Fusarium wilt of watermelon, caused by the fungal pathogen Fusarium oxysporum f. sp. niveum (FON), is the most severe soilborne disease in watermelon worldwide and has been a serious threat to watermelon production in the United States. The goal of my proposed research is to determine fungicide resistance, mode of FON seed infestation, and genetic and phenotypic diversity of FON in the southeastern United States. Assessment of sensitivity of the isolates to fungicides indicated that all isolates were sensitive to prothioconazole at 10 g/ml, while some isolates were resistant to thiophanate-methyl at 100 g/ml. Sequencing a portion of the -tubulin gene of resistant or sensitive isolates to thiophanate-methyl indicated that fungicide resistance was associated with a point mutation at nucleotide position 200 resulting in a substitution of phenylalanine by tyrosine. To determine how seeds in watermelon fruit can be infested by FON during the watermelon growing season, greenhouse and field experiments were conducted in 2014 and 2015 where watermelon flowers and immature fruit were inoculated with FON. Inoculation of the pericarp of immature fruit, pistil or peduncle resulted in both internal and external seed infestation under both greenhouse and field conditions. Simple sequence repeats (SSR) markers were used for analysis of genetic diversity of the FON isolates. Discriminant analysis of principal components (DAPC) of 99 isolates grouped the isolates into eight major clusters with two prominent clusters (1 and 8). Cluster 1 consists of a total of 14 isolates out of which 85.7% of the isolates came from FL. However, the majority of isolates (92.4%) in cluster 8 came from GA. Both DAPC and population structure analysis demonstrated that the phylogenetic groups are closely associated with geographical location of pathogen isolation. The majority of the isolates in cluster 1 and cluster 8 either belonged to race 2 (35.6%) or race 3 (45.8%). Additionally no relation between phylogenetic groups and races identified was observed. Overall, information gained from this dissertation research will add to basic and applied information in FON-watermelon pathosystem. In time, the findings can be incorporated into the integrated disease management options against this disease.
