Numbers ofV. tricorpuspropagules in soil showed considerable variation according to the time of the year when samples were taken. This is considered to be a true fluctuation, rather than chance variation, or periodic suppression by antagonistic fungi, and is in contrast to earlier evidence that the related species,V. albo-atrumandV. dahliae, are present in soil mostly as resting bodies. Instead, it is suggested that the fluctuation in number of propagules is due to microsclerotia producing short-lived spores. Under laboratory conditions it has been shown that conidia and microconidia are produced directly by microsclerotia, and it is suggested that such bodies are responsible for the fluctuation in number of propagules in soils.
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