Toxicity and role of terpenoid phytoalexins in verticillium wilt resistance in cotton

Hemigossypol (HG), methoxyhemigossypol (MHG), desoxyhemigossypol (dHG) and desoxymethoxyhemigossypol (dMHG), the four major terpenoids formed in the stem stele of Verticillium dahliae-infected, wilt-resistant Seabrook Sea Island (SBSI) cotton, were tested at pH 6·3–7·5 in liquid nutrient media for toxicity to V. dahliae. The terpenoids dHG, HG, dMHG, and MHG at 25 °C killed all conidia after 18–40 h at 10, 45, 25 and 60 μg ml⁻¹, respectively; and all mycelia after 48 h at 15, 32, 25 and 45 μg ml⁻¹, respectively. Inhibition of conidia germination also occurred at concentrations well below the fungicidal concentrations. Dimethylsulfoxide at 2 or 5% was required to solubilize HG, MHG and dMHG at fungicidal concentrations. Only dHG had the water solubility apparently required to reach fingicidal concentration in the aqueous medium of infected xylem vessels and thus account for the death of V. dahliae conidia and mycelia in most infected vessels in the stem stele of SBSI cotton 10 days after inoculation. The dHG in the stem stele at 10 days after inoculation was in excess of fungicidal concentration.