Alternaria solani is a causal fungus of early blight disease of potato and tomato. Two phytotoxins, alternaric acid and solanapyrones, were isolated from the different strains of the fungus. Biosynthetic studies on these two phytotoxins have been done, and in the biosynthesis of solanapyrones, it was proved that a biological Diels-Alder reaction is involved through the polyketide pathway and the participation of the enzyme Diels-Alderase has been proved for the first time.