Pine wilt is a disease caused by a nematode, the pinewood nematode. The disease can kill a mature pine in one season and often does in Illinois. That problem was discussed in issue no. 5 of this newsletter. Pine wilt is found on almost all pine species grown in Illinois, except white pine. In the past 25 years, the Plant Clinic staff has recovered pinewood nematodes from white pines only once, and in that case the tree was already dead. It was speculated that the nematode had been moved to the tree by the Sawyer beetle after the tree was dead.
Many of you who grow white pines in nurseries or landscapes have experienced pine-wilt-like symptoms, but on your white pines. This condition is most likely white pine decline. A decline is a situation that 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. White pine decline frequently results in tree death.
White pine problems seem to be present throughout the state, especially in central and southern areas. Symptoms vary but generally include some pattern of needle yellowing or browning, shriveled bark on branches or trunk, sap exudate on branches, and in some cases death of the tree. Affected trees have ranged in size from 2 feet to more than 20 feet, indicating that trees of any could be affected.
Over the years, the Plant Clinic staff has assayed white pine samples for the presence of pinewood nematodes; has cultured for fungal pathogens of needles, stems, and roots; and has inspected for insect infestations or injuries. The only common factor seems to be root decline. Few live white roots have been found on affected trees, but fungal pathogens cannot be correlated with poor rooting. It appears that roots are on the decline for other reasons. Some possibilities include heat, drought, flooding, deep planting, and sudden extremes in temperature and moisture.
White pines are understory trees that thrive in the cool, moist, welldrained soils of Wisconsin although they grow with intermittent success in Illinois. Many of the problem trees we have seen in Illinois 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. Excessive rains may contribute to root injury and decline by saturating the soil and causing a lack of soil oxygen.
We have already seen more white pine problems this year. 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 natural mulch (such as shredded bark) 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 or offcolor 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 aboveground and our theory is wrong, at least in your case.
Another factor that may be involved in Illinois is the pH of the soil. Our landscape soils have a fairly high pH level, whereas pines prefer more acidic soils. It may be helpful to fertilize with an acid fertilizer specifically packaged for pines or acid-loving plants. Follow the directions so as not to burn the roots by applying too much fertilizer. Fertilization is usually recommended in early spring or late fall.