fresh isolates from agricultural fields. The two fungi could be distinguished using qualitative characteristics and microsclerotial size. Molecular analysis and morphology on potato dextrose agar confirmed all identifications made on soil dilution plates.