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 in cotton (Gossypium spp.) worldwide. FOV race 4 (FOV4) is a highly virulent nominal race of this pathogen and is a significant threat to cotton production in the western and southwestern US and potentially the entire Cotton Belt. A field survey in three southern counties of New Mexico was conducted in 619 cotton fields in 2018-2020 to identify FOV4. 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 were identified to be FOV4 from 10 cotton fields and confirmed with a positive 500 bp fragment that is 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; and each FOV4-infested field contained the same FOV4 genotype. Ten representative FOV4 isolates with 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 post inoculation under temperature-controlled conditions at 21-22°C. Based on disease severity rating (DSR), mortality rate (MR) and the area under the disease progress curve (AUDPC) value, all the 10 isolates were pathogenic to both cotton cultivars and differed in virulence in that 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 in developing an effective strategy to manage FW in both the state of New Mexico and entire southwestern Cotton Belt.
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