CORVALLIS, Ore. – Two major fungal diseases are responsible for killing ornamental junipers, and summer is when damage is most obvious. Oregon State University Extension plant pathologists advise how to deal with both diseases.
The two major diseases in landscape junipers (Juniperus spp.) are Phomopsis twig blight and Phytophthora root rot, said Melodie Putnam, diagnostic plant pathologist at the OSU Plant Clinic. Insects can also inflict damage.
"The two diseases are fairly easy to tell apart,” Putnam said. “Phomopsis twig blight only affects new growth (small twigs), while Phytophthora takes out whole branches.”
Phomopsis is a fungus that infects only immature foliage; older foliage is resistant. Infection usually occurs in the spring when there is abundant moisture from spring rains, and when the young foliage is expanding. Infection can also occur later in the year if humidity is high and the plants have been pruned, which stimulates new growth.
Phomopsis forms a canker that restricts the flow of water to the branch tip, resulting in death of anything beyond the canker. An early indication of infection is an off color to the foliage. It looks paler green than normal and somewhat faded. Fading will rapidly progress to browning, especially if the weather turns warm. Only the tips of the branches will be affected, perhaps the last six inches.
To help control twig blight, prune out and burn blight-affected twigs and branches. Avoid wounding an infected juniper's twigs. Also avoid irrigating in the evening and overhead watering. Do not over fertilize. Space plants far enough apart for good air circulation. Look for resistant varieties to plant.
In contrast, Phytophthora root rot affects entire juniper branches via the root system, Putnam said. "Wet weather fosters outbreaks. Initially there will be an off-color appearance to affected branches. They will gradually fade in color from green into the red-brown hues."
Branches "fed" by diseased roots will show the symptoms first. The entire branch will be affected. With time, more and more branches will show symptoms, until eventually the entire shrub will die in a mass of brown foliage.
To avoid Phytophthora root rot in junipers, improve drainage where junipers are grown. Remove and destroy any infected plants to avoid spreading the disease to new ones. Contact your local county agent of the OSU Extension Service for recommended chemical controls in your area: http://extension.oregonstate.edu/find-us
Also be alert for insect damage in ornamental junipers, Putnam warned. Cypress tip moths and related insects bore into leaves, causing them to turn brown and die. Affected tissue is only about an inch or so long. Damage by these insects is easily distinguished from the twig blight or root rot by the small holes in the brown leaves and twigs, easier to see when using magnification, and by the presence of "frass," or insect droppings.