|Fusarium oxysporum f. sp. vasinfectum (FOV), the causal agent of cotton Fusarium wilt, has been causing losses in the cotton industry around the world for more than 125 years. Large genetic diversity exists within this pathogen which is also known to form a disease complex with at least three nematode genera to cause increased disease and crop damage. The overall objective of this research was to 1) identify the genetic diversity and temporal distribution of FOV that exist within a single Alabama cotton field and 2) evaluate a potential chemical management option for its effect on the entire FOV-nematode disease complex.
In chapter 1, an in-depth review is provided of current literature with insight into the diversity of FOV races/genotypes that affect cotton farms in the United States and explains methods that are used to characterize them. This chapter also provides a synopsis of the current understanding of the disease complex formed between FOV and plant-parasitic nematodes such as Meloidogyne incognita.
Chapter 2 is a survey of FOV isolates collected from the National Cotton Fusarium Wilt Evaluation Field to document the population diversity and temporal distribution of FOV races and genotypes that can exist within a single Alabama cotton field. This cotton field was found to contain eight different races and genotypes of FOV at varying levels. Seven of these races/genotypes were found during both years of testing and the genotype LA-112 was found only during 2019 testing. The most predominant races/genotypes found, which account for more than 93% of the total collected samples, were race 1, 2, 8, and LA-108. Other genotypes found (LA-110, LA-112, LA-127/140, and MDS-12) were only found at very low levels. For many of the races/genotypes, including all of the predominant races/genotypes, a correlation was observed between the frequency of isolation and accumulation of growing degree days. This trend indicates that for the races/genotypes found, disease incidence increased in the latter part of the cotton growing season.
In Chapter 3, evaluations were made of the use of ReklemelTM (a new chemical nematicide) for its ability to lower M. incognita population density, its effects on FOV, and its usefulness in management the FOV-nematode disease complex. The objectives of this study were 1) evaluate the impact of ReklemelTM on the growth of FOV isolates in vitro and 2) assess cotton growth, yield, and disease incidence with the application of ReklemelTM under greenhouse and field conditions. In vitro testing revealed that FOV isolates were affected by ReklemelTM at varying rates among the races/genotypes that were tested. EC50 (effective concentration to reduce 50% of mycelial growth) values ranged from a low of 14.6 mg/L with race 2 to a high of 187.6 mg/L with the genotype LA-108. In greenhouse testing, ReklemelTM significantly reduced M. incognita population density but had no significant effect on Fusarium wilt incidence. However, in the field, ReklemelTM reduced both M. incognita population density and Fusarium wilt incidence. This reduction in FOV incidence was not observed with the treatment of Velum TotalTM which had statistically similar reductions in M. incognita egg population density.
Lastly, the Appendix is a study to determine the yield loss of cotton by the nematode Rotylenchulus reniformis on seven commonly available cotton cultivars and estimate the benefit associated with the application of a chemical nematicide. Field trials were established during 2017 and 2018 in two adjacent fields; one was infested with R. reniformis and one where R. reniformis was not detected. In both fields, seven cotton cultivars were planted with and without Velum TotalTM (1.02 L/ha). Rotylenchulus reniformis was estimated to cause a yield loss of 50% between the two years of the study. Application of the nematicide Velum TotalTM significantly reduced the population density of R. reniformis in both years of the study but only increased yields during the 2017 cotton season.