What Causes Brown Spots on Arborvitae?

Identification

• A number of trees from the cypress family including arborvitae are prone to the phomopsis blight fungal disease caused by Phomopsis juniperovora. New foliage is more prone to the disorder. The disease is less frequently seen on established plants.

Favorable Conditions

• The fungal spores causing the disease are most active during extended periods of wet and warm weather and during periods of high humidity. High rates of infection occur between April to June and during late August and September. The dry periods of summer do not favor the spread of infection. Early spring rains spread the infection throughout the infected plant and to other plants.

Damage

• Phomopsis blight appears as yellow to brown spots on the tips of young foliage. Sunken, gray-colored lesions or cankers on the edges of the diseased tissue follow these spots. As the infection progresses, small, black spots containing the fruiting bodies of fungus are visible in the cankers. The disease kills the smaller sized stems measuring 1/3 inch or less in diameter. Larger branches are less likely to be affected. The fungal spores start to ooze out of the fruiting bodies in the form of tendrils during very warm, humid weather.

Control

• Buy resistant varieties and healthy plants in order to minimize chances of disease. Plant trees in well-ventilated areas and avoid the use of overhead sprinklers as the prolonged moistures on foliage increases susceptibility to infection. Chemical control options include the use of the fungicide benomyl. Apply fungicide as a preventative treatment one to two weeks prior to the new growth period in early spring.