Time is the incubation period between inoculation and tissue extraction. Each number is the average from eight samples with standard deviation. Terpenoids were induced in both cultivars at all times tested after inoculation with V. dahliae . As expected, the levels of terpenoids were higher in the moderately resistant cultivar, Deltapine 50, than in the susceptible cultivar, Rowden, at all times tested. These results suggest that the reduction in Verticillium wilt found in plants treated with T. virens was caused by a mechanism other than an earlier induction of defense responses, at least as far as terpenoid phytoalexins are concerned.
Further work is needed to determine whether other resistance pathways are induced.
Katz et al. Other defense-related systems in cotton might respond differently. Our results show that seed treatment with T. virens can reduce symptoms of Verticillium wilt on cotton in the greenhouse. This reduction occurs even when the pathogen is inoculated directly into the cotton stems, and may be due to induction of resistance in cotton by T. virens that can continue beyond the seedling stage. Further research will be conducted with other pathogen systems to determine whether this is a true systemic resistance response, providing reductions in the severity of diseases that are strictly foliar and have no potential for a direct interaction with the T. virens colonizing the plant root.