Detection of novel pathogenic variants of Fusarium oxysporum f.sp. lactucae in California

Authors: Nayak, Santosh; Richardson, Kelley; PUTMAN, ALEX - University Of California; Leblanc, Nicholas; Martin, Frank; LI, NINGXIAO - University Of California; McCreight, James

Submitted to: Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/3/2024
Publication Date: N/A
Citation: N/A

Interpretive Summary: Lettuce has great contribution in California’s economy, valued at over 3 billion dollars in 2022. Fusarium wilt, a disease caused by a soilborne fungal pathogen, is a major concern for lettuce worldwide. While four pathogenic races of this pathogen are recognized, the US has only dealt with race 1 until now. However, this study reveals the detection of novel variants in California, potentially representing new races, posing a significant threat to lettuce production. This research emphasizes the need for advancing Fusarium wilt management strategies, including breeding efforts to combat Fusarium wilt effectively and develop more resilient lettuce cultivars.

Technical Abstract: Fusarium wilt, caused by the soilborne fungal pathogen Fusarium oxysporum f.sp. lactucae (FOL), is an economically important disease of lettuce (Lactuca sativa L.) throughout the world. Four pathogenic races of FOL are reported, though only race 1 is known to exist in the United States. Recently, California lettuce growers have expressed increasing concern about Fusarium wilt, as race-1 resistant cultivars exhibit susceptibility, and cultivars classified as susceptible display reduced disease severity. To determine whether such changes in disease patterns are responses to potentially novel variants, infected plants were collected from commercial fields in the Salinas Valley and Santa Maria region. FOL was isolated from this material and a series of pathogenicity tests were performed using a set of FOL race differential cultivars over two years (2022 and 2023) in controlled conditions. Pathogenicity tests revealed two new FOL variants, Fol621s and 916, that elicited novel disease reaction patterns on the standard differentials that have not been previously described. Isolate Fol621s was less aggressive on race 1-suceptible ‘Banchu Red Fire’, whereas 916 incited severe Fusarium wilt on race 1-resistant ‘Costa Rica No. 4’. Moreover, 916 exhibited high aggressiveness, and has the ability to induce disease in other race 1-resistant cultivars. This study is the first documentation, to our knowledge, of the occurrence of any FOL isolate in the United States inciting Fusarium wilt on FOL race 1-resistant lettuce. Our work provides valuable information critical for the development of Fusarium wilt management strategies, including broad-spectrum resistance breeding efforts against multiple FOL races.