Survival of Verticillium albo-atrum on Nonsuscept Roots and Residues in Field Soil

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Abstract
Parasitic colonization of barley [Hordeum vulgare] roots by V. albo-atrum (microsclerotial type), the cotton wilt pathogen, was not a significant survival mechanism under field conditions. Although up to 5.6 colonies/100 cm of barley root were recorded by plating on selective media during the growing season, 1-6 mo. after harvest buried root residues of immune crops such as alfalfa [Medicago sativa], barley, corn [Zea mays] and sorghum did not contain V. albo-atrum microsclerotia. Barley and cotton [Gossypium hirsutum] stems were not colonized after incubation periods of 1-9 wk in naturally infested soil. No difference was found in inoculum density of V. albo-atrum during the growing season in rhizosphere (RS) or nonrhizosphere field soil (NRS) from alfalfa, barley, corn and sorghum. Few V. albo-atrum colonies were recovered from nonsuscept rhizoplanes following a 72 h air drying period. Nonsuscept rhizosphere and rhizoplane isolates of V. albo-atrum were pathogenic to cotton. Symptoms of Verticillium wilt never were observed on nonsuscepts growing under field conditions, nor was the fungus isolated from plant tissue above soil line.

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