POPULATION DYNAMICS OF Verticillium dahliae AND Fusarium oxysporum f. sp. vasinfectum OVER TIME AND THEIR IMPLICATIONS FOR DISEASE DEVELOPMENT IN COTTON

Verticillium wilt and the Fusarium wilt-Root-knot nematode complex are economically important diseases of cotton. Verticillium wilt is caused by a soilborne fungus Verticillium dahliae Kleb. The fungus can survive in soil and/or in plant debris for more than 20 years as microsclerotia (ms). Verticillium dahliae has a broad host range. Fusarium wilt of cotton is caused by a soilborne fungus Fusarium oxysporum f. sp. vasinfectum Atk. (Fov). In West Texas the occurrence of Fusarium wilt is in conjunction with the root-knot nematode, Meloidogyne incognita. Fov can survive in soil for over 10 years in the form of chlamydospores. Management options are limited for both the diseases. Strategies that influence soil inoculum density may help manage these diseases. This study focuses on understanding the relationship between inoculum density in soil at planting and wilt development in cotton. To examine the influence of Verticillium dahliae infested peanut residue amount on inoculum release in soil and Verticillium wilt development in subsequent cotton, a microplot study was conducted in 2008 and 2009. The hypothesis was that Peanut residue infested with V. dahliae will increase microsclerotia density in soil and Verticillium wilt on subsequent cotton will also increase. The effects of infested peanut residue amount on percent germination of cotton and on Verticillium wilt incidence were monitored in both cropping seasons. Microsclerotia density in soil was also quantified to investigate the release of inoculum from infested peanut residue over time. Infested peanut residue was collected from a field with a history of Verticillium wilt and used to artificially infest microplots at the rates of 370, 925, 1850, 2775, 3700, 18,495, and 37,000 kg/ha. Non-infested microplots served as a control. Treatments were arranged in a randomized complete block design with nine replications. Microplots were planted with a cotton cultivar, Stoneville (ST) 4554B2RF, susceptible to Verticillium wilt. Increasing infested peanut residue amount had a negative effect on percent germination of cotton seed with a slope of -1.88 (R2 = 0.90) in 2008, and - 4.0 (R2 = 0.98) in 2009. A positive effect was found between increasing infested peanut residue amount and Verticillium wilt incidence in cotton, with a slope of 3.07 (R2 = 0.97) in 2008, and 6.43 (R2 = 0.99) in 2009. Soil samples collected before incorporation of infested peanut residue artificially were void of V. dahliae inoculum for all the microplots. Densities of ms in the soil were found to increase significantly with increasing amount of V. dahliae infested peanut residue over time, with a slope of 0.42 (R2 = 0.99) in April 2009, 0.85 (R2 = 0.89) in November 2009, and 1.32 (R2 = 0.89) in April 2010. Results show the importance of removing infested peanut residue which otherwise may serve as a source of inoculum for subsequent cotton crops. To examine the consequence of cultivar selection on soil population dynamics of Verticillium dahliae over time and implications for Verticillium wilt development in cotton, a microplot study was conducted over the 2008 to 2010 growing seasons. The hypothesis tested was that the choice of cultivar will affect soil inoculum density of V. dahliae and disease incidence. ST 4554B2RF was used throughout the test as a susceptible cultivar and either (AFD) 5065B2F or an advanced breeding line was used as the partially resistant cultivar. Microplots were augmented with field soil naturally infested with V. dahliae. ST 4554B2RF when planted in three sequential seasons increased V. dahliae populations in soil from 1.3 to 11 ms/cm3; however, V. dahliae populations in microplots planted to the partially resistant cultivars over three seasons increased from 1.4 to 3 ms/cm3. Disease incidence increased from 8% to 58% over 3 yrs for ST 4554B2RF and from 0% to 5% for AFD 5065B2F or advanced breeding line over the same period. Yield was highest after 3 yrs of AFD 5065B2F or a breeding line and lowest after 3 yrs of ST 4554B2RF. Yield was related primarily by the current year cultivar, pre-plant V. dahliae densities, and disease