ACCUMULATION OF HOST-SPECIFIC TOXIN PRODUCED BY COCHLIOBOLUS-CARBONUM DURING PATHOGENESIS OF MAIZE

During penetration and colonization of susceptible maize leaves, the foliar pathogen Cochliobolus carbonum race 1 released the cyclic tetrapeptide toxin required for genotype-specific pathogenicity. Inoculation fluids on the leaf surface and extracts of leaf tissue contained the host-specific toxin by 12 and 18 h, respectively, after inoculation. Quantities of the toxin in inoculated leaf tissue increased during the development of necrotic lesions. The following preparations did not contain detectable quantities of the host-specific toxin: extracts of leaf tissue from susceptible or resistant isolines inoculated with the weakly virulent race 2; leaf extracts of the resistant genotype inoculated with race 1; and extracts of non-germinated conidia of either race. The results indicate that the pathogen produces and releases the toxin by the time of leaf penetration and that, in the susceptible interaction, the toxin accumulates by the time defence responses in the host must be suppressed. © 1991 Academic Press Limited.