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Assessment of isolates of Fusarium oxysporum f. sp. vasinfectum as seedling pathogens to cotton using a rolled towel assay.

Josue Diaz: California State University, Fresno

<div><i>Fusarium oxysporum </i>f. sp.<i> vasinfectum </i>(FOV) race 4 can be an extremely virulent wilt pathogen to susceptible cotton varieties. Within the United States, FOV race 4 currently is geographically limited, first identified in California in 2001 and Texas in 2017. In FOV race 4 infested fields the fungus has also been known to cause damping off and seedling mortality. To better assess disease impacts under California field conditions, more needs to be known on the relative pathogenicity of FOV races as early seedling pathogens. Therefore, the objective of this study was to assess the reaction of cotton to seedling infection with different isolates of FOV using a rolled towel assay. During the summer of 2017, isolates of FOV were isolated from symptomatic cotton seedlings from three FOV race 4-infested commercial fields in California. Sixteen isolates were used to inoculate seed of the FOV race 4-moderately resistant Upland cultivar FM-2334 and susceptible Pima cultivar PHY-830. Seeds were individually inoculated with 100 µl of a 1×10<sup>6</sup> conidia/ml suspension. At ten days, seedlings were rated using an ordinal rating scale (1= no disease, 5= dead). There was a significant difference for isolate and cultivar (P<0.0001). Based on the ordinal rating scale, averages for isolates ranged from 2.1-3.4 and 3.7-4.3 for the FM-2334 and PHY-830, respectively. Based on these results, the use of the rolled towel assay should be examined as an early screening method for FOV resistant cotton.</div>