Phytotoxins from alternaria alternata, a pathogen of spotted knapweed

The fungus Alternaria alternata was found to be a host-selective pathogen of spotted knapweed, Centaurea maculosa, a major weed pest in rangelands of the northwestern United States and southwestern Canada. Examination of the extracts of fungal cultures revealed three classes of phytotoxins—diketopiperazines, tetramic acids, and perylenequinones. The isolation, identification, and phototoxic activity of the tetramic acid and perylenequinones are described; two of the perylenequinones, alterlosins I and II, are new compounds. Members of the three classes were compared individually and in combination with respect to phytotoxicity and host specificity.