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Minnestoa Extension Service |
Oak wilt is a fungal disease that infects the vascular tissue of oak trees. As the fungus infects the tree, the tree tries to defend itself by producing defense compounds. These compounds plug the water-conducting tissue causing the tree to wilt and eventually die.
All species of oak trees are susceptible to oak wilt. Trees in the red oak group, including red and pin oaks, are highly susceptible and usually die within a few weeks. Trees in the white oak group, including white and bur oaks, are more resistant and may survive for one or more years following infection.
To identify trees infected with oak wilt, watch for wilting leaves in the upper canopy. Wilting leaves may develop yellow margins, while the interior portion of the leaf remains green. As the tree continues to wilt, leaves turn brown and fall from the tree.
Since oak wilt causes drought-like symptoms, it may be easily confused with other stress-related factors. These factors include construction damage, drought stress, or insect colonization. In addition, oak wilt is often confused with the common springtime disease, anthracnose. In contrast to oak wilt, anthracnose causes spotting, curling and browning of the leaves in the lower canopy. In rare cases oak wilt may cause brown streaking of the inner sapwood. This streaking is a good diagnostic symptom for detecting infected trees.
To obtain positive identification for questionable trees, contact the University of Minnesota Plant Disease Clinic or the Minnesota Department of Agriculture for sampling guidelines.
Preventing
Oak wilt is a fungal disease that causes wilting and eventually death of oak trees. Trees are initially infected by sap-feeding beetles like the picnic beetle. Wounds created by storms or pruning serve as a feeding site for these beetles and as an entry point for the fungus. Once the tree has been infected, the fungus moves through grafted roots to healthy trees nearby. As the tree dies, the fungus reproduces. The fruity smell of fungal mats attracts picnic beetles, which carry fungal spores to fresh wounds.
To prevent infection, avoid wounding trees during April, May, and June when beetles are active. If trees are wounded during these months, prevent beetles from interacting with fresh wounds by painting them. Apply a water-based (latex) paint or shellac within minutes. These materials act as a protective barrier; they do not help the tree recover. Since the chance for infection still exists during July, August, September, and October, the optimum time for pruning trees is when they are dormant.
To manage existing infections, use a vibratory plow with a five-foot blade to sever below ground root grafts and remove infected trees. Infected trees should be destroyed or covered with heavy plastic through June of the following year. The systemic fungicide Alamo has been used as a preventative treatment in highly valued red and white oaks, and as a curative treatment in white oaks.
For help with identifying, preventing and managing oak wilt infections contact your local community forester. Financial assistance may be available. Copyright © 1998 Regents of the University of Minnesota. All rights reserved.
BUCKTHORN
Article courtesy of the Minnesota Extension Service
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Department of Natural Resources |
In September 1999, the Minnesota Department of Agriculture declared Rhamnus cathartica (European or common buckthorn) a restricted noxious weed. This means that sale, transportation, or movement of this plant is prohibited statewide by the Minnesota Department of Agriculture. Rhamnus frangula (glossy buckthorn) joined Rhamnus cathartica as a restricted noxious weed in January 2001. Both species are considered invasive because they are capable of rapidly spreading and replacing native species throughout Minnesota. For help identifying buckthorn, see Yard & Garden Brief H402B, Common Buckthorn Identification.
European buckthorn was first brought to Minnesota from Europe in the mid-1800s as a very popular hedging material. Shortly after its introduction, it was found to be quite invasive in natural areas throughout Minnesota. The nursery industry stopped selling European buckthorn in the 1930s, but many buckthorn hedges may be found in older neighborhoods throughout Minnesota. Glossy buckthorn, also from Europe has been sold by the nursery trade in two different forms. Both forms reach a height of 12 feet. The cultivar 'Columnaris' has a narrow and tall form, while the cultivar 'Aspenifolia' spreads up to 10 feet and has narrow leaves that give it a ferny texture.
