Etiology, importance, and distribution of Verticillium wilt of cotton in southern Spain

Surveys for Verticillium wilt in 142 cotton fields in 1981 to 1983 and 1985 indicated that the disease is widespread in the Guadalquivir Valley, southern Spain, where it occurs in 80.0 to 82.5% of the fields. Verticillium wilt was most prevalent in the upper valley in the 1981 to 1983 surveys and in the lower valley in the 1985 survey. Analyses of dry soil samples collected in spring 1985 by means of an Andersen sampler detected Verticillium dahliae propagules in 35.1% of the fields surveyed. Inoculum density in soil was much higher in the lower valley (average 37.1 CFU/g) than in the upper and central areas of the valley (average 4.7 CFU/g). The increasing importance of the disease in the lower valley corresponds to the spread of a defoliating pathotype of V. dahliae in this area during the early 1980s. Two groups of isolates, including 100 mild nondefoliating and 90 severe defoliating, were distinguished among 191 isolates by means of morphological, physiological, and pathogenicity tests. The remaining isolate was identified as nondefoliating with intermediate virulence. All defoliating isolates were obtained from the lower valley, while nondefoliating isolates were widespread. Isolates of the defoliating pathotype produced elongated and rounded microsclerotia on water agar, were able to grow on sanguinarine-amended potato dextrose agar, and fluoresced under UV light. Isolates of the nondefoliating pathotype formed only rounded microsclerotia on water agar, were inhibited greatly on sanguinarine-amended PDA, and did not fluoresce under UV light. Furthermore, the defoliating isolates had an optimum temperature for in vitro growth of 24 to 27 degrees C compared with 21 to 24 degrees C for the nondefoliating isolates.