Pines grow in most every state of the U.S., and are planted for many reasons. They offer year-round color, protect homes from wind and snow, subtle fragrance, harborage for wildlife and a great backdrop to help show off ornamentals planted in front of them. Unfortunately, they are susceptible to several maladies.
Without question, pine wilt is one of the most devastating diseases of the recent decade. Pine wilt has the capacity to completely kill a tree within two years. If that wasn’t bad enough, even more frustrating to property owners is the deceiving nature of the disease. In most cases, a tree can be healthy looking in April and May, start looking a bit off-color in June, and be entirely brown by July. All pines can become infected, but most cases involve Scots or jack pine.
Pine wilt is similar to Dutch elm disease in that it is carried to the tree by an insect. The carrier for elms is the elm bark beetle. The culprit for pines is the pine sawyer beetle, which carries the actual killer, the pinewood nematode. Once infected, the sap flow throughout the tree declines rapidly, causing death. It typically affects trees that are more than 10 years old.
The pinewood nematode is transferred throughout the disease cycle in two phases. Phase one is relatively straightforward. The sawyer beetles feed on young shoots of healthy trees. Then, the nematodes that are in the bodies of the beetles enter the pine tree through feeding wounds in twigs. Once inside the tree, the nematodes multiply and clog the resin canals, which quickly results in tree death.
Phase two begins when the sawyer beetles lay their eggs in the bark of dead or dying trees. The larvae develop, bore inward and begin feeding on blue stain fungi that have been transmitted by bark beetles that were attracted to the dead trees. After feeding, the nematodes move to the sawyer beetle pupae and are eventually carried along when they develop into adult beetles, after which phase two is complete, and the process starts over again with phase one.
Various approaches have been examined to control pine wilt, including insecticide injections and topical protective sprays. A satisfactory degree of control can be achieved with an injection of abamectin, or Greyhound insecticide. The injection must be made before the onset of symptoms and will generally protect the tree for two growing seasons. Be sure to follow all label directions.
A moderate degree of control can be achieved in a stand of pines if infected pines are removed as soon as possible after the onset of symptoms. To be certain that the nematode is not transferred by the sawyer beetle, it is recommended to burn, bury or chip the logs of the infested tree. Using the trees for firewood is not prudent, as the beetles can continue to emerge from the logs over time.
Sphaeropsis tip blight
Formerly known as Diplodia tip blight, this disease causes the new shoots to die before extending fully. Of course, this is a serious outcome, because unlike deciduous trees, all future growth to sustain the tree comes from the apical meristem at the ends of the branches. If an insect, disease or adverse environmental condition causes an oak or beech tree terminal to die, new growth will sprout from lateral buds and can usually be directed to replace the damage. The long-needled pines, such as Austrian and ponderosa pine, are most susceptible.
The first recognizable symptom is a dotting of brown throughout the silhouette of the tree. Closer inspection reveals that the fungus has killed the new shoots. The disease can be further affirmed by pulling the needles loose from the killed shoot. If infected, they can be removed with a gentle tug. Look closely at the base of the removed needle; several small, black spores will be present if Sphaeropsis is involved. Secondly, inspect a cone from the damaged tree, either fallen or attached. Again, small, black spores are likely to be present on the outer scales of the cones. Trees that have been infected for several years are likely to contain several branches that are entirely dead.
Sphaeropsis blight can be controlled by applying cover sprays of copper sulfate, propaconazole and thiophanate-methyl in mid-spring. Thorough coverage of the needles is required. If the disease has heavily infected the tree, consider two applications of fungicide, applied two weeks apart.
Dothistroma needle blight
A disease that is equally as problematic in aesthetic terms as the first two maladies is Dothistroma needle blight. Infected trees often appear wind burned or scorched from extreme heat. All pines can become infected, but Scots and Austrian are most susceptible.
As the name would indicate, the symptoms begin with discoloration of the needles. The blighting takes two forms:
1. Small, olive brown to dark brown markings that extend the circumference of the needles, encircling them as if the needle was wearing a thin, flat wedding band.
2. Needles that are brown, starting in the middle and extending to the tip. Some needles are completely brown.
Because the disease does not typically kill the buds, it is generally less worrisome than pine wilt or Sphaeropsis tip blight. However, a pine depends on its needles to photosynthesize and send sugars and carbohydrates throughout the rest of the plant. The more brown needles that a tree has, the less chance it has to remain a healthy part of the landscape.
Control Dothistroma needle blight in much the same as for Sphaeropsis tip blight. Because the disease overwinters on old needles, thorough coverage is required to prevent the disease from spreading. Bordeaux mixture and copper sulfate can be used. Be sure to follow all label directions.