The involvement of mechanical barriers in the resistance response of a field-resistant and a field-susceptible potato cultivar to Verticillium dahliae

Abstract

The association of phytotoxin production by the pathogen Verticillium dahliae with the production of wilt symptoms in susceptible potato cultivars has been well documented. However, certain cultivars exhibit field resistance although isolated tissues and cell cultures of these cultivars demonstrate susceptible responses when treated with phytotoxins from V. dahliae. Therefore more than one type of resistance, other than biochemical insensitivity to phytotoxins such as hypersensitivity and mechanical barriers, may be present. We investigated several defence mechanisms, including hypersensitivity, suberization, wound periderm formation and phytoalexin production in tubers of a field-susceptible (Kennebec) and a field-resistant (Reddale) cultivar, both of which are sensitive to V. dahliae phytotoxins. Although the total number of suberized cell layers and wound periderm formed were not different between the two cultivars, the response was more rapid in the resistant cultivar. Tuber tissue discs of the resistant cultivar exhibited more rapid and intense hypersensitive browning than the susceptible cultivar when inoculated with V. dahliae, whereas treatment with the elicitor arachidonic acid produced similar responses in both cultivars. High levels of the phytoalexin rishitin where produced in the susceptible cultivar when inoculated with the fungus, but only trace levels of rishitin were found in the resistant cultivar regardless of treatment. These results indicate that rapid mechanical defensive mechanisms play an important role in the resistance of potato cultivars to seed-piece infection by V. dahliae, while in the cultivars studied the production of phytoalexins is not correlated with resistance.