ON OUTDOORS: WHITE PINES ARE DYING

Exotic Eastern white pine (Pinus strobus) is used in landscaping and windbreaks throughout much of Missouri. Its long, slender, soft needles give the tree a graceful look appropriate for many different settings. It has been considered a good choice for planting in our climate.

Recently, however, concern has been raised over reports of white pines exhibiting wilt, decline and unexpected death after 20 to 30 years of otherwise normal growth. The origin of this problem is not always clear and in most cases comes on with little or no warning. It should be noted that white pine is not a native of Missouri. To successfully put our finger on the problem will undoubtedly require an appreciation of this fact.

What follows is a brief discussion of some problems recently associated with white pine and recommendations for their management.

White pine root decline caused by a fungus has been positively diagnosed in a number of cases in Missouri. Symptoms include yellow, sparse and small leaves; uniform browning of needles; small twig growth; delayed bud break; and branch dieback. Trees up to 20 years old appear to be most commonly affected. Resin oozing from the base of the truck is typically visible for a few years prior to the onset of foliar symptoms.

The fungus has been described as an opportunistic one that causes most damage to plants stressed by adverse sites, root wounds, insects, polluted air, or other root pathogens. The disease is tightly linked to the prevailing site conditions. White pines become susceptible to the root disease when planted in wet areas, such as low spots in the landscape, heavily irrigated lawns, or along roadside ditches.

Pine wilt, caused be a nematode, has been isolated from white pine in at least two locations in Missouri. The needles initially turn gray-green, then to straw-colored brown. Resin flow is disrupted as the tree succumbs to the disease. Scattered branches may be infected initially, ultimately spreading to remaining branches. In severe cases, whole trees can turn brown within a few weeks. Pine wilt kills more Scots pine, the beetles carrying the nematodes responsible for this disease may move to a less-preferred host, namely white pine. The extent to which this may be occurring is still unknown. Drought conditions were the overriding forest health concern in Missouri in 2000. Dry weather patterns that began in July 1999 continued through the first half of 2000.

Above-normal temperatures and below-normal precipitation generally continued through the winter and spring of 2000. The winter of 2000 was the tenth warmest on record. It was followed by the driest April recorded in Missouri in 206 years. Many cases of mortality of single or small groups of trees were reported during 2000. Often no immediate cause was determined, but drought effects are suspected of having played a role. The relatively warm temperatures of the fall of 2000 and the winter of 2001 could be expected to cause increased moisture loss from evergreen foliage, thus increasing the already high drought stress. These compounding stress factors can be expected to contribute to white pine problems in the near future.

Chemical control of the above problems is not realistically possible. Regardless of what ultimately kills the white pine, removal and destruction of an infected tree is warranted because dead trees are attractive to bark-inhabiting insects that may affect surrounding trees. Try to determine the cause of decline or death before planting another white pine in the same spot.

When planting white pine, avoid low spots so that trees are not standing in water after heavy rains, or plant in an area that has good drainage. Avoid planting on sites that have a pH greater than 7.0 or where the pines are exposed to persistent splash from deicing salt. Make sure the trees have plenty of space for root expansion, and plant where they are not in deep shade. Use mulch around the tree to conserve moisture and reduce competition from weeds. In areas where trees are drought stressed also consider timely application of moisture. If the site is unsuitable for a white pine, consider alternative trees.

Joe Garvey is district forester with the Missouri Department of Conservation.