Not all browning and loss of conifer needles is caused by infectious diseases. Environmental and site stress can also cause discoloration and death of needles.
The wet conditions of 1993 caused root injury to many plant species, including conifers. Roots in saturated soils are deprived of the oxygen they need to grow and function. As a result of poor root health, discoloration of needles or dieback of large branches may occur. Trees do not necessarily need to have been in standing water to sustain injury. We have observed injury on trees in areas that are fairly well drained but have poor internal drainage, such as high-clay soils. White pine and concolor fir have been the most common samples arriving in the Plant Disease Clinic that show this type of injury. Although bud break this spring is an encouraging sign, it is difficult to assess the potential for tree recovery at this time. We basically have to "wait and see".
Winter desiccation is also a common cause of needle discoloration. Conifers, when subjected to the sun and winter winds, loose moisture. Because the ground is frozen, these trees cannot replace all the moisture that is lost. As a result, drying (desiccation) of tissue occurs. Needles appear yellow brown. Winter desiccation injury is often most severe on trees in exposed site or on the south and west sides of trees.
This article originally appeared in the May 4, 1994 issue, p. 62.