Evaluation of cotton cultivars for resistance to pathotypes of Verticillium dahliae

After the recent detection of serious losses caused by Verticillium wilt of cotton, incited by the defoliating pathotype of Verticillium dahliae in the Aegean Region of Turkey, 28 of the most commonly grown cotton cultivars (Gossypium hirsutum L.) of Turkey, were evaluated for the presence of field resistance to wilt. Six-week-old plants were inoculated with a cotton nondefoliating (ND) or a cotton defoliating (D) pathotype of V. dahliae under controlled conditions. Resistance was evaluated on the basis of external symptoms by calculating areas under disease progress curves. The percentage of plants killed and of those which recovered from the disease was used as additional parameters for including a particular cultivar into a defined category. Most of the evaluated cultivars were susceptible, although at different levels, to both pathotypes of V. dahliae. All cultivars were more susceptible to the D than to the ND pathotype. The most promising cultivars in the experiments appeared to be Carmen and ST-373. Carmen showed differential resistance: it was susceptible to the D but resistant to the ND pathotype. ST-373 was moderately susceptible to both pathotypes of V. dahliae. A resistance related phenotypic reaction to the disease was quantified by using six growth parameters (plant height, number of nodes, leaf weight, stem weight, leaf to stem ratio, and total shoot weight) measured 13 d after inoculation. The percentage decrease in leaf–stem ratio and leaf weight were found to be the best indicators of resistance. Results obtained in this study will be useful to quantify resistance to V. dahliae and identify the best parameters to phenotype in genetic studies.