Phytotoxin production by Alternaria linicola and phytoalexin production by the linseed host

Summary. Alternaria linicola produced a wide range of secondary metabolites when grown in a defined culture medium. Reverse phase chromatography fractions produced disease‐like symptoms on linseed cultivars and a range of non‐host species indicating the presence of phytotoxic components. Characterised via thin layer chromatography, these included the non‐host specific phytotoxins tenuazonic acid, alternariol monomethyl ether, tentoxin and two destruxin‐type compounds (which closely resembled destruxin A and destruxin B). The identity of four of the compounds was confirmed by two dimensional thin layer chromatography and proton nuclear magnetic resonance spectroscopy. In a second experiment, Linum leaf material infected with conidia of A. linicola and blastospores of Melampsora lini was extracted using a facilitated diffusion extraction technique. The resultant extracts contained a number of compounds which were fungitoxic to Cladosporium cladospiroides and, to a lesser extent, Alternaria brassicicola. One such compound corresponded to the phytoalexin coniferyl alcohol. Quantitative differences in the amount of the fungitoxic compounds produced between the inoculated and uninoculated resistant and susceptible host genotype combinations suggested that the production of fungitoxic compounds was greater in response to attempted colonisation. On this basis it is proposed that phytoalexin production is a component of the resistance reaction. The results from these investigations are discussed in relation to recent research on the ecology of the pathogen and the possible roles of phytotoxin production by the pathogen and phytoalexin production by the host on disease development.