Nicole Lukasko, PhD
Postdoc
Department of Microbiology and Plant Pathology, University of California, Riverside, California, USA
Riverside, California, United States
Santosh Nayak
US Department of Agriculture, Agricultural Research Service, Sam Farr United States Crop Improvement and Protection Research Center, Salinas, California, USA
Salinas, California, United States
Kelley Richardson
U.S. Department of Agriculture, Agricultural Research Service, Sam Farr United States Crop Improvement and Protection Research Center, Salinas, California, USA
Salinas, California, United States
Jason Stajich
Department of Microbiology and Plant Pathology, University of California, Riverside, California, USA
Riverside, California, United States
Alexander Putman
Department of Microbiology and Plant Pathology, University of California, Riverside, California, USA
Riverside, California, United States
Lettuce is one of the most valuable agricultural commodities in California, with a production value of $3.7 billion in 2024. A recent increase in prevalence of Fusarium wilt, incited by Fusarium oxysporum f. sp. lactucae, led to the detection of novel pathogenic variants isolated from the Salinas Valley and Santa Maria production regions. While race 1 is the only documented race in the United States, the variant isolates were able to cause severe Fusarium wilt symptoms on the race 1-resistant differential cultivar Costa Rica No. 4. High-throughput pathogenicity screening revealed that isolates with the variant phenotype can be found in multiple production regions, suggesting a widespread presence that may significantly impact disease management practices. To further characterize the potentially novel race, two variant isolates (VSP-0916 and VSP-1420) were whole-genome sequenced using an Oxford Nanopore PromethION flow cell. Genomes assembled with Flye yielded N50s above 3 Mbp and BUSCO completeness above 99%. Comparative genomic analyses, including phylogenetics and effector repertoire characterization, were conducted against representative race 1 and race 4 isolates. Findings from this research will contribute to the development of diagnostic tools for lettuce growers, guide resistance breeding strategies, and expand our understanding of Fusarium oxysporum f. sp. lactucae populations affecting lettuce production in California.