The phytopathogenic fungus Alternaria eichorniae attacks water hyacinth, an economically significant aquatic weed. The novel phytotoxin alteichin was isolated from liquid cultures of this fungus and its structure was deduced by X-ray crystallographic analysis. Altheichin is a doubly hydrated form of 4,9-dihydroxy perylene-3, 10-quinone. A single step dehydration of alteichin to anhydroalteichin is catalyzed both by acid and by a crude enzyme preparation from water hyacinth.

Two Cercospora beticola toxins, isolated from cultures of the sugar beet pathogen, were identified as tautomeric compounds and named cebetin A (1) and cebetin B (2). The structure of cebetin B (2) and its absolute configuration were determined by single-crystal X-ray diffraction. The structure of cebetin A (1) was elucidated on the basis of UV-visible, NMR, and mass spectral data. The molecular backbone of cebetins is totally different from that of the known phytotoxin cercosporin and comprises partially hydrogenated xanthone and anthraquinone moieties connected through two seven-membered rings.The xanthone and anthraquinone systems contain respectively a chlorine atom and an epoxide function. Cebetin B crystallizes in the orthorhombic space group P22{2, with a = 14.449 (3) Á, b = 22.269 (2) A, c = 14.061 (2) A, V = 4524.3 A3, Z = 4, and Z), = 1.369 g/cm3. The structure was determined from 3670 diffractometer data and refined to a final R = 0.066.In the crystal, cebetin B molecules exist as a dimeric complex, (C3, 2 013€1•2£ 3 )2 £2, where the two molecules of cebetin B share two Mg2+ ions and are related by a 2-fold axis. Spectroscopic data showed that cebetin A is a metal-free keto-enol tautomer of cebetin B. Both cebetins A and B at a concentration of 1 ppm were lethal to sugar beet cells in suspension culture in the presence of light. This effect was not observed in the absence of light.

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Secondary metabolite production by the phytopathogen Alternaria cinerariae has been investigated resulting in the isolation and identification of αβ-dehydrocurvularin and curvularin as products of this fungus. The phytotoxicity of these compounds is demonstrated.