Control Cankers on the main stem will usually kill the tree above the canker. No control is possible. Cankers on the branches at least 4 inches from the main stem (trunk) should be removed from the tree. This will prevent the canker from reaching the main stem and killing the tree. Most blister rust cankers in the Lake States occur within 9 feet of the ground so pruning has the added advantage of removing the needle-bearing surface of the tree that is most vulnerable to infection. Where infection occurs in the upper crowns of large trees, pruning for control is impractical. Caution! Check your area for white pine weevil infestation. An investment in blister rust canker control may not be practical where weevil damage is severe unless the weevil can also be controlled. In Forest Stands Pruning to control white pine blister rust is normally warranted only in the northern parts of the Lake States where the disease is most severe. For best blister rust control begin pruning when trees are about 4 years old and 2 feet in height. Of course pruning also provides the additional benefits of better wood quality. Here is how to decide if pruning is needed, regardless of hazard zone. 1. Consider stands of white pine more than 3 years old that have more than 200 crop trees per acre and living needles within 18 inches of the main stem in the lower 9 feet of the tree. 2. Determine the number of white pine crop trees per acre that have no cankers on or within 4 inches of the trunk. 3. Determine the number of crop trees per acre with at least one canker 4 to 18 inches from the main stem. Do not count cankers above lower seven living whorls. 4. Divide the infected crop trees (3) by total crop trees (2 + 3) to determine the percent of infection during the last 5 years. 5. Divide total infection percent (4) by 5 (years) or the total number of whorls on a tree minus 2, whichever is less, to determine annual infection rate. 6. Prune only those stands that have an annual infection rate greater than that shown to be acceptable in the tabulation below: White Pine Per Acre (Number) Acceptable Annual Lethal Infection (Percent) 200-299 1 300-399 2 400-499 3 500-599 4 600-649 5 650-699 6 700-749 7 750-799 8 800-849 9 850+ 10

7. Re-examine stands every 10 years until the lower 9 feet of the bole are free of living limbs. Prune a maximum of 200 crop trees per acre for natural stands and 350 trees per acre for plantations. Prune all limbs flush to the trunk from the lower half of the tree or to 9 feet, whichever is less. Prune all accessible branches above 9 feet that have a canker 4 to 18 inches from the stem.

Example
1.	10-year-old white pine stand.
2.	320 crop trees per acre.
3.	65 crop trees with cankers 4 to 18 inches from the main stem in the lower seven whorls.
4.	65/320 = 20 percent for 5 years
5.	20 percent/5 years = 4 percent per year
6.	320 crop trees per acre at 4 percent annual infection. Hence, pruning is appropriate.

Protecting Ornamental and Christmas Tree Plantings The principles of controlling blister rust on trees grown for ornamental purposes (landscaping, Christmas trees, etc.) are the same as for forest trees, but the practice is somewhat different. By definition such trees should maintain the maximum amount of foliage. So, home and cabin owners and Christmas tree growers who want to protect their white pines from blister rust should remove only those branches that contain cankers 4 or more inches from the trunk. Repeat the process whenever new infections are detected. Trees with cankers on or within 4 inches of the trunk are doomed to die anyway, so any effort to preserve them is futile. For Additional Help Contact your State Department of Natural Resources, Forestry or Plant Pathology Extension Services, or your State Department of Agriculture, Plant Industry Division. Technical References Anderson, Ralph L. 1973. A summary of white pine blister rust research in the Lake States. USDA For. Serv. Gen. Tech. Rep. NC-6, 12 p. North Cent. For. Exp. Stn., St. Paul, Minnesota. Brown, H. Daniel. 1972. Guidelines: pruning white pine in the Lake States for blister rust control. S&PF Field Office Rep. S-72-9, 13 p. For. Serv., St. Paul, Minnesota. Weber, R. 1964. Early pruning reduces blister rust mortality in White Pine Plantations. USDA For. Serv. Res. Note LS-38, 2 p. Lake States For. Exp. Stn., St. Paul, Minnesota. Photographs of Ribes uredia and telia and pycniospore droplets on white pine, courtesy of Robert Patton, Department of Plant Pathology, University of Wisconsin, Madison, Wisconsin.

Authors Thomas H. Nicholls Principal Plant Pathologist North Central Research Station St. Paul , MN Robert L. Anderson Forest Insect and Disease Management Staff Northeastern Area, State & Private Forestry Newtown, PA

1977

U.S. GPO 1989-656810

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