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The Botanical Gardens at Asheville is a good source for information on native plants and the natural world in which we live.  Our knowledgeable staff and volunteers are eager to share their experience.  In addition our library is open to the public.  Also, excellent reference books are available for sale in our gift shop

My hemlocks are covered with a white powdery substance.  What is it? Is this dangerous to the trees? What should I do?

Your trees are probably infected with the Hemlock Woolly Adelgid, Adelges tsugae, which has been in the United States since 1924.  This introduced insect is a serious pest of eastern hemlock and Carolina hemlock.  

White cottony sacs at the base of the needles are good evidence of a hemlock woolly adelgid infestation.  These sacs resemble the tips of cotton swabs.  The sacs are present throughout the year, but are most prominent in early spring.

By sucking sap from the young twigs, the insect retards or prevents tree growth causing needles to discolor and drop prematurely.  The loss of new shoots and needles seriously impairs tree health.  Defoliation and tree death can occur within several years.

The most effective treatment for the hemlock woolly adelgid is to spray the affected trees by a skilled arborist.

Each year toward the end of summer I notice brown leaves on trees along roads and highways throughout Western North Carolina. Is this caused by pollution or car exhaust or some chemical sprayed along the right-of-way, or is it just early fall coloration?

The brown coloration you are observing is not caused by chemicals or pollution, nor is it a case of early fall coloration. What you are seeing is the damage caused by a tiny insect, the Locust Leaf Miner, which inhabits the leaves of Black Locust Trees (Robinia pseudo-acacia) in our area. This insect lives inside of Locust leaves and spends its time chewing long and winding tunnels though the juicy tissues between the upper and lower epidermis. The tunnels, of course, damage the delicate leaf tissue and affected portions of the leaves die and turn brown. 

Locust Leaf Miner damage is most noticeable along roadsides because Black Locust trees tend to predominate on highly disturbed sites, such as along highway right-of-ways. Black Locust is a member of the pea (legume) family of plants, a family noted for forming a partnership with nitrogen-fixing bacteria in the soil. The roots of Black Locust trees shelter colonies of bacteria which capture nitrogen from the atmosphere and incorporate it into organic molecules, which enrich the soil in the area surrounding the roots. Thus, Black Locust can thrive on sterile soils, such as bulldozed roadbeds from which all fertile topsoil has been removed. Many Appalachian highways are therefore lined with Black Locust trees, giving motorists a first hand view of Leaf Miner damage. 

Leaf Miners have been plaguing Black Locust trees in our area for at least the last quarter century, with no sign of letting up. So far, the Black Locust trees have tolerated this annual assault. Damage is most noticeable late in the growing season, from mid-July through September. Then the pale brown of damaged Locust leaves is eclipsed by the multi-hued leaves of an Appalachian autumn.

What are Invasives and why are they considered to be a problem?

Invasives are plants not native to the Southeastern United States, but have been accidentally or purposely introduced by people.  The majority of these introduced plants pose no threat, but many, without the natural controls that keep them in check in their native homelands, are taking over our landscape.  These alien invaders out compete and gradually displace our native plants.  This affects native wildlife and the overall health and stability of our environment.  Kudzu, for example, has consumed an estimated sever million acres in the South, replacing thousands of native species!  Land managers suggest you avoid planting the worst of the invaders in your garden.  

What is killing the Oak Trees?

Sudden Oak Death (SOD), ramorum leaf blight and ramorum dieback are all names for a recently-discovered plant disease caused by the fungus-like microorganism Phytophthora ramorum. This disease has killed tens of thousands of oak trees in the coastal areas of California and a small area of Oregon. The host list is broad and continues to expand. It includes common native and ornamental landscape plants like camellia, viburnum, pieris, rhododendron, pyracantha, mountain laurel, and leucothoe. SOD does not usually kill these non-oak hosts. Instead, depending on the plant, it may cause symptoms such as leaf spots, defoliation, twig and branch dieback, or blighting. These symptoms can be easily confused with those caused by other plant diseases or environmental stresses. Laboratory analyses are necessary to confirm the presence of SOD in all cases. 

Despite the name, Sudden Oak Death disease is not just restricted to oaks.  Camellias, rhododendrons, Pieris, mountain laurel, viburnum, and lilacs are among 60 different plant hosts or potential carriers of the disease.

Although Sudden Oak Death (SOD) is a forest disease, the organism that causes this disease is capable of infecting a large number of woody ornamental plants that are commonly sold by nurseries and planted into urban landscapes.  Only above ground plant parts are affected. The roots of infected plants remain healthy.

On oaks, the organism causes bleeding cankers on the trunk that can eventually girdle and kill the tree. On the majority of host plants, however, the disease causes leaf spots and twig dieback, but very rarely results in plant death.

In 2004, the NC Department of Agriculture & Consumer Services confirmed that the fungus-like organism causing Sudden Oak Death, Phytophthora ramorum (pronounced Fi-TOFF-thor-ra ra-MOR-um), was introduced into the state in shipments of infested nursery stock from southern California.

Symptoms:

Camellia
The appearance of the leaf spots can be quite variable on Camellia. Generally the leaf spots are light to dark brown in color, may appear to be greasy, and often start at the tip of leaf and progress down the middle of the leaf toward the point of attachment. As the disease progresses, concentric rings of dead tissue may be observed on the infected leaves. Infected camellias may also drop their leaves, leaving sparse or barren stems.

Viburnum
The disease on Viburnum is somewhat variable, but the leaf tips are often affected first, and the brown discoloration spreads toward the base of the leaf in a V-shaped pattern. Tissue may initially appear soft and wet, but will become dry with age. Small branch tips may also be killed, causing the foliage to wilt. (Note: hole punches in leaves were made to remove leaf disks for analysis for Phytophthora ramorum the cause of Sudden Oak Death.)

Rhododendron
On rhododendron, brown leaf spots with diffuse margins often start at the tip of the leaf and move toward the petiole along the midrib. The tips of the branches may also become infected and spread from the petioles to the leaves resulting in v-shaped lesions starting at the base of the leaf. Affected branch tips may turn brown and wilt. (Note: hole punches in leaves were made to remove leaf disks for analysis for Phytophthora ramorum the cause of Sudden Oak Death.)

If you think you bought a SOD-infected plant . . .

The NCDA&CS has established a free testing service to assist homeowners and landscapers who have concerns about SOD. Contact them for more information.