When nematodes take flight: Multi-scale dispersal of the beech leaf disease nematode in forest ecosystems

Beech leaf disease, caused by the invasive foliar nematode Litylenchus crenatae, is an emerging threat to Fagus grandifolia across North America. Since its initial detection in 2012, the disease has spread to a total of 16 U.S. states and the Canadian Province of Ontario; however, the mechanisms underlying nematode dispersal remain poorly understood. This study evaluated multiple potential dispersal pathways across two forest sites in central Pennsylvania through integrated field surveys, environmental monitoring, and experimental validation. Weekly sampling was conducted to characterize nematode population dynamics in relation to temperature, precipitation, wind, and humidity. Long distance water-mediated dispersal was assessed through creek sampling, while biotic dispersal was investigated through field sampling from migratory bird feet during high-moisture events and a controlled greenhouse transmission experiment. Wind dispersal was further examined in a pilot study using unmanned aerial vehicle-assisted rainwater collection during precipitation events. Nematode presence was detected across all investigated pathways, supporting dispersal via water flow, rain splash, and biotic vectors. Funnel-based sampling recovered Litylenchus crenatae up to 90 m from infected trees, demonstrating movement beyond the immediate canopy area. Confirmed downstream transport of nematodes suggests waterways may facilitate long-distance dispersal across forested landscapes. Litylenchus crenatae was recovered from the feet of migratory birds, and greenhouse experiments confirmed that bird-associated nematodes could infect healthy beech seedlings, providing evidence for avian-mediated transmission. Desiccation assays demonstrated that live Litylenchus crenatae specimens could survive up to 10 days in the absence of available moisture, indicating that the nematodes may remain viable during transport on bird feet or exposed surfaces. In addition, unmanned aerial vehicle-assisted rainwater sampling detected Litylenchus crenatae within canopy-associated precipitation samples, providing additional evidence for short-distance aerial dispersal during wind and rain events. Together, these findings demonstrate that beech leaf disease spread is driven by multiple interacting abiotic and biotic dispersal mechanisms operating across spatial scales.