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Differential Accumulation and Distribution of Antifungal Sesquiterpenoids in Cotton Stems Inoculated with Verticillium dahliae. N. A. Garas, Western Regional Research Center, ARS, USDA, 800 Buchanan Street, Berkeley, CA 94710; A. C. Waiss, Jr., Western Regional Research Center, ARS, USDA, 800 Buchanan Street, Berkeley, CA 94710. Phytopathology 76:1011-1017. 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-76-1011.

Stems of four cotton cultivars representing Gossypium barbadense and G. hirsutum were inoculated in the greenhouse with conidial suspensions of mild (SS-4) and severe (T-1) strains of Verticillium dahliae. Vascular tissues of the infected, tolerant cultivar Acala SJC-1 contained sufficient amounts of antifungal substances on the third day after inoculation to completely inhibit mycelial growth of the SS-4 strain. Four sesquiterpenoid phytoalexins, desoxyhemigossypol (DHG), hemigossypol (HG), desoxy-6-methoxyhemigossypol (DMHG), and 6-methoxyhemigossypol (MHG), concurrently accumulated for several days in the vascular tissues of all inoculated plants. No sesquiterpenoids or antifungal activity were detected in stele tissues of injured, uninoculated plants. The highest levels of the methylated sesquiterpenoids (DMHG and MHG) were present in the vascular tissues of the most resistant cotton cultivar (Seabrook Sea Island) soon after inoculation and for several days thereafter, and the lowest levels were present in the most susceptible cotton (70-110). Intermediate amounts of both compounds were detected in the two Acala cottons, which are tolerant only to the mild pathotype of the fungus. The accumulation of HG and DHG was almost always lower in the most resistant cotton line than in all of the other cottons tested. Inoculation with the mild strain (SS-4) induced slightly higher levels of total sesquiterpenoids than did inoculation with the severe T-1 strain. Results demonstrate that the level of cotton resistance to Verticillium wilt is directly related to the amount of methylated sesquiterpenoids produced by the infected vascular tissues. Furthermore, these results suggest that the severity of symptoms induced by the T-1 strain is probably due to its capacity to overcome the toxicity of these compounds.

Additional keywords: bioassay for fungicides, HPLC analaysis of sesquiterpenoids, phytoalexins.