Pine Wilt Disease Fact Sheet No. 2.915 Gardening Series|Diseases by W.R. Jacobi and N. Tisserat* The pinewood nematode turn from gray-green to tan and eventually Quick Facts (Bursaphelenchus xylophilus) (PWN) is native brown. Dead needles remain attached to to North America and is found in dying or the tree through the winter. On Scots pine • Pine wilt is a lethal disease dead pines where it primarily feeds on blue the entire tree usually wilts and dies within caused by a native nematode stain fungi; thus the PWN does not cause a few months, whereas wilting on Austrian (Bursaphelenchus xylophilus), tree mortality in its native hosts. However, pines may at first be restricted to a portion vectored to trees by a wood when introduced into exotic pines, the of the tree and the wilting process may be borer insect–the pine sawyer PWN systemically colonizes the vascular prolonged. When cut, diseased wood is very beetle (Monochamus spp.). system and causes a fatal wilt disease. The dry to the touch as the infection process PWN was initially observed killing exotic interferes with resin production. In contrast, • Exotic pines including Scots, pines in Missouri in the late 1970s, but its copious resin flow is often associated with Austrian, and mugo are distribution expanded west into Kansas and trees damaged by physical injury or insect susceptible to infection by Nebraska during the past three decades. attack. Trees killed by pine wilt also develop the nematode. Native pines It was first detected in several Colorado blue stain in the wood. However, blue stain is are not susceptible to this communities along the Front Range in 2006 not always diagnostic because staining may pathogen. and in Grand Junction in 2012. The disease be found in pines killed by other factors and is now a potential threat in many regions of trees without blue stain may contain large • High summer temperatures the state where extended periods of high early populations of the PWN. are required for the nematode summer temperatures occur. to develop in the beetle and Field Diagnostic Techniques within infested trees. Hosts Pine wilt symptoms can easily be • The disease is now a threat Pine wilt is a lethal disease of the widely confused with other mortality agents in in both eastern and western planted Scots, Austrian and mugo pines. Colorado’s pines. Pine wilt may be the cause Colorado. It can be prevented Native ponderosa, limber, bristlecone, if the following are noted: by timely sanitation and whitebark and lodgepole pines, as well as • Rapid change in needle color from chemical injections. spruce and firs, are not susceptible to the green to gray-green to brown or light disease. These native conifers have resistance tan, beginning in late summer to the nematode most likely because they • Wood from recently affected trees is have evolved in association with each other very dry, brittle and lacks any resin but that resistance can be compromised by accumulation when cut. No evidence periods of drought. of exit holes, sawdust, or pitch tubes associated with bark beetles or wood borers in branches or the trunk Symptoms • The wood is discolored by blue stain Infection and colonization of pines by the fungi PWN starts in June or July but observable • Mechanical or chemical injury has symptoms don’t normally appear until late been ruled out as a possible cause summer or fall. Lack of water, resulting from dysfunction of the water-conducting system, causes the needles to initially turn grayish- Diagnosis green. As the disease progresses, needles The only certain way to diagnose pine wilt ©Colorado State University is to submit a wood sample to a diagnostic Extension. 2/14. lab for extraction and confirmation of the www.ext.colostate.edu *W.R. Jacobi, Colorado State University Extension presence of PWN. Diagnosticians usually specialist and professor, department of bioagricultural require for analysis a one inch (2.5 cm) thick sciences and pest management; N. Tisserat, Extension sample of wood from the trunk (pie slice, specialist and professor. 2/2014 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. 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 Figure 1. The pinewood nematode has a non-parasitic phase in which it feeds on blue stain fungi in adult takes only 4 to 5 days under favorable stressed, dying or dead trees (depicted by yellow trees). This is the primary life cycle of the nematode conditions of adequate wood moisture, on native pines and also occurs in exotic pines. The nematode may be introduced into small twigs on temperature and nutrient availability. The 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 development of the nematode switches to a does not occur in native pines and no disease develops. dispersal phase after tree death and occurs only in the presence of Monochamus beetle pupae within the wood. These third-stage wafer, etc.) or a wilted branch that is 6 larvae aggregate on the wall of the pine inches (15 cm) long and 2-3 inches (5-7.5 sawyer pupal chamber in the xylem and cm) in diameter taken nearest the trunk. enter the respiratory system of the young Because the distribution of the PWN in a adult beetle. Larvae can molt into adults tree can be non-uniform, it is best to collect within 48 hours after transmission to a at least two samples from different parts of conifer host. the tree. The fungal-feeding phase (mycophagous phase) of the pinewood nematode life cycle Disease Cycle occurs in freshly cut softwood and in dead and dying conifers. In North America, this The PWN is transmitted by the adult is the most common phase and is usually beetles of several species of wood borers, the result of secondary transmission called pine sawyers or long horned beetles during egg-laying by pine sawyer beetles Figure 2a. Two pine sawyer beetles common in (Monochamus spp., Cerambycidae). These Colorado may transmit the pinewood nematode. infested with the pinewood nematode. The Spotted pine sawyer (Monochamus clamator). large insects are attracted to weakened or pinewood nematodes feed on blue-stain dying trees, or recently cut logs, where they fungi and other fungi that typically invade mate and lay eggs. The beetles will deposit cut timber or dead and dying trees. 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 Management bark but then tunnel into the wood where they pupate. The PWN larvae, introduced Sanitation by infested female beetles when they are Trees killed by pine wilt in late summer laying eggs, invade the thoracic spiracles or fall should be removed and destroyed and tracheae of the young adults in before the emergence of pine sawyers in late numbers as high as 289,000. Adult beetles, May of the following year. contaminated with the PWN, emerge Do not leave stumps above ground level from the tree during the summer, leaving Figure 2b. The white spotted sawyer beetle and do not save infested wood for firewood (Monochamus scutellatus). a round, ¼- inch-diameter exit hole. These or transport infested logs. adults feed briefly on the bark of young, Diseased trees should be chipped, pine, usually at pressures ranging from burned or buried. There is minimal 30-50 psi (2-3.4 atmospheres), prior to risk in moving the nematode in wood infection. They are not effective if the tree chips; therefore chips can be used for is symptomatic or once the PWN has landscaping purposes. colonized the tree. Several commercial Because adult pine sawyers infested injection systems are available, but pine with PWN may be attracted to any freshly injections are almost always done by cut pine logs, even native pine logs, and the professional arborists. Injections can PWN can potentially persist and multiply be made anytime the weather is warm in these cut logs by feeding on blue stain (to allow for uptake and translocation) fungi, do not store any fresh pine wood although late fall is perhaps the ideal (firewood, logs, etc.) near exotic pines. time. Injections should be completed by early April and before pine sawyer beetle Figure 3. An adult male pinewood nematode. Note the spear-like projection called a stylet Tree Selection emergence. Yearly injections provide in the head (bottom of picture) and the The incidence of pine wilt will increase the greatest protection, but the cost and distinctive stirrup-shaped structure called the spicule near the tail. in the future if the current, warmer climate potential damage associated with the trend continues. Therefore, limit future injection process are issues to consider. planting of the highly susceptible Austrian Very good control has also been achieved and Scots pines, especially in areas where by injecting every other year.