Tree Services Magazine - October, 2008

FEATURES

Juniper Tree Diseases

By John C. Fech

Photos courtesy of the University of Nebraska unless otherwise noted.

Junipers and cedars are a standard of many landscapes. While some are rugged and rough in appearance, others add beauty and grace to the landscape through color, form and texture. Unfortunately, this group of woody plants is subject to many maladies, including various infectious diseases.

Most junipers have three types of foliage, although the differences can be subtle. Recognition of these differences can be helpful when determining which fungal organism is responsible for injury. The sharply pointed terminals of juvenile needles are commonly found on seedlings and new growth while spur needles comprise the bulk of the older tissues throughout the tree. Spur needles are flattened, blunt and clasping in arrangement and commonly seen on short branches. Whip foliage, characterized by long shoot growth, also occurs on the ends of branches.

Cercospora blight in a windbreak. Cedar-apple rust on fruit.

Cercospora needle blight

The symptoms of Cercospora needle blight are usually seen on the older foliage of cedars and junipers. Initial infection will begin on the spur needles of previous years’ growth in the interior framework of the tree in midsummer. As the disease progresses, all of the needles of the spurs become infected, turn brown and die by the end of the growing season. Once dead, they fall from the tree and produce the characteristic appearance of thin to bare inner foliage, while the branch tips have healthy green foliage. Young, juvenile shoots and whip foliage are somewhat resistant to the infection.

Close inspection of the dead or dying shoots will reveal the fruiting bodies of the fungus. They are released from the tree over a long period during the growing season, especially during wet weather. Moisture is necessary for dispersal, germination and penetration of the fungus into the foliage.  

Control of Cercospora needle blight is possible through fungicide application during the infection period. An initial application in early summer followed by a second application a month later is usually adequate for control. Fungicide products containing mancozeb or Bordeaux mixture have been shown to be effective in controlling this malady. Camelot, a fungicide containing copper salts of fatty and rosin acids, is a third option. Heavily infected trees may need to be sprayed two years in a row.

Cedar-apple rust.

Phomopsis twig blight

Unlike Cercospora needle blight, Phomopsis twig blight expresses itself with dead shoot tips and healthy older growth. This disease can be especially damaging to new seedlings and liner stock in nurseries. In the landscape, it causes major concerns for clients who want their trees to be entirely green with no blemishes.

As you may imagine, not only is Phomopsis twig blight aesthetically unappealing, it can cause a distortion of growth, as well as damage to the crown of the affected trees. Once they become infected, the needles become dotted with small yellowish spots. Soon afterwards, the fungus invades the stem tissue, which often results in death of the young shoots through girdling. Later, infected stems change from light green to reddish-brown to ashen grey. Within three to four weeks after infection, fungal fruiting bodies develop on the needles and stems. They are visible to the naked eye, taking the form of small, black, pimple-like structures on the host tissue.

Phomopsis twig blight is one of the many diseases whose spread is enhanced by rain splash and cool, moist spring weather. Because the spores need only seven to eight hours to germinate, penetrate the epidermis of the needle and initiate infection, the disease can be transferred easily to nearby trees.

Control of twig blight should be directed towards preventing infection through applying fungicides prior to a flush of growth from branch tips and pruning to remove infected tissue. In general, most junipers have two flushes of growth: one mid-spring and one in late summer or early fall. To time fungicide applications properly, schedule them to coincide with growth flushes. Propiconazole, mancozeb and thiophanate-methyl are the recommended control products. Removal of the infected branches and tips will reduce the overall amount of inoculum and the likelihood of infecting nearby trees, as well as reinfection of the existing tree. After removal, remove and discard branch and tip tissues. Leaving them in the landscape could facilitate spread of the fungus.

Encourage clients to avoid overhead irrigation of cedars and junipers. If necessary, apply irrigation water in the landscape only in the morning hours. Doing so will allow the foliage to dry quickly and reduce the potential for infection. If new plantings are planned, consult with nurseries regarding disease-resistant cultivars. Many have been reported to offer some degree of resistance to infection.

Photo courtesy of Dan S./www.stock.xchng.com.
Apples and crabapples often require several fungicide applications to prevent damage

Cedar-apple rust

Perhaps the showiest of all foliage diseases, cedar-apple rust requires two hosts to persist in the landscape. Cedars and junipers are the host to large, orange, gelatinous fruiting structures that hang from the branches. During the spring, spores from the orange structures are dispersed through rain splash to the second host, crabapple or apple trees.

After landing on the leaves of the second host, the spores germinate and penetrate the leaf surface. Cedar-apple rust produces fruiting structures on the apple/crabapple also, however they are much smaller than on the juniper/cedar, about the size of a pencil eraser. These structures also release spores, which can travel back to the cedar tree to complete the life cycle. Fruiting structures are produced on both leaves and fruit.

Fortunately for your clients with junipers and cedars, cedar-apple rust is rarely damaging. Trees can be heavily adorned with fruiting structures year after year and remain unscathed. Crabapples and apples are not as resistant however. Most cultivars exhibit some degree of susceptibility, and suffer more aesthetic and physiological damage from the fungus.

In cool, moist springs, cedar-apple rust can cause enough infection on susceptible cultivars to cause significant defoliation. The consequence of losing a large number of leaves is not only a lack of beauty and curb appeal in the landscape; the lack of foliage also reduces the photosynthetic capacity of the tree, causing it to transfer an inadequate volume of carbohydrates and sugars to the roots. Control measures are usually

Control measures are usually not necessary on junipers and cedars, yet apples and crabapples often require several fungicide applications to prevent damage. A spray program consisting of three to five applications, starting at petal drop and at intervals of seven to 14 days, can provide a satisfactory degree of control. Many products are registered for control including mancozeb, myclobutanil, propiconazole, thiophanatemethyl and chlorothalonil. Be sure to read and follow all label instructions.

John C. Fech is a horticulturist, certified arborist and frequent contributor located in Omaha, Neb. Frederick P. Baxendale is a professor and extension entomologist with the University of Nebraska-Lincoln.