ISSUE 1 May 6, 1999
TREES - Ash Yellow
Ash yellows phytoplasmas were recently discovered in this region.
These phytoplasmas cause a disease called ash yellows in ash and lilac witches broom
in lilac. Phytoplasmas are bacteria that survive only in the food conducting tissues of
host plants or in vectoring insects. The pathogen is transmitted by sap-sucking insects
such as leafhoppers. Ash yellows phytoplasmas
have been shown to significantly reduce growth and cause dieback of white ash and green ash in the northeastern United States. Although ash yellows can cause dramatic changes in tree functions over one or two years, performance of established trees is usually reduced very gradually over several years.
The most characteristic symptom of ash yellows is proliferation of branches from a single point (witches-brooms). These witches-brooms develop on large limbs or trunks and appear bushy. Leaves on witches-brooms tend to be small, chlorotic, and may be simple (ash trees normally have compound leaves).
Most infected trees do not have witches-brooms, but there are no other diagnostic symptoms. If a tree is performing poorly, ash yellows should be considered as a possible cause. Infection can be verified through the NDSU Plant Diagnostic Lab for a fee. Samples should be taken from witches-brooms, but may be taken from roots if no witches-brooms are present. Contact the Diagnostic Lab for packaging information.
Ash yellows phytoplasmas were first confirmed in North Dakota green ash trees in 1993 and lilacs in 1997. Beginning in 1996, NDSU Plant Pathologist Dr. Jim Walla coordinated a regional project including six states and three Canadian provinces to determine the incidence and impact of ash yellows in Great Plains and Rocky Mountain green ash trees. Community forests, rural plantings, and native woodlands were all sampled during the project. This project discovered that "ash yellows phytoplasmas are basically everywhere in central North America."
We do not know if ash yellows is causing damage in North Dakota. An association was found between the presence of ash yellows phytoplasmas and increased dieback in green ash, but we do not know if the phytoplasmas caused the dieback. Walla is currently investigating the effect that ash yellows has on natural regeneration of green ash. He will soon be screening cultivars to identify tolerance. If found, tolerant trees may be used as parents for future windbreak trees.
Where possible, good plant care principles such as proper irrigation, fertilization, and pest control may prevent infected trees from declining.
The presence of this pathogen is a concern since ash and lilac are among the best and most important woody plants in this region. However, we dont know yet if ash yellows will damage plants here. Walla recommends considering multiple species in new plantings and reducing the ash component (if high) during windbreak, woodland, or landscape renovation. Ash and lilac should continue to be planted as they are important components of our native forests, windbreaks, and urban landscapes.
Now is not a good time to remove healthy stems from trees, but the fresh green foliage provides an excellent opportunity to identify dead stems which should be removed. Dead and broken limbs stand out in otherwise healthy trees. They will not benefit trees and may provide sites for insects and disease-causing organisms to enter. Any limbs broken by winter snow loads or ice storms and any other dead stems should be removed as soon as possible.
Winter Injury of Evergreens
Every year there are questions about pine, spruce, and other evergreens turning brown in the spring. The most common cause of browning in evergreens is drying of the foliage during winter and early spring. If the trees ability to pull water up from the frozen soil is not great enough to meet the needs of the foliage, that foliage will die from the tip back. Oftentimes, greater injury will be seen on the side of the evergreen facing the prevailing winds. These winds can dry out foliage quickly, especially on warm, sunny winter days.
Winter injury may be increased by nonhardy stock, previous drought conditions, herbicide injury, weak root systems, stress from insects and diseases, or other forms of stress. To reduce the potential for winter injury in the future, water trees as needed from spring thaw until the ground freezes late in the fall.
Trees which have suffered browning often recover. Since the stems and buds are better protected from winter injury, new growth will often emerge in the spring. The overall condition of the plant should not be assessed until after the spring growth has been allowed to take place.
BUTTERFLY AND HUMMINGBIRD GARDENING INTERESTS TAKE FLIGHT!
The stresses of life are plentiful these days, and it is common for most of us to seek asylum in some manner. Why not try butterfly and hummingbird gardening? If such an idea conjures up hordes of munching caterpillars of Biblical proportions laying waste to your garden, think again. Many gardeners are opting for more natural landscapes that embody the planting of wildflowers on their property, in some cases, replacing areas that were difficult to mow. Many of the flowers that are part of a wildflower mixture, such as purple coneflower and beebalm, attract both of these acrobats of motion.
In creating a natural habitat in some areas of your landscape, you are inviting a diversity of nature, equipped with natural controls. For example, the butterfly weed (Asclepias tuberosa) attracts monarch butterflies and green peach aphids, but then this plant is also host to the small milkweed bug that is a predator to the aphid. Besides, the hummers that are attracted to this same garden do not live by nectar alone! They too, are bug eaters, helping to keep unwanted insect damage under control.
All of this "naturalness" implies the very intelligent use of pesticides, if any at all, in regions where this type of attraction is desired. The butterflies and hummingbirds are very sensitive to insecticide use, so if there are some unwanted insect pests present, use insecticidal soaps that do not leave a toxic residue when dry.
For those who do not want a naturalized area, but wish to attract these interesting, mobile works of landscape art, you can do so by strategically placing one or more of the following plants somewhere in your landscape setting where you can enjoy their activity. Four oclock (Mirabilis jalapa), Trumpet vines (Campsis radicans), Cardinal Flower (Lobelia cardinalis), New England aster (Aster novae-angliae), and French marigold (Tagetes patula).
Movement of colors in the landscape setting enhances the design intent and helps attract the attention of the viewer. With the colorful butterflies and acrobatic hummingbirds visiting your property as a result of these plantings, it sure would at least beat watching the evening news and be a lot less stressful!
Extension Horticulturist and Turfgrass Specialist