The effect of inoculum density, management practices, and environmental factors on the development of Verticillium dahliae Kleb. on cotton

Verticillium dahliae Kleb, a soil borne fungus that infects susceptible plants such as cotton (Gossypium hirsutum), causing significant yield loss. The fungus survives as microsclerotia, which are the inoculum spores that cause infection to plant roots, then moves into the vascular system choking the plant of water and nutrients causing Verticillium wilt symptoms. The objective of this thesis was to review the major factors influencing the development of V. dahliae by looking at them in a greenhouse and field experiment.
The greenhouse experiment, divided into two tests (Test 1 and Test 2), looked at the interaction between irrigation amount, disease incidence and the cotton yield response when planted in the presence of V. dahliae. The disease incidence showed no significance to irrigation amount. Cotton yields were significant to irrigation amount, as irrigation increased so did the yield. The cotton yields were also significant to V. dahliae, cotton planted in soil inoculated with V. dahliae had lower yields compared to cotton planted in non-inoculated soil for all irrigation rates. For Test 1 and 2 the average lint yield for the non-inoculated plants were 27 g and 13 g, and the inoculated plants average lint yield were 23g and 5g per plant.
The field experiment, used data collected from 2012 to 2016 on the Helms Field at the Texas A&M Research Station in Halfway, TX to build predictive models for Verticillium wilt incidence and cotton yield in the presence of V. dahliae. The predictive models: Verticillium wilt Incidence (%) = 72.44 + 0.07 Irrigation Amount (mm year-1) + 0.15 July Total Cumulative Heat Units (CHU) – 0.17 September Total CHU + 1.67 Inoculum Density + 0.6 Volumetric Water Content (VWC) + 5.28 Tillage, having a R2 = 0.65. The yield model: Yield (kg ha-1) = 2419.63 + 2.39 Irrigation amount + 8.21 August Monthly CHU + 5.2 VWC – 3.42 Disease Incidence, having a R2 = 0.73. The models were validated on two cotton fields near Plainview, TX and Floydada, TX. Both models under predicted the actual values for Verticillium wilt incidence and Yield. This was due to the low irrigation amount at the Helms Field, compared to the irrigation amounts used at the validation fields.