Phomopsis Twig Blight on an Arborvitae

Cause

• Phomopsis twig blight affects a number of members of the cypress family, including arborvitae. The fungal disease is caused by Phomopsis juniperovora. Other host plants of the fungus include a variety of junipers, eastern redcedar, fir trees and the European larch.

Symptoms

• Early symptoms of the disease start to appear during late spring and early summer with the dieback of new twigs. Affected new twigs, the primary sites of infection, change color from yellowish- green, to reddish-brown and finally to ash gray. Yellow spots are seen on twig bases. Older, mature needles are resistant to the disorder. With disease progression, the fungus move into shoots, girdling the stems measuring less than 1/3 inch in diameter.

Damage

• The fungal spores persist for extended periods in and around the tree causing repeat infections year after year. The needles and stems on affected plants brown and die. Left uncontrolled, the disease gradually kill the older trees. Favorable conditions of growth lead to the rapid decline of younger trees and shrubs. The fungus spreads most during extended periods of wet and moist weather conditions. Trees that are excessively fertilized are highly prone to infection.

Management

• Use resistant tree varieties and avoid planting in poorly drained soils. Prune and remove affected areas, taking care to prune during dry weather. Minimize the use of overhead irrigation as prolonged presence of moisture on foliage increases chances of infection. Start fungicidal treatment at the initial signs of disorder for best control. Treat trees as new growth appears and after pruning. Recommended fungicides include the use of copper-ased products, potassium bicarbonate or propiconazole.