Quantifying Verticillium wilt risk based on microsclerotia density in recently expanded cotton-growing areas of the Northern High Plains of Texas

The recent expansion of cotton acreage in the semi-arid Northern High Plains of Texas has raised concerns about the potential widespread distribution of <i>Verticillium dahliae</i>, the causal agent of Verticillium wilt, into these areas. These concerns were primarily driven by the use of lower seeding rates; lower summer air temperatures; the absence of completely resistant cultivars; and the region's proximity to the Southern High Plains, where the pathogen is endemic. This study was conducted to assess Verticillium wilt risk, based on microsclerotia density, in the Northern High Plains. Soil samples were collected from 26 cotton fields across 10 counties, and viable microsclerotia in 40 cm³ of soil per field were quantified using a plating assay with semi-selective media. Microsclerotia were detected in 88.4% of fields: 38.5% of fields were categorized as high risk (microsclerotia/cm³ ≥ 10), 23.1% as moderate risk (3 < microsclerotia/cm³ ≤ 9.9), 26.9% as low risk (0 < microsclerotia/cm³ ≤ 3), and 11.5% showed no detectable risk (microsclerotia = 0). All isolates/phylotypes tested belonged to the defoliating pathotype. Management recommendations tailored to the risk categories are discussed. The detection of microsclerotia in 88.4% of the fields surveyed, with 38.5% categorized as high risk, indicates a concerning level of inoculum and highlights the need for continued surveillance and further research on phenotypic and genotypic characterisation.