English boxwood (Buxus sempervirens) is a popular garden and hedge shrub, but it can be plagued by a host of fungal diseases, insect pests and root problems if not given proper care. Learning to distinguish symptoms of common boxwood diseases and protecting your boxwood from weather or pest damage will ensure a long life for these formal shrubs.
English boxwood can be damaged by a host of insects. Spider mites strike most often in summer and cause boxwood leaves to yellow. Boxwood psyllids curl up in boxwood leaves, causing the leaves to cup. Boxwood leafminer causes plant leaves to appear blistered as young larvae feed on the leaves. Ring, lesion, root knot or boxwood spiral nematodes can damage boxwood at the roots. Shrubs with a severe nematode infection display leaf browning and dieback of branches.
If your boxwood receives high levels of winter sun, frost or wind, its leaves can turn yellow to orange. To guard against frost damage, erect a burlap barrier. When spring returns, new growth will be green in color but the frost-damaged parts of the boxwood will remain red or brown. Prune away damaged growth.
Boxwood may experience root rot when planted in poor-draining soil. Gardeners may not notice root rot until it has advanced, since the initial symptoms of root browning and softening occur underground. Above ground, boxwood shrubs will stop growing, tips of branches will die back, and leaves will turn yellow or brown. There is no cure for root rot once it has advanced, aside from improving the soil drainage.
Boxwood decline causes plants to slowly decline and eventually die. This disease only affects English boxwood and predominantly hits shrubs that are 20 years or older. The foliage on affected shrubs changes from green to light yellow, then tan. Shrub stems and twigs change color, and underground the roots of the boxwood decay.
To prevent boxwood diseases, plant shrubs in soil that drains well. Water your plants only when necessary, but make sure the watering is deep. Frequent, shallow watering is not effective. Do not plant boxwood in an area where previous boxwood plants have died, since the soil may still be contaminated. Treat pest-infested boxwood with an insecticide that targets that pest or apply a soil fumigant for nematodes.