Graduate Thesis Or Dissertation
 

Survival of Verticillium dahliae in soil

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  • Three Oregon soil types (Chehalis silt loam (CSL), an alluvial soil from the Willamette Valley; Ayre sandy loam (ASL), a volcanic pumice soil from the Central high desert; and Klamath fine sandy loam (KFSL), a basaltic mineral soil from the Klamath Basin) were collected adjacent to fields which had been removed from potato production due to Verticillium wilt caused by the soil-borne fungus Verticillium dahliae kleb. The soils were infested with V. dahliae inoculum composed of individual microsclerotia (MS) or microsclerotia imbedded in stem pieces (SPMS). The infestation level was 100,000 propagules/g soil. The inoculum was derived by fragmenting and screening field-infested potato stems. Each infested soil type was transported to, and incubated at, each of the above geographic locations. Changes in the V. dahliae populations were studied over a five-year period. The inoculum potentials of the surviving N. dahliae populations in soils which had been infested with 1000 propagules/g were bioassayed with eggplants after five years. V. dahliae microsclerotia freshly added to moist soils showed an initial flush of sporulation with a subsequent population decline. Population lows were reached after 71 or 256 days incubation, depending on soil type and geographic location. After reaching lows of 14-45 percent of the original population, all V. dahliae populations showed gradual increases through the remainder of the five-year test period. Final populations were 34-63 percent of the original population. CSL soil and the Corvallis environment exerted a strong influence on V. dahliae sporulation. V. dahliae populations in all three soil types infested with MS inoculum showed periodic population fluctuations of up to four-fold at the Corvallis location through the five-year study. ASL and KFSL soil types suppressed sporulation of V. dahliae at the Redmond and Klamath Falls locations. Organic matter breakdown and subsequent release of V. dahliae propagules from stem piece inoculum was related to soil type and geographic location. After 20 months incubation, organic matter breakdown ranged from 7 percent in the CSL soil at the Corvallis location to 88 percent in the ASL soil at the Klamath Falls location. After five years incubation the inoculum potentials varied with soil type and geographic location. Only 20 percent of the eggplants grown in the CSL soils from Redmond showed symptoms, whereas 100 percent of the plants grown in the KFSL soil from the Corvallis and Klamath Falls locations showed symptoms. The three soil types at three levels of infestation were studied over a 290 day period under controlled conditions of soil moisture and temperature in the laboratory. Upon initial infestation with microsclerotia there were population increases in direct proportion to the level of infestation. Unlike the field experiments, no periodic population fluctuations occurred during the 290 day incubation period. The percent V. dahliae survival after 290 days was inversely proportional to the initial infestation levels in the three soils, while percent survival between soil types was in the order of CSL>ASL>KFSL. A linear regression analysis of Verticillium wilt symptom development in eggplants grown in the three soil types, infested at three levels showed a significant difference between the lowest infestation level and the two higher levels and also between soil types. Symptom development in the KFSL soils were significantly slower and less severe. The number of infection loci/g root was not proportional to the inoculum density in the soil. Infection loci/1000 propagules/g of soil were 4.0 in KFSL soils and 2.4 in ASL soils. Eggplants grown in V. dahliae infested soils showed increased fresh weights at each increased infestation level.
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  • File scanned at 300 ppi using Capture Perfect 3.0 on a Canon DR-9050C in PDF format. CVista PdfCompressor 5.0 was used for pdf compression and textual OCR.
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