Genetic Diversity, Pathogenicity, and Nematode Interactions of U.S. Fov

Fusarium oxysporum f. sp. vasinfectum (Fov) is a genetically diverse group of soilborne fungi that causes Fusarium wilt of cotton. Discoveries of race4 in Texas and new genotypes in Georgia are serious concern for cotton production. Pathogenicity assays based on infested-soil inoculation, infested-soil inoculation with interaction of nematodes, stem-puncture inoculation, and root-dip inoculation assays enabled distinction of two major pathotypes among the U.S. Fov: vascular competent vs. root rot pathotypes. Genetic diversity among isolates was assessed utilizing vegetative compatibility (VCG) tests and DNA sequence analysis of EF, PHO, BT genes. Fifteen VCG’s were found among vascular competent pathotype isolates, with VCG 2 (race 2), 17, and 9 (race 8) as the most widely distributed groups. No VCG 6 (race 6) was found in the U.S. However, VCG 15 and 16 shared identical sequences with VCG 6. In a survey in Georgia, 8 VCGs were found and belonged to vascular competent pathotype, with the VCG17B, 17C and 21 being the most prevalent ones. Vascular competent isolates caused disease in stem puncture assays, but failed to cause disease in the infested-soil assay. When root-knot nematodes were added to the infested-soil assay, isolates caused severe wilt in ‘Rowden’. In contrast, root-rot pathotype isolates caused disease in infested-soil assay in the absence of nematodes, but failed to cause disease in stem-puncture assay. VCG 3 (race 3), 4 (race 4 and 7), and 11 (Australian Biotype) belong to this root-rot pathotype, but VCG 3 and 11 were not found in the U.S. Only VCG 4 was found among the collected diseased plants from Texas race 4 fields. These Texas VCG 4 isolates are highly pathogenic to Pima S-7. The presence of two pathotypes in the U.S. requires breeding for nematode resistance for control of vascular competent pathotype and breeding for resistance for Fov 4.