Pine Wilt Information

Pine Wilt Killing Pines

 Mark Harrell, Forest Health Program Leader, Nebraska Forest Service

A disease called pine wilt is killing hundreds of pines every year in the eastern and south central parts of Nebraska.  About 95% of the trees killed by the disease are Scotch pines, and most of the remaining 5% killed are Austrian pines.  Most native pine species, as well as other conifers like spruces and firs, are resistant to the disease and are only rarely killed by it.  But many pine species not native to North America are susceptible.  Japanese red pine, Japanese black pine, and Luchu pine in Asia, and Maritime pine in Europe are highly susceptible to pine wilt.  Other pines not native to North America may be susceptible also, but not enough information is known about them at this time to be sure.  Until we have seen other non-native pines survive for many years in areas with pine wilt, we should consider them potentially susceptible to the disease.

Pine wilt is caused by the pinewood nematode, a microscopic worm-like organism that lives in the wood of declining and dying pines and other conifers.  As the nematodes feed, they interrupt the flow of water in the tree, causing the tree to dry up and die.  In a susceptible tree, the disease is fatal, and the tree typically dies within two to three months after becoming infested by the nematode.  The first visible symptoms of pine wilt are the needles turning grayish green then straw brown in color, and the needles may remain on the tree for a year or more.  Some individual branches may show the symptoms first, or the tree may change color uniformly, but in either case the tree usually turns completely brown within just a few weeks.  If branches are cut from the tree, the cut surfaces are typically not sticky to the touch and may feel dry.  Samples of wood from a diseased tree that are checked by a diagnostician will usually contain large numbers of nematodes.

The pinewood nematode is carried from diseased trees to healthy trees by an insect called the pine sawyer beetle.  The larva (immature stage) of the beetle is a borer that tunnels in the wood of dying and dead trees.  When the larva matures and becomes an adult, nematodes in the wood near the insect move onto the body of the beetle.  As the beetle chews its way out of the tree and flies to a healthy tree to feed, it carries the nematodes with it.  Thousands of nematodes may be carried by a single beetle.

For the first few weeks after leaving a dead tree, adult beetles feed on the bark of young shoots of healthy pines and other conifers.  As a beetle feeds, nematodes leave the beetle and enter the tree through the feeding wound.  If the tree is susceptible, the nematodes reproduce, spread throughout the tree, and the tree begins to die.  As a tree dies from pine wilt, it becomes attractive to pine sawyer beetles as a suitable tree for laying eggs.  When the eggs hatch, the larvae tunnel into the nematode-infested wood and begin the next cycle of the disease.

In Nebraska, hundreds of pines are dying every year from pine wilt generally south of a line that passes through Norfolk and Kearney.  Many of the trees that have been killed have had significant historical or landscape value, such as some over 100 years old in Nebraska City.  Elsewhere in Nebraska, isolated cases of pine wilt have occurred in or around North Platte, Tryon, Halsey, and Valentine, and the nematode has been found as far west as Chadron.  Overall, the nematode has been found over most of North America, which suggests the nematode is native to North America and is not an introduced pest.    

Control of pine wilt is difficult, and current strategies involve mostly trying to slow its spread.  Trees dying from the disease should be destroyed as soon as possible to prevent the beetles from emerging and carrying nematodes to nearby healthy trees.  This can be done by burning, burying, or chipping the dead or dying trees.  Wood from dead trees should not be stored or transported to other areas, because beetles can continue to develop and emerge from the cut logs.  If a tree is discovered with pine wilt during the spring or summer, the tree should be destroyed quickly because only a month may pass from when the browning begins to when the beetles begin to emerge.  If diseased trees are not discovered until after September 30, they should be destroyed before the end of April, because the beetles begin emerging in May.  Since pinewood nematodes can also be found in trees that have died from other causes, all dying Scotch pines should be removed and destroyed regardless of the cause of death.

Trees of all ages can be killed by pine wilt, but the disease is much less common in trees less than 10 year old.  For this reason, Scotch pines can be usually be used successfully as Christmas trees, but the trees need to be monitored regularly, and those that die need to be removed and destroyed quickly.  Since Scotch pines become more susceptible to the disease after about 10 years, new Scotch pines planted in areas where the disease is common may live 10 to 20 years but are not likely to reach maturity.

The only chemical treatment available for pine wilt that has been found to provide some protection against the disease is a trunk injection of abamectin (Greyhound, Pinetect).  This treatment provides some protection but will not always prevent pine wilt from killing a tree.  The cost of the treatment often makes it suitable only for high value trees.

For more information about pine wilt, see the publication: Pine Wilt – A Fatal Disease of Exotic Pines in the Midwest online at www.extension.iastate.edu/Publications/SUL9.pdf