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Locally severe outbreaks of Fusarium wilt of cotton (Gossypium spp.) in South Georgia raised concerns about the genotypes of the causal pathogen, Fusarium oxysporum f. sp. vasinfectum. Vegetative complementation tests and DNA sequence analysis were used to determine genetic diversity among 492 F. oxysporum f. sp. vasinfectum isolates obtained from 107 wilted plants collected from seven fields in five counties. Eight vegetative complementation groups (VCG) were found, with VCG 01117B and VCG 01121 occurring in 66% of the infected plants. The newly recognized VCG 01121 was the major VCG in Berrien County, the center of the outbreaks. All eight VCG resulted in significant increases in the percentages of wilted leaves (27 to 53%) and significant reductions in leaf weight (40 to 67%) and shoot weight (33 to 60%) after being stem punctured into Gossypium hirsutum 'Rowden'. They caused little or no significant reductions in shoot weight and height or increases in foliar symptoms and vascular browning in a soil-infestation assay. Soil infestation with Meloidogyne incognita race 3 (root-knot nematode) alone also failed to cause significant disease. When coinoculated with M. incognita race 3, all VCG caused moderate to severe wilt. Therefore, the VCG identified in this study belong to the vascular-competent pathotype, and should pose similar threats to cotton cultivars in the presence of the root-knot nematode. Use of nematode-resistant cultivars, therefore, is probably the best approach to control the disease in Georgia.

A highly virulent race 4 (Cal race 4) of Fusarium oxysporum f. sp. vasinfectum was identified in California cotton fields in 2001, and has since been found in increasing numbers of fields. Cal race 4 isolates contain a unique Tfo1 transposon insertion in the PHO gene that was not found in other F. oxysporum f. sp. vasinfectum genotypes. Based on this insertion, a multiplex polymerase chain reaction method was developed to detect the Cal race 4 pathogen. A panel of F. oxysporum f. sp. vasinfectum isolates representing different vegetative compatibility groups (VCG) and DNA sequence types was assembled to test the specificity of the detection method. In all, 16 of 17 Cal race 4 isolates produced a 583-bp amplicon; the other isolate produced a 396-bp amplicon reflecting the absence of the Tfo1 insertion. This isolate was a moderately virulent pathogen among Cal race 4 isolates. In total, 80 other F. oxysporum isolates associated with cotton and 11 other formae speciales of F. oxysporum produced only the 396-bp amplicon. The method also distinguished Cal race 4 isolates from India race 4 isolates and China race 7 isolates, which did not possess the unique Tfo1 insertion but otherwise had identical DNA sequences, and all belong to VCG0114. The method is capable of detecting the pathogen directly from infected stem tissues even before external symptom appears and, thus, provides an effective tool for timely identification of infested fields and seed lots, and should help reduce dissemination of Cal race 4 in the U.S. Cotton Belt.