Cotton Cultivar Effects on DNA-Based Quantification of <i>Fusarium oxysporum</i> f. sp. <i>vasinfectum</i> Race 4 in Soil

The soilborne fungal pathogen Fusarium oxysporum f. sp. vasinfectum race 4 (FOV4) poses a risk to U.S. cotton (Gossypium spp.) following detection in California, Texas, and New Mexico. This study evaluated the influence of cotton cultivar resistance on relative variation in soil FOV4 DNA quantity, a proxy for inoculum density, in a naturally infested field over two years. Field trials included Upland (G. hirsutum) and Pima (G. barbadense) cotton cultivars and measured plant mortality, root necrosis, and lint yield. Bulk soil FOV4 DNA was quantified by qPCR at three timepoints each year: planting, midseason (first flower), and late season (crop maturity). Susceptible Pima cultivars exhibited the highest mortality, most severe necrosis, and lowest yields, whereas resistant Pima and Upland cultivars had lower necrosis with variable mortality and yield. Commercial Upland cultivars with differing degrees of susceptibility showed intermediate mortality and necrosis yet high yields. Midseason soil FOV4 DNA levels varied among cultivars in both years (p=0.011, 0.028), with elevated levels in a subset of susceptible cultivars. Late season DNA levels varied among cultivars in 2021 (p=0.017) but not in 2020 (p=0.080) and did not correspond to resistance designations. Within cultivars, mortality correlated with FOV4 DNA levels at planting (p&lt;0.001, McFadden’s pseudo-R 2 [R 2 MF ]=0.810) and late season (p&lt;0.001, R 2 MF =0.802), necrosis with late season DNA levels (p&lt;0.001, R 2 =0.849), and yield showed no correlation with FOV4 DNA at any timepoint. These findings relate host response to DNA-based soil pathogen quantification and emphasize resistance variation as a determinant of FOV4 disease ecology and management success.