Pyrenophorin, the diketone analogue of pyrenophorol, was first reported as a metabolite of the oat pathogen Pyrenophora avenae showing cytostatic properties. Pyrenophorin is a simple macrocyclic dilactone with phytotoxic and antifungal activity. Pyrenophorin inhibits seed germination but once the seed is germinated, pyrenophlorin enhances root development but causes abnormal chlorophyll retention in leaf sections. Pyrenophorin is more potent but less selective than pyrenophorol. Its mechanism of action involves electron misdirection and generation of reactive oxygen species. Pyrenophorin is a potent antifungal, significantly reducing the growth of M. violaceum and S. cerevisiae at 5 μM.