Oak Wilt Management—What Are the Options? (G3590) R-12-01-5M-150
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G3590 Lake States Oak wilt management— Woodlands what are the options? Jane Cummings Carlson Trees at risk Biology and spread A. Jeff Martin housands of oaks in woodland of oak wilt and urban settings die from oak Mats of fungus, known as “pressure wilt every year. Widespread in pads,” develop under the bark of trees Wisconsin, Minnesota and that have died from oak wilt. Mats form Michigan, the disease is caused most often in spring, approximately 9- by the fungus Ceratocystis 10 months after a tree dies from oak fagacearum. Figure 1 shows the extent of wilt. These mats force the bark to crack Toak wilt in Michigan, Minnesota and open. The fungus produces a sweet Wisconsin. odor that attracts sap-feeding beetles Trees from the white and red oak such as Nitidulids. The beetles pick up groups, both found commonly in the fungal spores by crawling on the mats. Lake States, are susceptible to oak wilt. Then they fly to healthy oaks to feed on Because trees in the red oak group fall sap flowing from fresh wounds, thus prey to the disease most often, this pub- infecting new trees. lication focuses on the red oak group. In April, May and June, fungal mats (“pres- sure pads”) grow under the bark of trees that wilted the previous summer. Mats are some- times present in late summer or fall. Figure 1. This map shows the distribution of oak wilt in Michigan, Minnesota and Wisconsin in 2000. Map produced by the USDA Forest Service, Northeastern Area–Forest Health Monitoring GIS Group. Fungal mats force the bark to crack open. The mats’ odor attracts sap-feeding beetles which spread the disease to healthy trees. The fungus invades the tree’s water-carrying system, causing leaves to wilt and fall. Wilting occurs most often in July and August, and occasionally in spring or fall. LAKE STATES WOODLANDS ____________________________________________________________________________________ As fungus invades a tree’s water con- 1. Do not harvest, prune or other- wood under the bark is a light color ducting system, the leaves turn dull wise wound oak trees from bud (white to green), the sample is fresh. If green, bronze or tan, and wilt from the swell up to two or three weeks the wood is brown or dark, it is too old top of the tree downward. Leaves fall past full leaf development (gen- to be useful. Wrap the samples in wax rapidly after wilting. Infected trees are erally April 15 to July 1). During paper and keep them cool until you bare in 4-6 weeks. this time, fungal mats are most mail them. Underground spread of oak wilt from abundant and oak trees most vulner- In Wisconsin, mail your samples to: infected to healthy trees occurs through able to oak wilt. Infection is less Plant Disease Diagnostic Clinic root grafts. Most root grafts form common later in the summer but can Dept. of Plant Pathology between oaks of the same species; red occur after July 1. Take a very cau- University of Wisconsin-Madison oak roots graft more commonly than do tious approach—limit any cutting 1630 Linden Drive white oak roots, and grafts between red activities from April 1 to October 1. Madison, WI 53706 and white oaks are very rare. If wounding does occur, apply a tree (608) 262-2863 wound paint immediately. These www.plantpath.wisc.edu/PDDC products normally inhibit proper Prevention, sampling In Minnesota, send them to: and management wound closure so limit your use of tree paint to these situations. Minnesota Dept. of Agriculture Plant Protection Laboratory Prevention 2. Do not move infected trees 90 West Plato You can take two precautions to lessen with the bark still attached (as St. Paul, MN 55108 the chances of oak wilt invading your firewood or logs) into your (651) 296-4749 woodlands. woodlands. Pressure pads may www.mda.state.mn.us form on the transported wood, attracting insects that spread In Michigan, mail to: oak wilt. State Pathologist Michigan Dept. of Agriculture Sampling Laboratory Division You may need a laboratory 1615 Harrison Rd. analysis to confirm the pres- East Lansing, MI 48823 ence of oak wilt. To obtain an (517) 337-5091 analysis, collect three twigs A fee may be charged for testing. Call (about 1⁄2 in. diameter and 4 the appropriate lab to determine in. long) from three different charges. branches with wilting leaves. You must send samples which still have live tissue. Scratch the sample branch with your fingernail. If the Don’t confuse oak wilt with the two-lined chestnut borer. Galleries, or feeding tunnels of the two-lined chestnut borer are visible in this photo. The insect causes symptoms resembling that of oak wilt with one notable difference—trees infested with the chestnut