P-455 - Pathogenicity of Fusarium oxysporum f. sp. vasinfectum Race 4 Isolates from Far West Texas

Following an outbreak of Fusarium wilt of cotton, caused by Fusarium oxysporum f. sp. vasinfectum race 4 (Fov4) in Far West Texas (El Paso and Hudspeth counties), growers rapidly adopted resistant Pima varieties. However, in 2024, Fov4 caused seedling mortality in resistant varieties in many fields. In this study, Fov4 was isolated from symptomatic and asymptomatic cotton plants and 14 isolates were subsequently evaluated for pathogenicity on three cotton varieties with varying levels of Fov4 susceptibility in a growth chamber experiment. Foliar symptom severity was significantly different between the susceptible Pima and susceptible Upland variety, while root necrosis and asymptomatic colonization were significantly lower in the susceptible Upland variety, compared to both Pima varieties. The tolerant Pima had less foliar symptom severity and asymptomatic colonization than the other varieties. However, more than 50% of the isolates caused significant root necrosis in the tolerant Pima compared to the control, whereas 21% to 50%  of the isolates caused significant root necrosis in the susceptible varieties. The ability of some isolates to induce significant root necrosis in the tolerant Pima variety raises concerns about the long-term effectiveness of tolerance as a management strategy. The differences in disease severity among isolates emphasize the importance of ongoing monitoring of Fov4 in affected regions, which is crucial for developing more effective strategies to mitigate its growing impact on cotton production.