Yu-Chen Wang
Plant Pathology Farm Advisor
University of California Cooperative Extension, Monterey
Salinas, CA, USA
Jack Koster
University of California Cooperative Extension, Monterey
Salinas, California, United States
Suzanne Rooney-Latham
California Department of Food and Agriculture, Plant Pest Diagnostics Center
Sacramento, California, United States
Cheryl Blomquist
California Department of Food and Agriculture, Plant Pest Diagnostics Center
Sacramento, California, United States
Wei Belisle
California Department of Food and Agriculture, Plant Pest Diagnostics Center
Sacramento, California, United States
Tyler Bourret
USDA-ARS, Mycology and Nematology Genetic Diversity and Biology Laboratory
Beltsville, Maryland, United States
Stunted mature lettuce (Lactuca sativa L.) with wilted outer leaves were observed at five commercial fields in Monterey Co. California in spring 2023 and 2024. Brown internal stem and crown lesions progressed into sunken cavities and plant collapse. Incidence was approximately 5 to 75%. Margins of discolored stem and crown tissue were surface sterilized and plated on PARP-CMA and colonies resembling Phytophthora were recovered. Sequences from the two primary barcodes, ITS and COI were obtained from isolates collected from five different host cultivars and locations, finding identical multi-locus genotypes. ITS chromatograms contained six double-peaks, indicating interspecific hybridization. COI found the lettuce isolates forming a clade with the provisional P. taxon castitis. If P. taxon castitis ITS is used as one parental ITS haplotype, the other haplotype is 1 bp different from P. lactucae and P. pseudolactucae ITS, suggesting one is the other parent of this hybrid taxon, provisionally introduced here as P. taxon ×salinaslettuce. Koch's postulate was satisfied by inoculating romaine and iceberg lettuce in the growth chamber, and symptoms similar to the original samples. This is the first report of P. taxon ×salinaslettuce detection on lettuce. Plans are underway to confirm the hybrid status and parentage of P. taxon ×salinaslettuce using genomics. This emerging pathogen may cause severe economic losses in CA lettuce production during the winter and spring growing seasons.