Production of a toxin stemphol by Stemphylium species

Five of 11 isolates of Stemphylium botryosum Wallr. from oilseed rape produced the phytotoxin stemphol when cultured on rice, with yields varying from 15.0 to 98.4 mg/kg, and three of them also produced the toxin on Czapek Dox (CD) liquid medium supplemented with cations (0.12-0.31 mg/L) and potato-dextrose (PD) broth (0.37-1.47 mg/L). In liquid cultures about 95% of stemphol was found in the mycelium, and toxin production was slightly increased when cultures were exposed to near UV light rather than being grown in the dark. The LD50 concentrations for stemphol against isolated cells of oilseed rape and chickpea were 8.4 and 7.0 microM, respectively. One isolate of S. majusculum (strain no. 135459) produced much greater amounts of stemphol, i.e., 22.8 mg/L, 535.3 mg/L, and 548 mg/kg when grown on CD, PD, and rice, respectively. Leaves of oilseed rape, artificially infected with S. majusculum and bearing lesions that occupied an average of 70% of the lamina contained 0.9 microgram stemphol per gram dry weight of leaf tissue.