Herbicide-induced resistance to plant wilt diseases

Pretreating tomato and eggplant seedlings with various dinitroaniline herbicides markedly increased their resistance to vascular wilts caused by Fusarium and Verticillium species. The present study compares the mechanisms of resistance to Fusarium oxysporum f. sp. lycopersici induced by trifluralin in susceptible tomato plants with those in monogenic resistant plants. It was found that fungitoxic compounds, extractable in ethanol, accumulated in herbicide-treated inoculated susceptible tomatoes and also in inoculated monogenic resistant plants. As these fungitoxic compounds only accumulate after inoculation and are not present in either uninoculated plants or in susceptible interactions they are unlikely to be metabolites of the herbicide and are probably phytoalexins. Trifluralin enables the susceptible plant to produce the compounds after inoculation and is therefore considered to be a sensitizer. The fungitoxic substances are different from the terpenoid phytoalexins known to be produced by solanaceous plants since they are insoluble both in chloroform and in water-saturated ethyl-acetate. Fungitoxic compounds, extractable with ethanol, also accumulated in cotton seedlings cv. Pima pretreated with trifluralin and inoculated with F. oxysporum f. sp. vasinfectum.