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Relationship of Cultivar Resistance to Distribution of Verticillium dahliae in Inoculated Cotton Plants and to Growth of Single Conidia on Excised Stem Segments. N. A. Garas, Research plant pathologist, Western Regional Research Center, ARS, USDA, 800 Buchanan Street, Berkeley, CA 94710; S. Wilhem(2), and J. E. Sagen(3). (2)(3)Professor and staff research associate, Department of Plant Pathology, University of California, Berkeley 94720. Phytopathology 76:1005-1010. Accepted for publication 9 April 1986. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1986. DOI: 10.1094/Phyto-76-1005.

Verticillium dahliae was recovered, in laboratory cultures, from apical vascular stem and leaf petiole tissues of various cotton cultivars within 3 days of basal stem-puncture inoculation with either a mild (SS-4) or a severe (T-1) strain of the fungus. This movement of V. dahliae in cotton stems was observed in young greenhouse-grown plants of all the susceptible, tolerant (Gossypium hirsutum), and resistant (G. barbadense) cultivars tested. Leaf symptoms appeared after 7-9 days. The pathogen was isolated less frequently from the resistant Seabrook Sea Island than from the tolerant or susceptible cotton cultivars (G. hirsutum). Compared with wilt-susceptible and wilt-tolerant cotton cultivars, excised stem segments of resistant plants strongly inhibited mycelial growth of V. dahliae when inoculated on cut surfaces with single germinated conidia. Of those stem segments on which the fungus grew, microsclerotial formation was sparse and delayed, whereas susceptible cultivars supported vigorous mycelial growth and abundant microsclerotial formation. The presence of V. dahliae in the upper stem and petiole tissues of both susceptible and resistant plants soon after inoculation, as well as the differential growth and microsclerotial formation of the fungus on cut stem surfaces of resistant versus susceptible cottons suggest that resistance is due to the activation of a host chemical response. These results are consistent with the information presented in the subsequent paper on the accumulation of methylated sesquiterpenoid phytoalexins, which appear to be the primary determinants of Verticillium wilt resistance in cotton.