borer usually keep their dead leaves, while This aerial view shows the characteristic pocket of dead trees, trees with oak wilt lose them. a hallmark of oak wilt. 2 ______________________________________________ OAK WILT MANAGEMENT—WHAT ARE THE OPTIONS? If sampling results dead trees that characterizes oak wilt. In are negative mixed stands, where oaks are scattered If your samples test negative for oak among other species, root grafts may wilt, check for the presence of the two- occur less often than in pure oak stands; lined chestnut borer which harms trees thus, spread may be slower or less con- by tunneling between the bark and spicuous. New pockets can appear in wood. This insect causes leaves to wilt your woodlands from overland spread and turn brown in in mid-summer by Nitidulid beetles feeding on fresh when larvae feed between the bark and wounds. wood of infested trees. Dead leaves gen- If you allow the disease to progress, you erally remain on the tree. A few should consider the impact on the branches or the whole crown may be health of neighboring trees. Even if root- affected. (Oak wilt always affects the graft spread to your neighbor’s trees is entire crown in red oaks.) unlikely, overland spread to a neigh- If sampling results bor’s yard or woodlot is a real possibil- are positive ity. Proper removal and utilization of infected trees reduces the hazard of There are two management strategies overland spread. You can find guide- from which you can choose. lines for removing and using dead and Passive management. If you allow infected trees in the section on active Oak wilt trenching. the disease to progress, it will spread to management. healthy oaks through the roots of dis- As oaks die, the open spaces and dead eased trees. In the Lake States, oak trees trees can provide valuable feeding and may grow in pure or relatively homoge- nesting sites for wildlife. When over- neous stands. You may also find them story trees die, the site often becomes mixed with species such as white pine, brushy for about ten years. Warblers, sugar maple, basswood and white birch grosbeaks, cuckoos, cardinals, grouse, (common in the northern hardwood rabbits, deer and shrews will be forest type), or hickory, elm, ash, aspen, attracted to the brushy area. Brown and black cherry (species common in creepers may nest under the sloughing the oak-hickory forest type). bark on dead trees. Dead trees will In pure oak stands, the disease moves furnish insects for birds, and larger outward from the original infected trees specimens may provide perches for through root grafts, killing more trees raptors. each year. This creates the pocket of The vibratory plow is a tracked or rubber- tired vehicle with a vibrating head attached at the rear. Crown wilt indicates the presence of oak wilt. A knife-like plow blade with a slight hook at the bottom is attached to the vibrating head. The blade is pulled hori- zontally through the soil, slicing the root connections. 3 LAKE STATES WOODLANDS ____________________________________________________________________________________ Active management. Since oak wilt A forest pest specialist, forester or Table 1. Minimum inter-tree distances spreads both underground and above arborist trained in oak wilt management based on combined tree diameters and soil ground, you must combat it with a two- should work with you to plan the type. pronged approach. To contain the barrier location. Choose a location for Combined disease: the barrier in early August, after most of DBH — Inter-tree distance — 1. install a root graft barrier; and the year’s infected trees have wilted. (inches) loamy sand sandy Barrier installation should take place 2. remove and properly use trees 2 3.1 3.9 before the soil freezes. inside the barrier. 4 6.2 7.8 To determine where to place the barrier, 6 9.3 11.6 Installing a root graft barrier. Root measure the diameters at 41⁄2 feet above 8 12.4 15.5 graft barriers break the root connections the ground (or diameter at breast 10 15.4 19.4 between infected and healthy trees. A height, DBH) of an infected tree and a 12 18.5 23.3 barrier should be at least four, and 14 21.6 27.2 nearby apparently healthy tree; add preferably five feet deep. Trenchers and 16 24.7 31.0 these figures to calculate the combined vibratory plows are the tools most com- 18 27.8 34.9 diameter. A sprout clump is represented monly used for this job. 20 30.9 38.8 as the sum of the diameters of all its 22 34.0 42.7 Locating the root graft barrier. stems. Measure the distance between 24 37.1 46.6 Properly locating the root graft barrier is the same two trees. Consult table 1 to 26 40.2 50.4 critical to stopping the underground determine the minimum inter-tree dis- 28 43.2 54.3 spread of oak wilt.