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

Highlights

• Commercial cotton cultivars (259) were placed into 4 classes for resistance to wilt and defoliation.

• Yield averages for the lowest to highest wilt class ranged from 1421 to 1204 kg/ha.

• Cultivars (135) were placed into 15 of 16 combination classes for wilt x defoliation.

• The most resistant combinations of wilt x defoliation resulted in the highest yields.

Abstract

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.

Introduction

Verticillium wilt causes substantial losses in many dicotyledonous plants. The causal agent, Verticillium dahliae Kleb, is a soilborne fungus that has a host range of more than 300 plant species (Pegg and Brady, 2002). The fungus is capable of persisting in the soil via the production of microsclerotia (Wilhelm, 1955). Management of Verticillium wilt requires a number of different tactics, most providing only limited control. Crop rotation with non-hosts has not been very effective at reducing high V. dahliae microsclerotia densities (Butterfield et al., 1978, Huisman and Ashworth, 1976). Rotation with sorghum can delay initial buildup of V. dahliae microsclerotia (Wheeler et al., 2014), resulting in less wilt, higher cotton yields, and better economic returns (Wheeler et al., 2012, Wheeler et al., 2016). Chemical control with fumigation has been practiced, but high usage rates are required to kill microsclerotia, limiting the use to high-value crops (Ben-Yephet and Frank, 1984, Woodward et al., 2011). Fungicides have not been widely used for managing Verticillium wilt (Bell, 1992), though some have been shown to reduce severity of Verticillium wilt symptoms in field studies (Bubici et al., 2006, Kurt et al., 2003). Selection of resistant or partially resistant cultivars is the most desirable method of managing the disease; however, highly resistant commercial cultivars have not been identified in Gossypium hirsutum grown in the U.S.

Efforts to assess Verticillium wilt resistance include the incidence or severity of foliar symptoms and vascular discoloration (Bassett, 1974, Karademir et al., 2010, Marani and Yaacobi, 1976). An index may include both incidence of wilt and percent defoliation within the same scale (Aguado et al., 2008). Fungal reproduction or growth in the plant has also been used as a direct measure of resistance (Frost et al., 2007, Pasche et al., 2013, Tsai and Erwin, 1975, Zhang et al., 2013). Chawla et al. (2012) found that microsclerotia (MS) densities increased slowly over a three-year period in soils planted with partially resistant cultivars, from 1.3 to 2.8 MS/cm³ soil, compared to a susceptible cultivar which increased to 11.1 MS/cm³ soil. Such relatively small changes in microsclerotia production in a cultivar within a growing season might be difficult to detect in cultivar trials due to spatial variability.

“Many cotton breeding programs have been frustrated by the fact that a cultivar might be resistant to Verticillium wilt, but is not as productive as less resistant cultivars. Therefore, the use of yield or its components associated with foliar symptoms is very important as an indicator of Verticillium wilt tolerance” (Aguado et al., 2008). The objective of this research was to represent cultivars as class values by their relative ability to reduce wilt symptoms (expressed as wilt incidence), and defoliation (expressed as percent defoliation); and determine the relationship between these class values and yield in fields infested with V. dahliae.