Extension Educator, Horticulture
Nematodes are threadlike microscopic round worms. Pinewood nematode causes a fatal disease of pines called pine wilt. This is one interesting nematode.
It rides from pine to pine inside the breathing tubes of different varieties of sawyer beetles. The nematode enters the tree through the feeding wounds made by the beetle. Once inside the tree, the nematodes reproduce rapidly. The tree's water conducting vessels become clogged and the tree quickly dies.
Scotch pine is highly susceptible to pine wilt. The disease can also be found in Austrian and Jack pines. Pine wilt is much less common in red and mugho pines. The beetle is attracted to trees over ten years old. Fifteen to twenty year old Scotch pines that die quickly are particularly suspect.
This disease causes a sudden decline and eventual death of the entire tree within a few weeks or months after the initial symptoms appear, according to Nancy Pataky in the U of I Extension Home, Yard and Garden Pest Newsletter. Most pine fatalities due to pine wilt occur from now to late fall and in spring.
A change in needle color may be the first indication of problems. The needles go from a light grayish green, to yellowish green, yellowish brown and finally completely brown. The color change may appear first on a single branch in the top of the tree or perhaps the entire tree. The brown needles will often stay on the tree for long periods. This disease affects the entire branch except in Austrian pine which can show a tip dieback.
There are other reasons why pines might die or decline in health – root damage from construction or water-saturated soils or needle blights. Any dead pines should be removed quickly. To find out if a pine tree has pine wilt, take a sample to the University of Illinois Plant Clinic at 1402 West St. Mary's Road in Urbana. The fee is $18.50. Send in branch samples that are one to two inches in diameter and at least six to eight inches long. Wrap in aluminum foil and be sure to include information such as age of tree, what kind of pine, when symptoms were first noticed, date of sampling, name and address of owner and any information on the planting site such as type of soil or low area.
The samples should come from branches with brown needles still attached. The Plant Clinic will be closing for the season September 15.
Sanitation is the most effective control for this disease. There are no known effective chemical controls for the disease, the nematode or the beetle. Affected trees should be burned or buried quickly in order to reduce spread.
Adult beetles emerge from dead trees in June and July with their nematode cargo. Beetles will then feed on twigs of healthy pines to repeat the disease cycle.
Trees should be removed to the ground line or deeper. Wood should not be saved for firewood. It can be chipped for mulch, although it should be composted for a few months or spread out to dry before using around pines.
Many factors such as mature size, site adaptability and winter hardiness should always be considered when selecting a tree. However, possibilities to replace dead pines are Norway spruce, white spruce, blue spruce, hemlock, douglasfir or cedar trees which are not susceptible to the disease. Pine wilt is rare in white pines, however white pines are having other problems with Illinois soils and temperatures.
Keeping trees healthy with proper fertilization, mulching and watering can help to prevent disease. Also diversify your landscape. Don't rely on one type of any tree even if it is listed as insect and disease free.