Reduction of Verticillium dahliae microsclerotia viability in soil by dried plant residues

Experiments were conducted to assess the eradicative effectiveness of a number of organic amendments from various botanical species on microsclerotia (MS) of defoliating (D) and non-defoliating (ND) Verticillium dahliae isolates in infested soil. All the substrates studied significantly diminished the viability of V. dahliae MS when applied as amendments to infested soil. The efficacy was found to depend on the particular species, incubation time, pathogen isolate and whether the soil was sterilized. The effect generally increased with increasing incubation time. The ND isolate of the pathogen was more markedly affected than the D isolate. Organic tissue debris from Diplotaxis virgata, Lavandula stoechas and Thymus mastichina in the non-sterilized soil proved highly effective in reducing the viability of MS of both isolates over time. The D. virgata debris provided the most consistent reduction of MS viability among all types of organic matter and no viable inoculum of either isolate was detected after 6 weeks incubation (wai) in sterilized and non-sterilized soil. The L. stoechas amendment killed the ND V. dahliae isolate propagules in non-sterilized soil. Although the viability of D isolate MS was reduced only 58.6% 3 wai, no viable propagules were detected 6 or 9 wai. Similarly, the control rate with L. stoechas was lower in sterilized than in non-sterilized soil for both isolates. No viable MS were detected after 3 weeks incubation in the non-sterilized soil amended with T. mastichina. Plant debris from C. albidus and C. salvifolius had an intermediate effect on MS viability, but affected the viability of MS of both isolates more markedly in sterilized than in non-sterilized soil. The eradicative effect of organic amendments on V. dahliae MS was confirmed by examining the development of disease incidence and severity of symptoms of Verticillium wilt in cotton plants growing in infested, amended soil. All organic materials reduced the disease incidence and severity in soils infested with both isolates. Thus, no diseased cotton plants were detected in soils that were infested with the ND isolate and amended with any of the organic substrates. Similarly, the C. albidus and L. stoechas treatments completely prevented development of the disease in soil infested with the D isolate. The other organic treatments applied to this isolate strongly diminished the final disease incidence levels, which fell below 16.7%.