Fusarium wilt (FW), caused by Fusarium oxysporum f. sp. vasinfectum (Atk.) W.C. Snyder & H.N. Hans (FOV), is one of the most destructive diseases of cotton (Gossypium spp.) worldwide. FOV race 4 (FOV4) is a highly virulent nominal race of this pathogen and a significant threat to cotton production in the western and southwestern USA and, potentially, the entire Cotton Belt. A field survey to identify FOV4 was performed in three southern counties of New Mexico in 619 cotton fields from 2018 to 2020. From 132 samples of cotton plants that exhibited wilt symptoms, Fusarium spp. were the most frequently isolated group of fungal species, with an isolation frequency of 57.4%. Eighty-four Fusarium spp. isolates were subsequently characterized by a DNA sequence analysis of three genes, EF-1α, PHO, and BT, encoding for translation elongation factor, phosphate permease, and β-tubulin, respectively. Forty-two isolates from 10 cotton fields were identified as FOV4 and confirmed with a positive 500-bp fragment diagnostic for FOV4. Twenty-six (62%) of the 42 FOV4 isolates were T type and the remainder (38%) were null type with and without a Tfo1 insertion in PHO, respectively. Each FOV4-infested field contained the same FOV4 genotype. Ten representative FOV4 isolates (one each from the 10 FOV4-infested fields) were evaluated for their pathogenicity on resistant Pima PHY 841 RF and susceptible Upland PHY 725 RF at 7, 14, 21, and 28 days after inoculation under temperature-controlled conditions at 21 to 22°C. Based on the disease severity rating, mortality rate, and area under the disease progress curve value, all 10 isolates were pathogenic to both cotton cultivars and differed in virulence; four isolates of the T genotype as a whole were more virulent than the six isolates of the N genotype. PHY 841 RF had significantly higher levels of resistance than PHY 725 RF to all FOV4 isolates. The results provide the first comprehensive account of the occurrence, distribution, and virulence of FOV4 in cotton production in New Mexico and will be useful for developing an effective strategy to manage FW in the state of New Mexico and the entire western and southwestern Cotton Belt.
Keywords: Fusarium oxysporum f. sp. vasinfectum; Fusarium wilt; cotton; field survey; race 4.