Extension Educator, Horticulture
We all have our driving distractions - cell phones, CD's, shiny objects. For me it's desolately doomed trees or delightfully divine trees. For better or worse I respond to each case with equal enthusiasm.
My 35 mile-an-hour diagnosis for declining Scotch pines is pine wilt. Pine wilt disease is caused by the microscopic round worms, pinewood nematodes. The nematodes ride from pine to pine in sawyer beetles. The nematode enters the tree through the beetles' feeding. 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 especially as they get to fifteen to twenty years old. The disease can also be found in Austrian and Jack pines and less commonly in red and mugho pines. White pines are resistant.
Sanitation is the most effective control for pine wilt. There are no known effective chemical controls for the disease, the nematode or the beetle. Affected trees should be burned or buried quickly before beetles emerge in June and July in order to reduce spread.
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.
So if it's not pine wilt, what's happening to white pines? As reported by Nancy Pataky, U of I plant clinic director, in a recent Home, Yard and Garden Pest Newsletter, white pines are most likely suffering from white pine decline. Pataky states decline involves many stress factors working together to cause tree decline and often tree death. Usually an infectious agent is not involved or involved only as an added stress.
Reported symptoms of white pine decline include pale, yellow or browning needles, shriveled bark on branches or trunk, sap on branches, and in some cases quick death of the tree. Old and young white pines are affected.
White pine decline may appear as a sudden decline of one tree in the midst of other healthy white pines. In fact, many homeowners report that nothing has changed in their established landscape that might cause tree decline.
In all cases where Pataky has been able to get to the roots of struggling trees, she finds no new white root tips, few root hairs, and an outer cortex that easily pulls off the roots. The trees are dying because the roots are dying. However, as far as Pataky has been able to determine in the lab, this is not a primary root rot infectious disease problem.
As Pataky reports much of the white pine problem appears to involve the site, the environment, and species requirements. White pines are understory trees that thrive in the cool, moist, well-drained soils of Wisconsin. Many of the problem trees have been situated on clay sites or exposed to the elements (planted in new housing developments or used as windbreaks). Planting too deep or extremes in temperature or moisture may have contributed to root injury.
Since the problem does not appear to be infectious, immediate removal of the still struggling white pine is not necessary. Care for white pines by watering during dry periods and mulching with 3-4 inches of wood mulch. However, keep in mind trees with reduced root systems will not be able to use the available water quickly enough to replace what is used by the foliage. Browning of foliage and death of branches may still occur. Fertilizing in spring or fall with an acidic fertilizer may also help.
For more information on evergreen insects and diseases http://www.urbanext.uiuc.edu/focus/index_evergreen.html