Assessing the Role of Asymptomatic Infected Trees in Pine Wilt Disease Spread in Japan—Insights from Tree Health Monitoring

To examine the role of asymptomatic infected trees in the spread of pine wilt disease (PWD), we established two study sites in a coastal black pine forest in 2020: one in a heavily damaged site and the other in a slightly damaged site. Half of the trees in each site were treated with a nematicide injection to suppress nematode activity. Tree health, assessed by resin exudation and external symptoms, was monitored for four years. In the slightly damaged site, asymptomatic infected trees emerged within 20 m of infected trees, and even with nematicide treatment, trees within 2 m of infected trees became asymptomatic infected. However, nematicide treatment allowed temporarily asymptomatic infected trees to survive or recover. These findings suggest that combining nematicide injection with the felling of neighboring infected trees can effectively suppress PWD.