Birch trees display colorful and textural bark qualities that are unusual among tree species. Some varieties have reddish and yellowish bark, but even more highly valued are the white-barked types. The U.S. Department of Agriculture Forest Service notes the two most pervasive insect pests of birch trees are the birch leafminer and the bronze birch borer. Birches also succumb to diseases that infect foliage, branches and trunks.
Birch leafminers are the larvae of tiny wasps called black sawflies. Adult wasps lay eggs in leaf litter on the ground that hatch into foliage-eating larvae. As they feed, they mine through birch leaves, creating pale tunnels. Eventually, tunnels turn brown and dead leaves often drop from trees. The University of California recommends planting birch species that are less susceptible to leafminers, such as river birch (B. nigra) and European white birch (B. pendula). Even though birch leafminers don’t kill trees, they detract from their appearance and they weaken their resistance to invasion from the bronze birch borer.
Unlike the birch leafminer, bronze birch borers can kill trees. These borers are beetle larvae that burrow underneath bark and feed on vascular tissue, which is responsible for nutrient transport in trees. Yellowing leaves at tree tops in late summer are usually the first symptom of borer invasion, because they are furthest from roots, which uptake nutrients. There are no chemical controls that kill feeding larvae, according to the Maryland Cooperative Extension, and affected branches must be pruned immediately to control the spread of these pests.
Fungal diseases from various pathogens cause leaf spots, blotches and blisters on birch trees. Cylindrosporium and Septoria cause small spots with no borders and Colletotrichum causes larger brown spots with darker borders. Anthracnose diseases, caused by Marssonina and Discula pathogens, form angular blotches instead of round spots. Taphrina species cause blisters that may initially be yellow, red or green but eventually turn brown. The Maryland Cooperative Extension notes that birch leaf diseases on mature trees do not generally require chemical pesticides. On younger trees, preventive copper or sulfur fungicidal sprays at bud break in spring may help these diseases from gaining a stronghold.
Branch and Trunk Diseases
Dieback and canker diseases from pathogens such as Melanconium betulinum, Nectria galligena and Botryosphaeria species attack bark and cambium layers of birch trees. Diseased tissue enlarges and kills living tissue underneath bark. Knotty growths form on bark and may girdle stems, or the bark may split, revealing dead wood underneath. No chemical controls manage these diseases. Stressed trees succumb to diseases more easily, so it’s important to maintain tree health by watering and fertilizing plants properly. Infected branches need pruning back to healthy wood to try and stave off the spread of these diseases.
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