by W.R. Jacobi and N. Tisserat* (2/14
- Pine wilt is a lethal disease caused by a native nematode (Bursaphelenchus xylophilus), vectored to trees by a wood borer insect–the pine sawyer beetle (Monochamus spp.).
- Exotic pines including Scots, Austrian, and mugo are susceptible to infection by the nematode. Native pines are not susceptible to this pathogen.
- High summer temperatures are required for the nematode to develop in the beetle and within infested trees.
- The disease is now a threat in both eastern and western Colorado. It can be prevented by timely sanitation and chemical injections.
The pinewood nematode (Bursaphelenchus xylophilus) (PWN) is native to North America and is found in dying or dead pines where it primarily feeds on blue stain fungi; thus the PWN does not cause tree mortality in its native hosts. However, when introduced into exotic pines, the PWN systemically colonizes the vascular system and causes a fatal wilt disease. The PWN was initially observed killing exotic pines in Missouri in the late 1970s, but its distribution expanded west into Kansas and Nebraska during the past three decades. It was first detected in several Colorado communities along the Front Range in 2006 and in Grand Junction in 2012. The disease is now a potential threat in many regions of the state where extended periods of high early summer temperatures occur.
Pine wilt is a lethal disease of the widely planted Scots, Austrian and mugo pines. Native ponderosa, limber, bristlecone, whitebark and lodgepole pines, as well as spruce and firs, are not susceptible to the disease. These native conifers have resistance to the nematode most likely because they have evolved in association with each other but that resistance can be compromised by periods of drought.
Infection and colonization of pines by the PWN starts in June or July but observable symptoms don’t normally appear until late summer or fall. Lack of water, resulting from dysfunction of the water-conducting system, causes the needles to initially turn grayish turn from gray-green to tan and eventually brown. Dead needles remain attached to the tree through the winter. On Scots pine the entire tree usually wilts and dies within a few months, whereas wilting on Austrian pines may at first be restricted to a portion of the tree and the wilting process may be prolonged. When cut, diseased wood is very dry to the touch as the infection process interferes with resin production. In contrast, copious resin flow is often associated with trees damaged by physical injury or insect attack. Trees killed by pine wilt also develop blue stain in the wood. However, blue stain is not always diagnostic because staining may be found in pines killed by other factors and trees without blue stain may contain large populations of the PWN.
Field Diagnostic Techniques
Pine wilt symptoms can easily be confused with other mortality agents in Colorado’s pines. Pine wilt may be the cause if the following are noted:
- Rapid change in needle color from green to gray-green to brown or light tan, beginning in late summer
- Wood from recently affected trees is very dry, brittle and lacks any resin accumulation when cut. No evidence of exit holes, sawdust, or pitch tubes associated with bark beetles or wood borers in branches or the trunk
- The wood is discolored by blue stain fungi
- Mechanical or chemical injury has been ruled out as a possible cause
The only certain way to diagnose pine wilt is to submit a wood sample to a diagnostic lab for extraction and confirmation of the presence of PWN. Diagnosticians usually require for analysis a one inch (2.5 cm) thick sample of wood from the trunk (pie slice, wafer, etc.) or a wilted branch that is 6 inches (15 cm) long and 2-3 inches (5-7.5 cm) in diameter taken nearest the trunk. Because the distribution of the PWN in a tree can be non-uniform, it is best to collect at least two samples from different parts of the tree.
The PWN is transmitted by the adult beetles of several species of wood borers, called pine sawyers or long horned beetles (Monochamus spp., Cerambycidae). These large insects are attracted to weakened or dying trees, or recently cut logs, where they mate and lay eggs. The beetles will deposit eggs only on dying or stressed trees or logs with the bark attached. The beetle larvae hatch within a week and initially feed in the bark but then tunnel into the wood where they pupate. The PWN larvae, introduced by infested female beetles when they are laying eggs, invade the thoracic spiracles and tracheae of the young adults in numbers as high as 289,000. Adult beetles, contaminated with the PWN, emerge from the tree during the summer, leaving a round, ¼- inch-diameter exit hole. These adults feed briefly on the bark of young, healthy pine shoots (called maturation feeding) before mating and finding suitable trees for laying eggs. During the maturation feeding process, the beetles create small wounds on twigs that allow the PWN to move from the insect and enter healthy susceptible Scots, Austrian and mugo pines.
Figure 1. The pinewood nematode has a non-parasitic phase in which it feeds on blue stain fungi in stressed, dying or dead trees (depicted by yellow trees). This is the primary life cycle of the nematode on native pines and also occurs in exotic pines. The nematode may be introduced into small twigs on healthy trees during maturation feeding by the pine sawyer beetle. In exotic pines, the nematode then multiplies systemically inside the tree to enormous numbers, and results in a lethal wilt. This process does not occur in native pines and no disease develops.