incidence. Adoption of a resistant cultivar for at least 2 years was necessary to maintain a low ms density in soil. Results from this study indicate that cultivar selection can impact ms density and incidence of wilt in cotton and should be considered when developing management strategies. Fusarium oxysporum f. sp. vasinfectum Race 1 populations in Texas are genetically diverse. To examine the impact of inoculum densities of genetically distinct Fusarium oxysporum f. sp. vasinfectum Race 1 isolates, Meloidogyne incognita, and cultivar on Fusarium wilt development in cotton an experiment was conducted with 12 Fov isolates at four densities (0 to 6.5 ×105 cfu/cm3 soil), Meloidogyne incognita densities (0 and 1,000 eggs/pot), and two cultivars (partially resistant ST 4554B2RF and susceptible Fibermax (FM) 9058F) in the greenhouse. The hypothesis was that the isolates of Fov collected from Texas will demonstrate variability in aggressiveness. FM 9058F had significantly (p = 0.0001) higher area under the disease progress curve (AUDPC) than ST 4554B2RF for all the Fov isolates tested with the interaction of Fov Race 1 isolates, their inoculum densities, M. incognita densities, and cotton cultivars. Fusarium oxysporum f. sp. vasinfectum isolates 3, 4, 5, 6, 7, 10, 11, and 12 exhibited significantly (p = 0.05) higher AUDPC than isolates 1, 2, 8, and 9 suggesting, that variability in aggressiveness exist among Fov isolates. Isolates 3, 4, 6, and 7 with cultivar FM 9058F showed significantly higher AUDPC in the absence of M. incognita at higher inoculum densities of Fov (6.5 × 105 cfu/cm3) compared to other Fov isolates tested which implies that disease incidence may be higher even in the absence of M. incognita when Fov inoculum densities in soil are high. Isolates 5 and 11 of Fov had significantly (p = 0.05) higher AUDPC in the presence of M. incognita (1000 eggs/pot) at lower inoculum density (2.5 × 103 cfu/cm3) than other Fov isolates tested suggesting, the root-knot nematodes were required to cause significant wilt incidence when inoculum densities of Fov were low in soil. Plant growth and symptoms expressions were affected by the susceptibility of cultivar. Susceptible cultivar, FM 9058F had significantly stunted plants, decreased root weight, shoot weight and total plant weight as compare to partially resistant cultivar, ST 4554B2RF. Plants inoculated with M. incognita (1000 eggs/pot) had root galls and were stunted with reduced shoot weight and total plant weight. Results from this study show that variability in aggressiveness exists among Fov isolates of West Texas. To examine the impact of planting combinations of susceptible (FM 9058F) and/or partially resistant (ST 4554B2RF) cotton cultivars, on soil inoculum density of Fov a microplot study was conducted over the 2008 to 2010 growing seasons. The hypothesis was that due to the use of resistant cultivar, soil inoculum densities of Fov and disease incidence will decrease over time. Field soil naturally infested with Fov was added to microplots in 2008 and M. incognita density was augmented each spring before planting. Inoculum density of Fov increased to 9.1 × 105 cfu/cm3 after 3 yrs of FM 9058F (SSS indicates 3 years of a susceptible cultivar); however, Fov density decreased to 2.9 × 105 cfu/cm3 after 3 yrs of ST 4554B2RF(RRR indicates 3 yrs of a partially resistant cultivar). Soil inoculum density in the microplots initially planted with ST 4554B2RF followed by FM 9058F planted for the next two years (RSS) was not different (9.2 × 105 cfu/cm3) from those planted to FM 9058F for three years (SSS) (9.1 × 105 cfu/cm3). Microplots planted with a SRR rotation had similar Fov densities (3.0 × 105 cfu/cm3) from those planted to a RRR rotation (2.9 × 105 cfu/cm3). Disease incidence increased from 18% in 2008 to 69% in 2010 with FM 9058F. Disease incidence was a function of both the cultivar grown in that season and preplant Fov soil density. Yield was highest after 3 yrs of ST 4554B2RF and lowest after 3 yrs of FM 9058F. Yield was related primarily by the current year cultivar, pre-plant Fov density in soil and cultivar history. Results from this study indicate that cultivar selection is the cornerstone for Fusarium wilt management in the fields infested with Fov and M. incognita.