While shrubs enhance the appearance of a landscape with varied color, texture and form, they are sometimes plagued by diseases that adversely affect their health. Most fungal or bacterial diseases vary from one species to another and do not kill infected shrubs. Timely and proper treatments are key to preventing spread. The best defense is to minimize environmental stresses and adopt proper cultural practices to strengthen the shrub so it resists disease.
Common shrub diseases include phomopsis blight, phytophthora canker, powdery mildew, anthracnose, phomopsis gall and verticillium wilt.
Phomopsis blight causes new shoots or tips to turn from light yellow to deep red and then ash gray before dieback in late spring or early summer. Phytophthora canker infects roots of shrubs, stunting growth and causing parts to wilt or die back. Removing the outer bark at the base of the shrub exposes cankers that emit a brown exudation. Powdery mildew causes infected leaves, fruit, buds and stems to display gray or white blotches that spread to cover the entire area if untreated, accompanied by bud deformation or dropping leaves in extreme cases. Symptoms of anthracnose include spots or patches on infected leaves in the summer, causing new growth to appear curled or deformed. Shrubs infected with phomopsis gall feature single or clustered round balls ranging in size from a pea to 1 inch. Verticillium wilt causes leaves of infected shrubs to fall off or curl, branches and stems to die back and xylem tissue to appear streaked.
Phomopsis blight infects savin juniper and Chinese juniper, while shrubs susceptible to phytophthora canker include Korean rhododendron, flame azalea and Japanese yew. Powdery mildew infects oakleaf hydrangea, late lilac and Amur honeysuckle, while anthracnose infects Amur and Japanese maples. Phomopsis gall destroys both early and border forsythia and flame azalea, while border privet, bottlebrush buckeye and American cranberry shrubs are susceptible to Verticillium wilt.
Shrub blights thrive in prolonged cool and wet periods. Plant susceptible shrub varieties in well-drained soils or improve drainage to control the disease, and prune out infected parts in dry weather. Sterilize pruning shears between cuts to avoid spread of disease. Because phytophthora is a soil-borne disease, effective control is achieved through improving soil drainage and irrigation practices. Spray shrubs infected with powdery mildew with a fungicide early in the growing season to prevent spread. Immediately remove and destroy or discard infected parts to control phomopsis gall. No chemical control method exists for controlling verticillium wilt. Fertilize and irrigate shrubs with symptoms of the disease and prune infected parts immediately.
Avoid powdery mildew infestations by planting shrubs in well-drained soils and avoiding overwatering or excess fertilizing. Immediately remove and discard fallen leaves from infected shrubs to prevent the spread of anthracnose. Prevent verticillium wilt from destroying precious shrubs by planting hardy ones naturally adapted to the climatic conditions of the area.