According to Extension and Outreach at Iowa State University, the most common cause of brown needles on conifer trees is known as winter browning, which can result if the trees have not stored adequate amounts of water in the fall to meet requirements throughout the winter. Photosynthesis continues throughout the winter, a process that requires water. Winter browning is often most visible on the south and west sides of conifers.
Winter browning most often develops in late winter or early spring. Cold temperatures and rapid temperature changes can increase the risk of winter browning. Prevent drought stress from affecting conifers by extra watering of the trees during the dry stretch of late summer and early fall. This watering schedule is much more effective in preventing winter browning than one heavy watering just before freezing temperatures in the late fall.
Another cause of brown conifers for evergreens growing near roads is road salt used to combat icy driving conditions in the winter. Some infectious evergreen diseases can also result in brown conifers. Some of the common evergreen needle diseases are Rhizosphaera needle cast in spruce trees, Dothistroma needle blight in Austrian pines and Diplodia tip blight in Austrian and Scots pine trees.