Pine Wilt Disease

Bursaphelenchus xylophilus (Steiner and Buhrer) (Nickle)

Host

• Pines, particularly Scotch pine

Damage Potential

• Moderate–severe

Symptoms and Signs

• Considerable decrease of resin flow from wounds
• Needles turn yellow, then reddish brown through the growing season (wilting)
• Needles may remain attached for several months following the sudden death of the tree
• The wood of affected trees dries out and completely lacks resin

Causes of Similar Symptoms

• Wood borers and bark beetles
• Diplodia tip blight
• Atropellis canker

Identification

This pest can only be identified using a microscope. Check dying trees for symptoms and, if pine wilt disease is suspected, have samples examined by a pest specialist to determine if nematodes are present in the wood.

Biology and Life Cycle

Pinewood nematodes spread from infected pines to healthy or stressed pines in the spring through contaminated pine sawyer beetles (Figure 1). These long-horned beetles acquire the nematode from infested trees during their development within the tree. When new adult beetles emerge in spring, they transfer nematodes to healthy trees through feeding or to diseased trees during egg-laying activities.

 The nematodes have two phases. The propagative phase occurs in the sapwood of infested trees and includes six stages from egg to adult. A single generation can be completed in as few as 5 days under ideal conditions. This enables the nematodes to rapidly develop extremely high populations (Figure 2). As they feed in the resin ducts and cambial tissues, the tree's water-conducting system fails, causing rapid wilt under dry conditions (Figure 3).

 The second phase of the nematode life cycle is the dispersal phase. This only occurs in the late stage of tree infection when pine sawyer pupae are present. Under these conditions, immature nematodes develop into a nonfeeding stage that attaches to the pupal cell of the beetle (Figure 4). When the adult beetles emerge, the nematodes will have contaminated their respiratory system and will be carried with them to new host trees. Nematodes leave the beetle and enter the shoots of a new tree through the wounds created by beetle-feeding activity. Within 48 hours of introduction to a new, healthy host, the nematodes have matured into reproducing adults.

Pinewood nematodes feed on epithelial cells and resin ducts of healthy trees. They also may feed on blue-stain fungi in dead and dying trees to sustain or build population levels (Figure 5). The fungus feeding cycle is the most common stage in North America.

Monitoring and Management Strategies

Plantation Establishment

• Avoid planting on traditionally dry sites.
• Do not plant susceptible pines in areas where mean summer temperatures are above 68°F.

Preseason

• No recommendations are available at this time.

Growing Season

• Maintain tree vigor through periodic fertilization and irrigation during times of dry weather.
• Any trees exhibiting symptoms should be analyzed by a diagnostic lab to determine the presence of the pine wilt nematode.
• At the end of the season, evaluate results and update records.

Control Options

Biological

• No recommendations are available at this time.

Mechanical

• Remove and destroy (burn, bury, and/or chip) trees to help prevent the spread of pests to nearby, healthy pines in the spring, before beetle emergence.

Biorational

• No recommendations are available at this time.

Chemical

• No recommendations are available at this time.

Next Crop/Prevention

• Buy and plant disease-free stock only.