Diversity and pathogenicity of fungal species associated with lettuce wilt and root rot in Vietnam

Since the beginning of 2023, an outbreak of Fusarium wilt-like symptoms on lettuce (Lactuca sativa L.) has been observed in Lam Dong province, Vietnam. Wilting consistently combined with yellow leaves, root rot and stunted growth are common symptoms of infected plants. Thirty-one fungal isolates were recovered from symptomatic lettuce plants and identified based on morphological and molecular characteristics (rDNA internal transcribed spacer (ITS) and the translation elongation factor (TEF-1α) gene sequences) as F. oxysporum (51.6%), F. incarnatum-equiseti species complex [FIESC: F. compactum (16.1%) and F. sulawesiense (3.2%)], and Plectosphaerella cucumerina (29.0%). The composition of these fungi varied between locations, with co-infection by P. cucumerina and Fusarium spp. observed in lettuce plants from the same field. Pathogenicity tests showed that all isolates were pathogenic to lettuce, causing reduced seed germination, induced collar rot, and decreased seedling survival and mass. Notably, F. oxysporum isolates caused collar rot earlier, significantly reducing seedling survival and indicating faster disease progression than P. cucumerina and FIESC. Additionally, there was wide variation in virulence and specialization of isolates at different stages of seedling growth: P. cucumerina L0311 primarily reduced seed germination, while F. oxysporum L0911 and L0331 strongly inhibited seedling growth. These findings suggest a shift in the pathogenic roles of the isolates during seed germination and seedling growth, with potential variation continuing post-nursery. This work is the first to report on the population and virulence profile of fungi causing lettuce wilt in Vietnam, providing essential insights for quarantine measures, disease monitoring, and management strategies.