White pine is a very time-consuming plant, at least from the perspective of a diagnostician. Questions about white pine outnumber those about most other host plants at this time of year. Still, the number of actual infectious disease problems on white pine is low. We do not see problems with needle blights or needle casts on this species in Illinois. It is rare for Sphaeropsis to infect stems and needles. We often state that white pines do not host pinewood nematodes. In actuality the Plant Clinic staff has identified the nematode on white pine twice in 24 years but only on trees that were already dead. And, although this species is obviously susceptible to white pine blister rust, that disease is rare in Illinois. We do see it every few years on white pines in northern Illinois. Given these facts, why are so many white pines in Illinois dying or growing poorly?
We have addressed this problem in past years and will probably continue to address it as long as white pines remain a large part of our landscapes. Symptoms we have observed include pale needles, an apparently sudden decline of the tree, death of trees in the midst of other healthy white pines, spongy bark, and no signs of any pathogens or insects. In fact, many homeowners report that nothing has changed in their established landscape that might cause tree decline.
In all cases where we have been able to get to the roots, we find no new white root tips, few root hairs, and a cortex that easily pulls off the roots. The root systems can be shaken and easily drop the soil in the root ball. The trees are dying because the roots are dying. As far as we have been able to determine in the lab, this is not a primary root rot disease problem.
What root rots affect white pine? Armillaria, Phytophthora, and Procera are the major fungal pathogens of pine roots in Illinois. Armillaria forms white mycelial mats on the roots and develops shoestringlike structures that grow under the bark on the trunk. The distinct signs of the fungus make it easy to identify. Both Phytophthora and Procera are common in very wet sites and cause a soft, dark rot of the roots. Phytophthora can be a primary pathogen, but Procera is generally considered a stress pathogen. We find each of these pathogens from time to time at the Plant Clinic, but the vast majority of white pines that we see are not infected with a root rot pathogen.
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, although they grow with intermittent success in Illinois. Many of the problem trees we have seen have been situated on clay sites or exposed to the elements (planted in new housing developments or used as windbreaks). It is also likely that site stress has contributed to the decline of these trees. The excessive rains of the past several springs also may have contributed to root injury and decline by saturating the soil and causing a lack of soil oxygen. If roots were injured as we are suggesting, they will not be able to absorb enough water in drought-stress situations.
Watering helps, as does the use of a natural mulch over the root system; but, without adequate root mass, plants will not be able to use the available water quickly enough to replace what is used by the foliage. The result will be sudden browning of foliage or off-color needles and death of branches. Because these problems in white pine are not usually the result of an infectious disease, immediate removal of the tree is not necessary. Instead, try to keep the tree watered and see how it responds. Also, try digging into a bit of the root system for a better picture of the situation. If roots are brown in cross-section and the outer layer easily pulls off or is not present, then root injury has occurred. If the roots are white and healthy, then the problem is above ground.
One other factor that may be involved in Illinois is the pH of the soil. Our soils have a fairly high pH level, whereas pines prefer more acidic soils. It may be helpful to fertilize with an acidic fertilizer specifically packaged for pines or acid-loving plants. Follow the directions so as not to burn the roots by applying too much fertilizer.