Figure 2a. Two pine sawyer beetles common in Colorado may transmit the pinewood nematode. Spotted pine sawyer (Monochamus clamator).
Figure 2b. The white spotted sawyer beetle (Monochamus scutellatus).
Once the nematodes are introduced into susceptible trees, they feed on the epithelial cells of resin ducts and move systemically throughout the sapwood of the branches, trunk, and roots, causing physical and physiological changes within the tree resulting in water blockage, wilting and eventual death. Vascular dysfunction can occur rapidly under dry conditions or with mean summer temperatures above 68°F (20°C). Disease occurrence is expected to be more common during hot summers.
The PWN has an egg, four larval stages and an adult stage. The sequence of egg to adult takes only 4 to 5 days under favorable conditions of adequate wood moisture, temperature and nutrient availability. The development of the nematode switches to a dispersal phase after tree death and occurs only in the presence of Monochamus beetle pupae within the wood. These third-stage larvae aggregate on the wall of the pine sawyer pupal chamber in the xylem and enter the respiratory system of the young adult beetle. Larvae can molt into adults within 48 hours after transmission to a conifer host.
The fungal-feeding phase (mycophagous phase) of the pinewood nematode life cycle occurs in freshly cut softwood and in dead and dying conifers. In North America, this is the most common phase and is usually the result of secondary transmission during egg-laying by pine sawyer beetles infested with the pinewood nematode. The pinewood nematodes feed on blue-stain fungi and other fungi that typically invade cut timber or dead and dying trees.
Trees killed by pine wilt in late summer or fall should be removed and destroyed before the emergence of pine sawyers in late May of the following year.
Figure 3. An adult male pinewood nematode. Note the spear-like projection called a stylet in the head (bottom of picture) and the distinctive stirrup-shaped structure called the spicule near the tail.
Figure 4. Symptoms of pine wilt include needle discoloration and rapid wilting in late summer or early fall.
Figure 5. Cross section of a Scots pine showing blue stain. Blue stain fungi often develop in wood of trees with pine wilt.
Do not leave stumps above ground level and do not save infested wood for firewood or transport infested logs.
Diseased trees should be chipped, burned or buried. There is minimal risk in moving the nematode in wood chips; therefore chips can be used for landscaping purposes.
Because adult pine sawyers infested with PWN may be attracted to any freshly cut pine logs, even native pine logs, and the PWN can potentially persist and multiply in these cut logs by feeding on blue stain fungi, do not store any fresh pine wood (firewood, logs, etc.) near exotic pines.
The incidence of pine wilt will increase in the future if the current, warmer climate trend continues. Therefore, limit future planting of the highly susceptible Austrian and Scots pines, especially in areas where it is impractical or too expensive to preventively protect trees with chemical injections. There currently are no selections of these pine species that are resistant. Deciduous trees or any of Colorado’s native conifers can be safely planted as replacements at sites where exotic pines have been killed from pine wilt.
Two nematicidal avermectin compounds (abamectin and emamectin benzoate) are labeled for the prevention of pine wilt. These products are directed towards killing/immobilizing the nematode and not for killing the pine sawyer beetle vector. They must be injected into the pine, usually at pressures ranging from 30-50 psi (2-3.4 atmospheres), prior to infection. They are not effective if the tree is symptomatic or once the PWN has colonized the tree. Several commercial injection systems are available, but pine injections are almost always done by professional arborists. Injections can be made anytime the weather is warm (to allow for uptake and translocation) although late fall is perhaps the ideal time. Injections should be completed by early April and before pine sawyer beetle emergence. Yearly injections provide the greatest protection, but the cost and potential damage associated with the injection process are issues to consider. Very good control has also been achieved by injecting every other year.
The use of foliar insecticide applications to prevent pine wilt has not been researched but it has been suggested that this treatment would kill pine sawyer beetles during the maturation feeding process, thus preventing introduction of the PWN into the tree. However, insect mortality would not likely be instantaneous, and any movement of the PWN into feeding wounds, regardless of whether the pine sawyer beetle was ultimately killed, could result in pine wilt development. Furthermore, many of the insecticides used in foliar applications may not be toxic to the nematodes. Because there are no data to support this type of treatment, foliar applications are not currently recommended for pine wilt control.
*W.R. Jacobi, Colorado State University Extension specialist and professor, department of bioagricultural sciences and pest management; N. Tisserat, Extension specialist and professor. 2/14
Colorado State University, U.S. Department of Agriculture, and Colorado counties cooperating. CSU Extension programs are available to all without discrimination. No endorsement of products mentioned is intended nor is criticism implied of products not mentioned.
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