We have recently received several samples in the Plant Disease Clinic from yews that are turning brown. Yews are typically very hardy plants, and are not susceptible to many diseases. However, several stress factors can cause yews to turn brown.
Yews don't like "wet feet" and can develop root problems if their roots are kept too wet. Although considered relatively drought-tolerant, too little water can also cause problems. Choosing an appropriate, well-drained site for the yew and watering during very dry periods is the best defense against these problems.
Winter damage also can affect yews. Winter injury occurs as a result of rapidly changing temperatures during the winter, bright sunshine, and inadequate water reserves in the root system of the plant. Plants usually show the first symptoms of winter injury in late winter through spring, and browning is most pronounced on the south and west sides of the plants. Although foliage turns brown, if buds remain green and viable, the plant may recover as the spring progresses.
Yews are quite sensitive to deicing salts used on roadways and sidewalks. Plants that have been affected by these salts typically turn brown starting from the side closest the area salted. Symptoms usually first appear in the spring. When salts have washed into the soil under a yew, leaching the soil with a large amount of water may help.
Wounds to the bark of branches can also cause portions of yews to turn brown. Such wounds can be caused by animals or inadvertent injury by people. Yews are not very tolerant of wounding. To diagnose this injury, the base of the plant must be examined carefully.
When yews turn brown, their site and recent history should be reviewed to determine a cause. Yews affected by salts or winter damage may recover over time, and it is wise to not prune out the brown tissue immediately in case the branch tips are still viable. When planted in a proper site and cared for, yews can be a valuable addition to the landscape.