Localized within- and between-tree variation in nematode distribution during latent state of pine wilt disease makes the disease status cryptic

The pinewood nematode (PWN), Bursaphelenchus xylophilus, is the causal agent of pine wilt disease, an epidemic disease that has severely damaged pine forests in East Asia. The disease has spread to northern areas in Asia and parts of Europe. To prevent disease spread as the forefront of damage prevention, a better understanding of infection status is highly important. Not all infected trees show disease symptoms, and such asymptomatic PWN-carrying trees are likely to be overlooked and can become a pathogen reservoir. To elucidate PWN infection status in asymptomatic trees, we performed PWN inspection of branches and trunks in 21 test trees in two different conditions: trees that had experienced PWN inoculation and those with suspected PWN infection that had experienced transient foliage discoloration. We detected PWNs in eight test trees (38%) and in 13 (1.5%) of a total of 843 samples. The difference in these percentages suggests that nematode inhabitation was highly localized within the trees, possibly owing to the restricted migration of PWNs. Our data demonstrated that trees that were once weakened but recovered their vigour can persist, as the asymptomatic carriers, in the forest. The implications for disease control are also discussed.