Fusarium oxysporum f. sp. lactucae Inoculum Dynamics and Dissemination in Greenhouse Lettuce Production Systems

Fusarium oxysporum f. sp. lactucae (Fol) poses a significant threat to lettuce production, particularly with the emergence of new variants. To investigate dissemination routes and soil inoculum dynamics, samples from seed, soil, irrigation water, nutrient solution, agricultural equipment and plant material were collected at commercial lettuce producers and nurseries in Belgium. Real‐time PCR assays targeting Fol races 1 and 4 were used for detection. Fol was frequently detected at lettuce producers in greenhouse soil, on equipment, in water samples and in nutrient solution, whereas no detection occurred at the nurseries. Using an enrichment protocol and real‐time PCR assays, Fol DNA was detected in 5 out of 14 seed batches analysed. A single attempt to isolate Fol from a DNA‐positive seed batch was not successful, while other, minor pathogens were isolated. Controlled experiments with potting soil artificially inoculated with Fol race 4 showed a short term decline in soil inoculum over time, accompanied by increasing root colonization and symptom development. A similar reduction in soil inoculum over time was observed during a field trial. Furthermore, field sampling revealed a markedly heterogeneous distribution of the soil inoculum, which aligned with observed disease hotspots. The results suggest that multiple pathways may contribute to local Fol persistence and spread. Continued attention to hygienic and phytosanitary measures as well as monitoring is necessary to support disease management in lettuce production systems.