Needles Turn Brown Branches Discolor
Soil problems - If Yews are planted in boggy areas or subjected to very wet winters and springs their needles will turn brown. This indicates that the soil is too wet for these plants and the root system is starting to die back. To minimize this problem, plant Yews on higher ground or a gently sloping site where drainage is better, or build raised beds for them.
Sometimes Yews begin to develop yellow needles on the tips of branches. This discoloration spreads, needles wilt, and the shrub dies several months later. Take a look at the roots. The bark of roots may be decayed. Research has determined that Yews do not tolerate soil that is very acid (pH 4.7 to 5.4), and this causes the environmental illness. Move affected shrubs to a location with more alkaline soil or correct the soil problem by sprinkling ground limestone on the soil over the shrub's root system to "sweeten" the soil and increase the pH to about 6.5.
Needles Turn Yellow Branches Die
Black Vine Weevils - Black vine weevils are a serious pest of Yews. Adults are brown beetles with long snouts. They have tear-shaped, hard-shelled bodies, averaging 3/8 inch long. They feed on leaves and bark, chewing out distinctive notches along the edges of needles. Weevil larvae, white grubs with brown heads, feed on Yew roots deep in the soil. These pests are hard to spot because they feed at night, living under the bark and debris on the ground by day.
Because these weevils "play dead" when disturbed, folding their legs and dropping off plants to the ground, they can be trapped. Gently beat the branches of the infested shrub and catch the startled insects when they fall onto a cloth spread below. Apply a sticky adhesive product to the stems of the shrub to prevent the adults from climbing up and eating the leaves.
Cottony Tufts on Foliage and Stems
Mealybugs - Mealybugs are 3/8 inch long, oval, flattened, covered with white waxy powder and adorned with short, soft spines around their sides. A heavy infestation will cover the trunk and branches of Yews, the young having over wintered in bark crevices. After hatching in late May, they crawl up the trunk to the leaves, where they suck plant sap by attacking Yew foliage. Sometimes root mealybugs feed on Yew roots. The characteristic cottony tufts where needles join branches are their egg sacs. Severely infested plants tend to look unsightly, do not grow well, and may die. Honeydew secretions from the insects' feeding encourage mold growth on foliage and attract ants and fungi.
Scale Insects - Several types of scale insects infest Yews. Some scale pests appear as somewhat convex reddish brown soft bumps on the needles where they join the stems. The first sign of a scale attack is often discoloration of the upper leaf surface, followed by leaf drop, reduced growth, and stunted plants. Upon close inspection telltale bumps are visible. Some species excrete honeydew, which coats foliage and encourages ants and sooty mold growth. Heavy infestations of this pest kill plants.