European buckthorn is the most problematic species of buckthorn because it is both drought and shade tolerant. These characteristics enable it to become invasive in woodland understories or at the edges of prairies or fields. In more recent years, glossy buckthorn has also been shown to be an invasive plant, problematic mainly in wetlands and moist woodlands. Both types of buckthorn have berry clusters that attract birds who then deposit the seeds in their droppings. This process allows for large quantities of buckthorn to become distributed over vast and diverse areas.
Both mechanical and chemical treatments may be implemented to remove buckthorn. While buckthorn does not resprout from underground roots, it can resprout from the buds at the base of stems if they are not cut close enough to the ground. It can be difficult to determine how close to the ground the cut should be made; using one of the herbicide treatments recommended below will prevent these buds from resprouting.
Physical removal
For buckthorn stands containing individual plants ⅜ inch in diameter or less, hand pulling may be the most effective method of removal. Uprooting the small plants can be aided by using a weed wrench or shovel to help loosen their roots. This is a practical method for small stands of buckthorn, but may not be practical for large stands due to the amount of labor involved. Buckthorn plants that are greater than ⅜ inch in diameter may be more difficult to pull out of the ground. To loosen their roots, soak the soil around the trunk for several days before pulling. Before you pull or dig buckthorn out of your soil, call Gopher State One to ensure there are no buried utilities in the area.
Stump Treatment
When controlling large quantities of buckthorn, cutting the stems, then painting or spraying the stumps with the herbicide glyphosate (commonly sold as RoundUp) is quite effective. You will need to buy a concentrated form containing 25 percent or higher amounts of the active ingredient, glyphosate. Stores may carry concentrated forms containing smaller percentages, so it is important to read the fine print on the label. Farm and landscape supply stores often sell this more highly concentrated glyphosate.
The proper time to cut buckthorn and apply glyphosate is when the plant is active or when the leaves are fully expanded and temperatures are above 32 F. This will ensure that the herbicide is taken up by the plant. The herbicide must be applied within 24 hours of cutting, but applying it immediately after cutting is best. In cases where more than a few plants are treated, it is beneficial to add a dye to the herbicide to mark cut stumps and bark you've already sprayed. Laundry blueing works well, is inexpensive, and is available at the grocery store. If you choose to use a herbicide as a means of buckthorn control, always read and follow label directions to prevent danger to yourself (the applicator) and to the environment, and to ensure that the herbicide will work effectively.
Frequent Pruning
When feasible, removal of buckthorn in the landscape is encouraged. When it is not feasible to remove buckthorn, such as the case for some homeowners with large mature hedges, one last option may be used to prevent the spread of buckthorn without removing the plants. Pruning or shearing frequently (at intervals of every 3-4 weeks) can dramatically reduce or eliminate the flowers and subsequent berries. This method is effective in preventing birds from eating the berries and distributing buckthorn seeds only if it is done frequently as suggested above. Copyright © 1998 Regents of the University of Minnesota. All rights reserved.
References:
Dziuk, Peter. 1995. Pest Alert Factsheet Buckthorn and its Control. Minnesota Department of Agriculture.
European (Common) Buckthorn. Minnesota Department of Natural Resources. September 2000.
More information on Buckthorn
Choosing the right tree for the right site:
"tough trees and shrubs for tough sites"
http://www.extension.umn.edu/distribution/naturalresources//DD7502.html
"recommended trees for southeast Minnesota"
http://www.extension.umn.edu/distribution/naturalresources//DD6574.html
"trees and shrubs for clay soil"
http://www.extension.umn.edu/projects/yardandgarden/ygbriefs/h408claysoil.html
Proper tree care:
"how to properly prune trees"
http://www.na.fs.fed.us/spfo/pubs/howtos/ht_prune/prun001.html
"properly staking trees"
http://www.cnr.umn.edu/FR/extension/stakingandguying.html
"fertilizing"
http://www.extension.umn.edu/distribution/horticulture//DG7410.html |
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