Field assessment of commercial cotton cultivars for Verticillium wilt resistance and yield

Verticillium wilt is responsible for substantial yield losses in cotton. The use of cultivars with partial resistance to Verticillium wilt is a preferred management strategy. Cotton breeders utilize rating scales based on severity of foliar symptom expression and/or defoliation when making cultivar selections. The objective of this research was to determine the yield relationship with cultivars grouped by both incidence of Verticillium wilt during the boll filling stage, and late season defoliation in fields with a history of Verticillium wilt. Small plot, replicated cultivar trials were conducted over an 11 year period in west Texas. Cultivars were divided into four classes (A–D) based on normalized wilt incidence (NW) and four classes based on normalized defoliation ratings (ND). Classes were determined through a mixed model analysis of cultivars, with T-test comparisons between a partially resistant check (Fibermax 2484B2F) and a susceptible check (Deltapine 0912B2RF). The A and C classes did not differ (P = 0.05) from the partially resistant and susceptible checks, respectively. Classes were used in a mixed model analysis with lint yield. In a model with only NW classes, lint yield decreased significantly as NW increased with least square mean values of 1421, 1385, 1284 and 1204 kg/ha, for classes A, B, C and D, respectively. In a model with both NW and ND classes, the A/A class combination of NW and ND had a significantly (P ≤ 0.05) higher lint yield (1776 kg/ha) than any other combination. NW/ND class combinations of A/B, B/A, B/B, and C/A had intermediate yields, and ND classes of C or D had the lowest yields. Cultivars with both low wilt and defoliation incidences should result in higher yields in fields infested with Verticillium dahliae. Furthermore, these two rating criteria could be used to standardize the process for developing resistant cultivars.