Pine Wilt (Pinewood Nematode) of Needled Evergreens
Click for larger image Dieback of scotch pine (Pinus) due to pine wilt disease (pinewood nematodes).

The pinewood nematode, Bursaphelenchus xylophilus, kills conifers, especially pines, of many species. This pest is endemic throughout the eastern half of the United States. Scotch pine (Scots pine) is the most commonly affected host. The nematode is transmitted by sawyer beetles, a wood-boring bark beetle, through their feeding wounds into the resin canals. Here, the nematode will reproduce and hinder the movement of water through water-conducting tissues, eventually causing the plant to wilt. Locally destructive outbreaks have occurred in forest and landscape plantings of non-native pine species in Missouri and Illinois.

Symptoms and Diagnosis

Early symptoms of pine wilt are often inconspicuous or slight, giving way to a rapid decline. The first visible symptoms usually include fading green color and/or slight yellowing of limbs. This may be evident on one or a few branches or may develop on all simultaneously. In addition, within 48 hours after infection, there will be a visible loss of resin flow as observed on a cut twig. Trees often die so rapidly that brown needles continue to cling to the twigs. Diseased pines commonly die in early summer or early fall; some species require 2 seasons. Confirmation of the disease is done by microscopic detection of the pathogenic nematode in an examination of infested wood samples from the main trunk or affected branches. For diagnostic purposes, branch samples should be not be less that one inch in diameter and collected from freshly killed areas where needles are brown, but still attached.

Life Cycle

Once a susceptible host plant has been inoculated with the pinewood nematode via bark beetle feeding, the nematode will feed and multiply in the resin canals. Populations of 1,000 nematodes per gram of wood may occur when foliar symptoms appear. When plant stress factors, such as high temperatures, drought, or attack by other pathogens, increase, nematode populations will rise. After the host plant dies, the nematode will transform into a non-feeding form and this stage can be picked up by wood-feeding bark beetle larvae. This dispersive stage of the nematode will migrate to the pupal chambers of the bark beetle, then enter the bodies of newly formed bark beetle adults that will seek other tree hosts, and repeat the nematodes’ life cycle.

Integrated Pest Management Strategies

1. Removal. Quick removal of an infected tree is important in order to break the nematodes’ life cycle. Infested wood should be burned or buried (not chipped and used as mulch) to prevent reproduction and overwintering of the bark beetles in infested wood. Periods of high temperatures in summer favor epidemics, as do periods of severe drought, but little else is known of factors that favor or prevent epidemics of pine wilt.

2. Select resistant varieties. Consistent with geography and climate, select evergreen trees that are more resistant to the pinewood nematode. These would include eastern hemlock; blue, Norway and Serbian spruce; and Jeffrey, pitch, Virginia, shortleaf, loblolly, and Eastern white pine. Avoid the more susceptible species such as Scotch pine (Scots pine), Austrian pine, Japanese black pine, Japanese red pine, jack pine, mugo pine, and long-leaf pine.

3. Maintain healthy trees. While even a healthy tree may become infested with the pinewood nematode, stressed trees are more attractive to bark beetle feeding. Watering during extended drought periods, periodic light fertilization, and avoiding damage to the root system through construction or traffic will all reduce the chances of borer infestation and nematode transmission.

NOTE: Since the bark beetles are active throughout the growing season, the use of insecticides does not offer a practical means for controlling the spread of the pinewood nematode in residential areas.

Organic Strategies

Strategies 1 and 2 are strictly organic approaches. Using an appropriate organic fertilizer would be a viable organic approach to Strategy 3.

More images:

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Core samples showing blue stain characteristic of pine wilt on pine (Pinus) caused by pinewood nematodes
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Pinewood nematodes in great numbers clog the vascular tissue of a pine (Pinus) with pine wilt disease
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Silvery stylet inside anterior of a pinewood nematode, which causes pine wilt disease in pines (Pinus)
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Rapidly browning pine (Pinus) with pine wilt disease caused by pinewood nematodes
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