Stemphylium solani, the causal agent of leaf blight of garlic (A Ilium sativum), produced phytotoxic metabolites in culture. A non-host-specific phytotoxin from culture filtrate of S. solani isolate DY-5, named SS-toxin, was extracted by ethyl acetate, isolated by bioassay-guided thin layer chromatography on silica gel, and purified by preparative liquid chromatography. SS-toxin produced necrotic lesions on detached garlic leaves, similar to that caused by S. solani. When wounded leaves were each treated with a 10-mu l droplet of toxin, Changbanpo, a garlic cultivar susceptible to leaf blight, showed necrotic lesions at 11 mu g/ml toxin, while a resistant cultivar, Qingganruanye, showed symptoms at 44 mu g/ml. The effective doses causing 50% inhibition (EC(50) values) of root growth and shoot elongation of the susceptible cultivar were 64.9 and 178.5 mu g/ml, respectively. The SS-toxin significantly inhibited mitotic activity of root tip cells of the susceptible cultivar from 22 mu g/ml and higher causing an abnormally high frequency of cells in interphase. Concentrations over 50 mu g/ml of SS-toxin were found to be significantly toxic to total chlorophyll, both chlorophyll a and b, of the susceptible cultivar. The plasma membrane cell wall interface, nuclear membranes, mitochondria, and chloroplasts were affected by SS-toxin in susceptible leaf cells. This is the first report of the purification and testing of a phytotoxin produced by S. solani.