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WHITE PINE BLISTER RUST
Description and Damage
White pine blister rust is the most serious disease of white pine in Maine. Its most commonly observed symptom is the presence of cankered (dead and sunken) areas on tree trunks and branches. Pitch commonly flows from these cankered areas and, in May, conspicuous yellowish-orange spores may be produced (image). Infected small trees are usually killed quickly, but large trees may survive for many years. In the woodlot the disease strikes indiscriminately, often taking the best crop trees or trees at critical spacings. White pine blister rust may effectively eliminate white pine regeneration on some sites.
Eastern white pine is the only native tree species affected, but all five needle pines are susceptible. All species of currants and gooseberries (Ribes spp.) are susceptible [see Guide for Identifying Ribes Bushes (1.5Mb pdf)]. Currants and gooseberries are significant because of their role in disease spread. [The white pine blister rust fungus (Cronartium ribicola) cannot spread from pine to pine, but must infect a currant or gooseberry plant first (image)].
Most efforts to control this disease in pine stands involve attempts to eliminate its alternate host plants (Ribes). The fungal spore which carries the disease organism from Ribes to pine is fragile, rarely surviving airborne transport for distances exceeding 900 feet, even less where vegetation interferes with spore movement. So if all Ribes plants can be killed ("eradicated") within 900 feet of pine to be protected, new infections are rarely a problem.
The most effective way to kill alternate host plants is through use of herbicides, although currants and gooseberries may be physically uprooted instead. For homeowners and small landowners, the easiest herbicide to use is glyphosate (Roundup). It is sold as a ready to use spray formulation but is most economical when purchased as concentrate (41% glyphosate) and diluted at the rate of 2 oz./per gallon of water. For larger landowners, triclopyr (Garlon 4) is recommended at 2-4 oz./per gallon of water. Ribes should be sprayed to wet, not to runoff, during the growing season between May and September. Be careful spray does not drift to desirable plants.
It is also sometimes possible to prune infected areas (cankers) from trees, thereby eliminating the disease. Branches are infected before main stems; therefore if a diseased branch is removed before infection spreads to the main stem, the disease is effectively eliminated from the tree. For this reason pruning of low branches is sometimes recommended in young plantations; this practice has the added bonus of permitting the production of knot free lumber. When pruning be sure not to remove more than one-third of the live crown at any one time.
It is also sometimes possible to excise (cut out) a canker on the stem of a high value ornamental tree, so long as the canker affects less than half the circumference of the tree. Make an elliptical-shaped cut through the bark around the canker extending 4 inches above and below the margin of the discolored zone and two inches out from either side. Remove the diseased bark and buffer zone of healthy bark. If the wound does not start to heal after the first year, additional bark may have to be removed.
Quarantines are in place to help control this disease. Maine law prohibits the planting and cultivation of currants and gooseberries in most of southern Maine, and prohibits the planting and cultivation of European black currants and their hybrids anywhere within the state. Please contact the Maine Forest Service for more specific information.
MAINE DEPARTMENT OF CONSERVATION
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