Many Fusarium oxysporum formae speciales produce small, infection-dependent effector proteins called “secreted in xylem” (SIX) proteins. These proteins are secreted into the xylem of a plant during the infection process and are thought to promote virulence. In this study, a collection of Fusarium oxysporum isolates composed primarily of different races of Fusarium oxysporum f. sp. vasinfectum (FOV) was screened for the presence of 14 SIX effector genes ( SIX1 to SIX14). Our results showed that some of the most virulent FOV races, FOV4 and FOV7, share a common SIX effector: SIX9. This effector is largely absent in other FOV races in North America, making SIX9 a potential target for rapid detection of these highly virulent FOV strains and enabling race-specific FOV quantification in infected host plants. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .