Evergreen shrubs are plants that retain most of their foliage, called needles, throughout the cold winter months. Gardeners commonly use evergreen shrubs as hedges, privacy screens and foundation plantings. The evergreen classification includes junipers, false cypress shrubs and cherry laurels. Evergreen shrubs are susceptible to a number of plant diseases.
Cytospora Canker of Spruce
Cytospora canker of spruce is a stem disease caused by a fungal pathogen (Cytospora kunzei) that typically attacks older evergreens. Initial symptoms include browning needles and dying lower branches. Higher branches show signs of damage as the disease progresses. Cankers frequently form at the bases of the affected branches. White, purple or amber resin patches occasionally appear on the bark around cankers. The canker disease pathogens spread by wind, rain or infected pruning tools. Control includes pruning and destroying affected branches.
Pine wilt is caused when pine wood nematodes (Bursaphelebchus xylophilus) invade xylem tissue. These nematodes feed on the plant tissue and reproduce in the resin canals. The nematodes breed rapidly, killing the tree by plugging the vascular tissue. Symptoms of pine wilt include fading and yellowing foliage, wilting needles, stunted plant growth and dead needles clinging to the branches. These pathogens typically spread by insects, particularly sawyer beetles. Affected trees should be removed immediately to avoid spreading this disease.
Many cedar and juniper shrubs contract the phomopsis blight diseases caused by a fungus (Phomopsis juniperovora). The blight pathogens first infect the new foliage, causing tip blight and stem lesions that often develop into cankers. Evergreens suffering from advanced stages of this disease sometimes form small black spots of fungal spores. These fungal spores spread by wind and rain and can survive in diseased tissue for up to two years. This evergreen disease is most prevalent during extended periods of warm, wet weather. Regular applications of fungicides during the growing season help to control phomopsis blight.
Cedar-hawthorn rust is a fungal disease (Gymnosporangium globosum) that affects cedars and junipers. The fungus first develops into tiny galls on the branches of its evergreen host. These growths develop on the tree’s needles and then move onto the twigs. The immature galls appear a reddish-brown color while the mature galls are scarred and gray-brown. Fully matured galls swell and sprout red-brown, jelly-like horns during rainy spring conditions. Severe rust problems often cause the foliage to yellow and prematurely drop. This rust disease is often controlled by applying fungicides before the spring bloom.
Diplodia Tip Blight
Diplodia tip blight is caused by a fungal pathogen (Sphaeropsis sapinea) that typically attacks the young needles. Early symptoms include brown or stunted needles that sometimes twist or curl. Resinous cankers often form on stems or needle bases. Older branches sometimes develop large, resinous cankers in wounds. This disease is often mistaken for drought or winter damage. Control requires the infected parts of the shrub to be removed.