Isolation, Purification, and Biological Activity of a Phytotoxin Produced by Stemphylium solani

Garlic leaf blight caused by Stemphylium solani is a serious disease in vegetable-growing areas of China (28). Symptoms of the disease start as small white spots (1 to 3 mm) enlarging to elongated purple necrotic lesions along the vein, followed by blighting of the leaf tips. Similar symptoms were produced by culture filtrates of S. solani, and a strong correlation was also found between garlic cultivar sensitivity to S. solani and sensitivity to S. solani culture filtrates (29), suggesting the possible involvement of phytotoxins in lesion symptom development. We recently found that S. solani isolated from garlic produced toxins that were non-host-specific, and caused symptoms normally associated with leaf blight on a variety of plants (29). The phytotoxic metabolites of several species of Stemphylium have been isolated or structurally characterized. The phytotoxins produced by S. botryosum include stemphyltoxin (I-IV), stemphyperylenol (3), stemphylin (4), stemphyloxin (I, II) (5,6,16), and stemphol (22). Stemphylone/ radicinin (9,11) and pyrenophorin (10) were found with S. radicinum, and stemphone (12) was found with S. sarcinaeforme. However, chemical synthesis of the aglycone from stemphylin has shown that its originally proposed structure requires revision (23,25). The partial chemical structure of SV-toxin produced by S. vesicarium is a benzaldehyde ring attached to some unknown groups (18). All these structurally characterized toxins are low molecular weight secondary compounds with diverse chemical structures. However, the toxin(s) secreted by S. solani has not yet been structurally elucidated. Culture filtrates of S. solani isolated from cotton were found to contain host-specific toxins that were proved to be nonproteinaceous (17). Preliminary chemical characterization of the SS-toxin produced by S. solani showed it to be a quinone compound containing phenolic hydroxyl groups, and the UV and IR spectra of SS-toxin are both different from those of other Stemphylium metabolites described above (30). The objectives of this study were to purify the major phytotoxin related to a virulent S. solani isolate, and to assess biological activities of this phytotoxin.