Novel phytoalexins including elemental sulphur in the resistance of cocoa (Theobroma cocoaL.) to Verticillium wilt (Verticillium dahliaeKleb.)

Four phytoalexins were identified from inoculated stems of cocoa genotypes resistant toVerticillium dahliae.Following purification by repeated flash chromatography and TLC, they were identified by NMR and GC-MS. The most abundant and polar compound was the triterpenoid arjunolic acid; two related phenolics were 3,4 dihydroxyacetophenone and 4-hydroxyacetophenone. The least polar was unambiguously identified as the most stable form of elemental sulphur, cyclo-octasulphur S8, by GC-MS and X-ray crystallography. Respective toxicities toV. dahliaeconidia 13585 after 10 and 3 days respectively, were present in the wood of stems at levels well above (>×13) those required for toxicity and they persisted for long (>50 days) periods; they were found only after infection whereas the phenolics were detected in control stems and were enhanced to similar extents by infection or wounding. Sulphur accumulation was localized to xylem cells. In contrast, condensed tannins, although increasing approx. two-fold after infection, were preformed, of low toxicity (ED50>383μgml−1) and concentrations in two resistant and one susceptible genotypes were not significantly different. This is the first report of phytoalexins inT. cacaoand of sulphur accumulation in plants linked with active defence.