The study of applied management options to enhance crop protection against Verticillium wilt in Gossypium hirsutum.
Type of DegreeMaster's Thesis
Entomology and Plant Pathology
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Verticillium wilt of cotton is incited by Verticillium dahliae, which colonizes the vascular cylinder of the plant and resulting in defoliation, stunting, and yield loss. The long-term goal of this project was to look closely at Verticillium wilt in Alabama and find multiple methods for management of this disease to enhance cotton yields. Specifically our objectives were: 1) assess the state cotton fields with wilt symptoms to determine Verticillium presence; 2) evaluate cotton cultivars for resistance or tolerance to Verticillium wilt in the field; 3) determine whether different soils types are conducive to Verticillium wilt disease severity; and 4) assess potential chemical fungicides for efficacy to Verticillium spp. in vitro. In Alabama, two different species were found in the state, Verticillium. dahliae and V. albo-atrum. Both species were found in the northern part of the state where the wilt is often a yield limiting factor in cotton production. However only V. albo-atrum was found in the mid to southern part of the state, where Verticillium wilt is rarely a concern in cotton. Cotton variety selection impacts Verticillium development in cotton. In 2013, plants displaying the least foliar symptoms were Stoneville (ST) 4747 GLB2, FiberMax (FM) 1944 GLB2, Phytogen (PHY) 339 WRF, and Deltapine (DP) 1044 B2RF. In terms of disease incidence for 2013, the resistant check FM 1944 GLB2 had the lowest number of plants with darkened vascular systems, while ST 4946 GLB2 and FM 1944 GLB2 showed the least symptoms in 2014. Varieties all displayed vascular staining and average percent of disease incidence ranged from 46-68%. In terms of disease incidence, ST 4946 GLB2 had the lowest number of plants with darkened vascular systems. Vascular staining and average percent of disease incidence ranged from 50-53% for PHY 339 WRF, ST 4747 GLB2 and ST 5032 GLB2. These percentages were statistically similar and indicated less disease than was noted for the resistant check FM 1944 GLB2. Ranking the cultivars by yield indicates ST 4747 GLB2 and DPLX 14R1455 B2R2 produced the greatest yield with a yield gain of 19% when compared to the resistant check FM 1944 GLB2. The cultivars DP 1137 B2RF, ST 4946 GLB2, and DPLX 13R352 B2R2 also increased yield on average of 5% compared to the check. Using the average price of cotton for 2014 of 76 cents per pound a $650 per acre revenue increase was obtained with ST 4747 GLB2 over the check FM 1944 GLB2. Gross income was also 53% higher for ST 4747 GLB2 than the most susceptible line DPLX 14R1456. Thus cotton variety selection has the greatest impact of yield and income in the Verticillium-infested field. In addition to the cultivar test a microplot trial was conducted to determine disease severity with and without irrigation in selected soil types commonly found throughout the state of Alabama. A significant interaction was found between the heaviest soil type, Houston Clay, and irrigation. The irrigated Houston Clay soil significantly supported a greater disease incidence compared to the other soil types with or without irrigation. To give producers some potential control options, fungicide active ingredients were evaluated in vitro for efficacy against V. dahliae and V. albo-atrum. Thirteen chemicals were tested at 100% labeled rates. In vitro, V. dahliae growth was best inhibited by Methoxy–carbamates, and Methoxy- acrylates + Triazoles chemical groups more specifically Quilt Xcel and Domark fungicides. V. albo-atrum growth was best inhibited by the same chemical groups (Methoxy – carbamates, and Methoxy- acrylates + Triazoles) however, specific fungicides with superior activity were Quilt Xcel and Headline. In summary, two species of Verticillium are found in the portion of the state where disease is most prevalent. ST 4747 GLB2 was the most tolerant cultivar producers can plant to combat yield loss. Producers who irrigate and are planting in heavier soils are at higher risk for Verticillium wilt in cotton. There are some potential fungicide treatments that possibly may reduce disease severity. Understanding infection risks and implementing different management options is the best way to combat this disease.