INDUCTION OF SYSTEMIC RESISTANCE TO TOBACCO POWDERY MILDEW BY TOBACCOMOSAIC-VIRUS, TOBACCO NECROSIS VIRUS OR ETHEPHON

Local infections of either TMV or TNV in tobacco plants cv. Havana 425 (hypersensitive to TMV) proved effective in inducing systemic resista nce to subsequent inoculation with the powdery mildew fungus Erysiphe cichoracearum DC. The proportion of leaf surface invaded by this patho gen and the amount of conidia it produced were both significantly lowe r in virus inoculated plants than in non-inoculated controls. However, the decrease in sporulation rate was less regularly observed than the reduction in leaf area infected. TMV was more effective than TNV in p rotecting tobacco plants from powdery mildew. E. cichoracearum is thus added to the list of challenge pathogens to which TMV or TNV are know n to induce resistance in the host plants. Necrotic lesions caused to the leaves by local treatment with Ethephon (an ethylene-releasing com pound) also conferred to tobacco some degree of systemic resistance to the same fungal pathogen, more frequently visible as a reduction of l eaf area invaded. The protection due to the Ethephon lesions was in pr esent experiments less marked than that of TMV. No effects against sub sequent powdery mildew infection were obtained when point freeze necro tic lesions were provoked on the plants.