Graduate Thesis Or Dissertation
 

Influence of host and non-host plants upon populations of Verticillium dahliae in soil

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  • 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 plant residues. 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 mint species. 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.
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