Extension Communications
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915
9/27/99
Contacts:
Mark Vitosh, Forestry Extension, (515) 294-6739, mvitosh@iastate.edu
Elaine Edwards, Extension Communication Systems, (515)
294-5168, eedwards@iastate.edu
Yard and Garden column for the Week Beginning Oct. 1
What's Wrong With Scotch Pines?
By Mark Vitosh
Extension Forestry Specialist
Iowa State University Extension
Scotch pine (Pinus sylvestris ), also known as Scots pine, has been planted for Christmas trees, windbreaks, wildlife habitat, and ornamental purposes for many years in Iowa. Identification characteristics of this tree include two slightly twisted needles (1-4 inches long) in a bundle, and in the top portion of the tree the bark of the branches and trunk have an orange color. In recent years, this species can be identified by its browning needles and quick death. In general, this type of quick death (within a few weeks to a year) is appearing on Scotch pine over 10 years of age throughout most of the state. Why are these trees suddenly dying?
In Iowa, there appear to be a number of factors and organisms that are having an impact on the decline of Scotch pine. One key factor that makes this tree less adaptable to Iowa's unpredictable growing conditions is that it is not a native plant. Since the late 1980s there have been a number weather extremes such as drought and flood conditions, and these types of extremes can stress plants to a point that makes them more susceptible to insect and disease problems.
There are a number of disease and insect pests that can attack Scotch pine, but the two that appear to be causing the most problems in Iowa include a small bark beetle (Ips spp.) and a wormlike critter called a nematode (Bursaphelenchus xylophilus). Bark beetles in general attack stressed and weakened trees. Masses of pitch (sap) coming from the bark, small holes in a scattered pattern in the bark, and sawdust on the bark and ground are indications of beetle attack. By the time these signs are visible the tree can not be saved.
The disease caused by the nematode is called pine wilt. There are a number of different susceptible conifers, but this problem is most commonly found on Scotch pine. The spread of the nematode is accomplished with the help of the pine sawyer beetle (Monochamus spp.), which is a wood-boring beetle. The female beetle lays its eggs under the bark of dying pines where nematodes already exist. When new adult beetles emerge from these dead trees their breathing tubes are full of little microscopic nematodes ready to jump off at the next pine tree the beetle visits. New adult beetles feed on young shoots of healthy trees, and the nematode enters twigs through the feeding wounds.
Once the nematodes enter susceptible species such as Scotch pine they quickly reproduce and begin to plug the water-conducting system of the tree. As the tree begins to lose its ability to move water, the needles on the branches begin to turn a mix of yellow and brown before they die. In some cases the branches brown one by one and in others browning occurs uniformly throughout the tree. In either case, susceptible trees with the nematode often die within one to three months.
Currently in Iowa it has not been determined which of these two pests is actually causing most of the death on Scotch pine. The bottom line for this situation is that there are no feasible controls currently for either of these problems. Consequently, Scotch pine should not be planted in Iowa for any use other than Christmas trees. Injecting trees with insecticides, fungicides, and fertilizers has not been proven to be effective, so these treatments are not recommended. It is very frustrating to stand by and watch these trees die, but at this time the best management method available is sanitation (tree removal).
Remove dead and dying trees immediately and destroy or chip all removed material. All material converted to chips should be composted for 6 weeks or more before use. When applying mulch keep it a few inches away from the trunk, and avoid using this chipped material around conifers. Reduce stress on remaining pines by avoiding site changes such as compaction and grade changes, and expand mulched areas around these trees when possible. Watering trees during extended dry periods may also be helpful.
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