Genotypic Evaluation of Current Field Populations of Fusarium oxysporum f. Sp. Vasinfectum Isolates from California

Fusarium wilt caused by Fusarium oxysporum f. sp. vasinfectum (FOV) race 4 has been a recurring and expanding threat to California. It was first identified in California in 2001 and was more recently reported in Texas in 2017 and New Mexico in 2019. It was previously thought that FOV race 4 was a clonal population in California, however our results confirm two common genotypes present in California cotton fields. Fusarium isolates were collected from symptomatic cotton plants in seven commercial fields in the San Joaquin Valley during the 2017 and 2018 growing seasons. Isolates were identified to species using DNA sequencing of the translation elongation factor gene (EF-1a). Based on the EF-1a gene, there were a total 138 isolates of F. oxysporum and two isolates of F. solani. For F. oxysporum isolates, FOV race 4 specific primers were used to genotype the isolates. Genotypes were identified based on the absence (N type) or presence (T type) of the insertion of the transposable element Tfo1 in the phosphate permease (PHO) gene unique to some FOV race 4 isolates. Our results identified 85 isolates as the T genotype and 45 isolates as the N genotype among FOV race 4 isolates. Eight isolates were identified as FOV race 3. Further studies are being conducted to determine if the FOV race 4 genotypes respond similarly to Pima and Upland germplasm with high to moderate levels of tolerance.