- The incitant of Verticillium wilt of peppermint (Verticillium
dahliae Kleb.) causes disease symptoms only in the genera Mentha
(the mints) and Monarda. This fungus is unable to grow saprophytically
through natural soil. V. dahliae survives in soil by colonizing
and forming microsclerotia in susceptible plants, living root tissue
of many resistant plants, and, to a limited extent, non-living
Fluctuations in populations of V. dahliae in the rhizosphere
and penetration of the roots of host and non-host plants were studied
in the greenhouse. Rhizosphere soil was collected from plants and
diluted into ethanol-streptomycin agar (ESA) by standard soil dilution
techniques. Rhizosphere soils from all plants tested had significantly
higher populations of V. dahliae than did non-rhizosphere
soils. V. dahliae populations in non-rhizosphere soil always remained
relatively stable over an eight week period. Resistant and susceptible mint species supported
in the rhizosphere, indicating that
similar populations of V. dahliae
susceptible mint species do not
selectively stimulate the pathogen near the root surface. Tomato,
a symptomless host of the mint strain of V. dahliae, also supported
large populations of V. dahliae in the rhizosphere, but plants such
as wheat, corn, and beans (non-hosts) tended to support fewer numbers
of Verticillium propagules in the rhizosphere than host plants.
Roots from the rhizosphere test plants were thoroughly washed,
coarsely chopped, placed in petri dishes, and covered with ESA to
determine relative numbers of root penetrations. There were no
statistically significant differences in numbers of root penetrations
between resistant and susceptible mint species, but if roots were
finely fragmented to release the contents of the vascular system,
many more V. dahliae propagules were recovered from susceptible
mint roots than from resistant mint roots.
of susceptible mints were invaded to a much
those of resistant mints. Symptomless host
fewer penetration sites on roots than either
The vascular systems
greater extent than
and non-host plants had
resistant or susceptible
Three mint species of low, medium, and high resistance were
grown in field soil infested with 10, 100, and 1000 V. dahliae microsclerotia
per gram of soil. Susceptible Mentha piperita L. exhibited
severe disease symptoms at all inoculum density levels. Symptoms on moderately resistant Hybrid 88-148 increased from
mild to moderate to severe with each increase in inoculum density.
Resistant M. crispa L. showed only mild to moderate symptoms at
all inoculum density levels. However, yields were drastically reduced
in all species at the lowest inoculum density compared to non-infested
controls. Higher levels of inoculum density reduced yields
still more, but not as much as the low inoculum density, which was
the most critical level of infestation for yield reduction.
Soil assays after two years' cropping showed that V. dahliae
populations increased over the original level of infestation only at
the low inoculum density, regardless of the mint species grown.
The amount of increase in V. dahliae populations was correlated
with susceptibility. Soil assays after a six month fallow period
showed that increases in V, dahliae populations at the low inoculum
density were temporary; all plots had populations lower than the original
level of infestation.
Susceptible indicator plants were grown in all plots after the
fallow period to measure inoculum potential. Disease symptoms
and reductions in yields were unusually severe in low inoculum density
plots on which susceptible M. piperita was originally grown.
The susceptible mint apparently maintained a higher inoculum potential
at the low inoculum density than did resistant species.