Isolates of Verticillium fungicola, collected over a
period of 45 yr from various geographical sites, were analysed for
their responses to four fungicides (benomyl, thiabendazole,
chlorothalonil and diethofencarb), and for colony morphology,
virulence and DNA polymorphisms. Three different colony morphologies
were observed for this population of isolates including fluffy, dense
and appressed. The majority of isolates had the appressed colony
morphology. Most isolates exhibited high levels of virulence. The
appearance in this population of both benomyl and thiabendazole
resistance was correlated with the introduction of benomyl for use on
mushrooms. Eighty-eight percent of the isolates were cross-resistant
between benomyl and thiabendazole and 80% exhibited negatively
correlated cross-resistance between benomyl and diethofencarb.
Seventy percent of the isolates exhibited negatively correlated
cross-resistance between thiabendazole and diethofencarb. The level
of resistance to chlorothalonil was relatively high even prior to its
introduction and overall the response to this fungicide appears to
have varied little over the last 45 yr. DNA polymorphisms, as
detected by randomly amplified polymorphic DNA (RAPD) analysis,
divided the isolates into four groupings. Limited correlations could
be drawn between RAPD groupings, fungicide response and geographical
origin. All recently collected isolates (1993–5) were extremely
similar in their response to the four fungicides and had appressed
colony morphology, high virulence and were members of RAPD group
four. The lack of variation in the recent isolates as compared to the
older isolates indicates that the V. fungicola population
may be becoming more homogeneous.