PRODUCTION AND FUNGITOXICITY OF THE TERPENOID PHYTOALEXINS IN COTTON INOCULATED WITH FUSARIUM-OXYSPORUM F-SP VASINFECTUM

Four terpenoid phytoalexins, desoxyhemigossypol (dHG), hemigossypol (H G), desoxyhemigossypol-6-methyl ether (dMHG) and hemigossypol-6-methyl ether (MHG), were identified with HPLC analysis of extracts from the moderately Fusarium wilt resistant cotton var. TAMCOT CAMD-E (Gossypiu m hirsutum) inoculated with Fusarium oxysporum f. sp. vasinfectum (F.o .v.). Concentrations of dHG, HG, dMHG and MG in stem steles at 10 days after inoculation were 45.1, 175.0, 38.3 and 1.6 mug g-1 fresh tissue , respectively. The bioassays demonstrated that all four phytoalexins were toxic to F.o.v. The ED50's of dHG, dMHG and HG were calculated as 8.8, 13.4 and 29.3 mug ml-1, respectively. The very low solubility of MHG in the standard assay medium prevented the determination of its E D50 value. Only dHG is water soluble at levels that appear necessary t o act as an effective phytoalexin. At 30 mug ml-1, dHG kills all conid ia and mycelia of F.o.v. Viable propagules of F.o.v. were recovered fr om the steles of inoculated plants 10 days after inoculation; however, the pathogen was restricted to a zone 15 cm above the hypocotyl inocu lation site. Thus, the fungistatic action of dHG appears to contribute to the resistance of cotton to Fusarium wilt by preventing the system ic distribution of F.o.v. propagules.