Twig Blight of Juniper

Twig Blight of Juniper

Symptoms include the dieback of the tips of branches that are equal to or smaller in diameter than a pencil. Infected foliage becomes pale, then reddish brown, then turns brown or ash-gray after death. To confirm your diagnosis of Phomopsis as the cause of twig blight, scrape away the thin bark until you reach living wood. There should be a sharp line between discolored dead wood and healthy wood. Also, look at the base of the ash-gray scale needles for small, black fruiting bodies. The fruiting bodies form the spores that cause new infections. In wet weather, spores ooze from the fruiting bodies and splash onto new foliage. The spores are produced year round and infect yellowish green juvenile foliage. The darker green mature foliage is resistant to infection.

Junipers generally have two flushes of growth under natural conditions, one in spring and another in summer. When new growth coincides with wet, warm weather or frequent overhead sprinkler irrigation, disease can become severe. Pruning, which stimulates the formation of new growth, can increase the severity of the disease by providing flushes of juvenile foliage susceptible to infection. Most severely diseased plantings are the result of excessive wetness, usually from being planted in poorly drained, shaded and overwatered areas. Under continuously wet conditions, an entire plant may die.

Two other fungi can cause twig blight in juniper: Cercospora sequoiae var. juniperi and Kabatina juniperi Schn. & v. Arx. The distribution and severity of the two diseases is not yet known. The symptoms are identical for the Phomopsis and Kabatina twig blights, but the Cercospora twig blight is different.

The Cercospora infections start on the oldest foliage on lower branches and the blight spreads upward. The branch tips usually remain healthy and green. Early symptoms occur on the leaves of the spur branches. The leaves brown at the tip, then become entirely brown and die. This foliage usually dies in late summer and the affected spur branchlets fall off in the fall, leaving a plant with an inner crown devoid of foliage. On the infected brown leaves, small, dark fruiting bodies form and produce the spores that cause new infections of the previous years' spur leaves. Wet weather is needed for spores to infect foliage, and the longer foliage remains wet, the more disease develops. The spores are produced from spring through fall, but most infection occurs in summer in some years and in late summer other years.

Cercospora twig blight is most destructive on Juniperus scopulorum and J. monosperma and seldom causes problems in J. virginiana in the landscape. On nursery seedlings, the disease is usually seen only on the grafted selections.

Control: Avoid overwatering and planting in poorly drained or shaded locations. Plant resistant varieties and cultivars. Resistance to twig blight is influenced by the provenance. Thus, cultivars listed as resistant in other states may be susceptible in Michigan. Fungicides may be needed. To control Phomopsis twig blight, spray a fungicide at 10 to 14-day intervals when new flushes of growth occur until new growth has matured to a dark green color. To control Cercospora twig blight, apply a very persistent fungicide about late June or early July.

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