Pine Wilt Disease

Bursaphelenchus xylophilus

1. Identification

Pine wilt is a lethal disease that affects mainly Pinus species, characterised by rapid wilting, loss of needle colour, interruption of resin flow and sudden death of the tree. The causal agent is the nematode Bursaphelenchus xylophilus, transmitted mainly by insects of the genus Monochamus (cerambycid beetles), which act as vectors during feeding and oviposition.

2. Causal agent

Nematode of the genus Bursaphelenchus, with Bursaphelenchus xylophilus being the species responsible for the disease. The species was originally described by Steiner & Buhrer in 1934, and was later redescribed and reclassified by Nickle in 1970. This nematode colonises the resin canals and the xylem, causing water blockage and rapid death of the tree.

• General characteristics: include a short life cycle, high reproductive capacity, dispersive forms associated with the vector (Monochamus spp.) and rapid multiplication in woody tissues and associated fungi. Its activity causes cavitation, loss of resin and failure of the conductive system
• Most cited species: Bursaphelenchus xylophilus is the only species associated with pine wilt; other species of the genus, such as Bursaphelenchus mucronatus, are morphologically similar but not pathogenic.

3. Main hosts

• Maritime pine (Pinus pinaster) – highly susceptible
• Stone pine (Pinus pinea) – moderate susceptibility
• Scots pine (Pinus sylvestris) – susceptible
• Other Pinus species may be affected
• Trees weakened by drought, fire, pests or recent cuttings present greater risk

4. Symptoms

• Crown: shows loss of needle colour, which change from green to greyish-green and later reddish-brown, accompanied by progressive wilting and generalised drying of the crown.
• Branches and trunk: present reduction or absence of resinous exudation, drying of the branches, loss of flexibility and, in advanced stages, rapid death of the conductive tissues.
• Wood: shows a drier and more opaque colouration, loss of resin, cavitation of the resin canals and interruption of the water flow, without presenting the marked discolourations typical of vascular fungi.
• Other signs: presence of emergence holes and galleries of Monochamus spp., which act as vectors; fine sawdust associated with larval activity may be observed.

5. Disease cycle

• The nematode survives in infected wood and inside Monochamus spp. adults.
• During the feeding of the insects on young branches, the dispersive nematodes are transmitted to healthy trees.
• Inside the tree, they multiply rapidly in the resin canals and xylem.
• The obstruction of the vessels and cavitation lead to water failure and death of the tree within weeks.
• Dead or recently felled trees serve as a breeding site for Monochamus, perpetuating the cycle.

6. Favourable conditions

• High temperatures (above 25–30 °C) that accelerate the nematode's cycle
• Trees weakened by drought, fires or mechanical damage
• High density of vectors (Monochamus spp.)
• Presence of dead, cut or decomposing wood that favours the reproduction of the insect
• Periods of prolonged water stress

7. Disease management

• Cultural: include the removal and destruction of symptomatic or dead trees, sanitary felling in infested areas, the elimination of dead or recently cut wood that may serve as a breeding site for Monochamus, and the adoption of management practices that reduce water stress and increase the vitality of the stands.
• Vector management: is based on the monitoring of Monochamus spp. populations, the use of attractive traps when recommended, compliance with legal periods for cutting and transporting wood and reduction of the insect's breeding foci.
• Prevention: involves avoiding wounds on the trunk, complying with phytosanitary standards for the movement of wood, maintaining stands with adequate densities and promoting practices that increase the resilience of the trees to environmental stress.

Note: there is no effective curative treatment for infected trees; management depends on prevention, vector control and rapid elimination of affected trees.

Bibliographic references

• EPPO (European and Mediterranean Plant Protection Organization). PM 7/4 (3) Bursaphelenchus xylophilus. EPPO Diagnostic Protocol.
• Sousa, E., Naves, P., Bonifácio, L., Rodrigues, J. M. (2015). Pine Wilt Disease: A Threat to European Forests. EFI Technical Report, European Forest Institute.
• Mota, M. M., & Vieira, P. (2008). The Pinewood Nematode, Bursaphelenchus xylophilus. In: Pine Wilt Disease: A Worldwide Threat to Forest Ecosystems. Springer.
• Rodrigues, J. M., & Fonseca, L. (2019). Pine Wilt Disease in Europe: Current Situation and Research Advances. Forest Systems.
• EFSA Panel on Plant Health (PLH). (2013–2021). Scientific Opinions on Bursaphelenchus xylophilus and Pine Wilt Disease. European Food Safety Authority.
• Naves, P., Sousa, E., Rodrigues, J. M. (2017). Monochamus galloprovincialis and the Spread of Pine Wilt Disease in Europe. Forestry Review.
• Vieira, P., & Mota, M. (2013). Biology and Epidemiology of Pine Wilt Disease in Europe. European Journal of Plant Pathology.