THE BLACKLEG FUNGUS - PHYTOTOXINS AND PHYTOALEXINS

This paper describes the systematic screening for phytotoxins produced by different isolates and strains of Leptosphaeria maculans the causative agent of blackleg disease of cruciferous plants. The isolation of sirodesmin PL, deacetylsirodesmin PL, and four novel phytotoxins, sirodesmins H, J, and K and phomalirazine, are reviewed. Nine different isolates of L. maculans were analyzed for phytotoxin production by TLC, HPLC, and H-1 NMR. Only.virulent isolates of L. maculans produced phytotoxins. The toxicity of sirodesmin PL and acetylated derivatives of sirodesmin PL was determined on cotyledons and leaves of host and non-host plants. The compounds were phytotoxic but not host-specific. The toxicity of sirodesmin PL, deacetylsirodesmin PL, and phomalirazine also was tested on protoplast and microspore cultures. Microspore cultures of a resistant cultivar (Brassica juncea) were more sensitive to phomalirazine (LD50 = 1.89-mu-M) than to sirodesmin PL (LD50 = 2.99-mu-M). By contrast, microspores of a susceptible cultivar (B. napus) were as sensitive to phomalirazine (LD50 = 1.01-mu-M) as to sirodesmin PL (LD50 = 0.84-mu-M). Necrotic leaf tissue inoculated with L. maculans was analyzed to determine whether the fungus synthesized phytotoxins in diseased plants. This investigation has led to the isolation of the phytoalexin spirobrassinin synthesized by a susceptible plant in response to infection by a highly virulent isolate of L. maculans. Sirodesmin PL elicited similar foliar response.