Detection of novel pathogenic variants of <i>Fusarium oxysporum</i> f. sp. <i>lactucae</i> in California

Abstract Fusarium wilt, caused by the soilborne fungal pathogen Fusarium oxysporum f. sp. lactucae (FOL), is an economically important disease of lettuce ( Lactuca sativa ) throughout the world. Four pathogenic races of FOL are reported, though only race 1 is known to exist in the United States. Recently, Californian lettuce growers have expressed increasing concern about Fusarium wilt, as some race 1‐resistant cultivars have exhibited susceptibility, and some susceptible cultivars have displayed reduced disease severity. To determine whether such changes in disease patterns are responses to potentially novel variants, we collected infected plants from commercial fields in the Salinas Valley and Santa Maria regions, isolated the fungus, and conducted a series of pathogenicity tests using a set of FOL race differential cultivars over 2 years (2022 and 2023) in controlled conditions. Pathogenicity tests revealed two new FOL variants that elicited novel disease reaction patterns on the set of differential cultivars that have not been previously described. Isolates Fol621 and Fol621s were less aggressive on race 1‐susceptible Banchu Red Fire, designated Variant‐1. Isolate VSP‐0916 incited severe Fusarium wilt on race 1‐resistant Costa Rica No. 4, designated Variant‐2. Moreover, VSP‐0916 exhibited high aggressiveness and the ability to induce disease in other race 1‐resistant cultivars. This study is the first documentation, to our knowledge, of the occurrence of FOL‐inciting Fusarium wilt on race 1‐resistant lettuce in the United States. Our work provides valuable information critical for the development of Fusarium wilt management strategies, including broad‐spectrum resistance breeding efforts against multiple FOL races.