Careful pruning of white pine limbs helps reduce damage from blister rust and white pine weevil. It also helps trees to grow straighter and produce higher quality wood products.
White pine blister rust
Blister rust is a fungal disease that kills branches, causes stem cankers, and can be fatal to the tree. It infects white pine through live needles during the late summer when temperatures are cool and a thin film of water forms on the needles. As a result, most infections occur within 9 feet of the ground where dew or moisture are most likely to collect. This disease is more common and more serious as you go further north and east in Minnesota, but can occur anywhere in the state where late summer weather is cool and moist.
Pruning to control white pine blister rust
Pruning is beneficial to any size tree. However, it is best to start when the trees are small (taller than 2 feet) and continue until all the branches on at least the lower 9 feet of the tree have been removed. It is important to find and remove even the lowest and smallest limb or sprig of needles. Cutting off infected limbs that are flagging (a large group of needles or an entire limb that turns orange) may help save the tree.
For blister rust control it is not necessary to paint over the branch stubs of the pruned limbs, clean up or remove the limbs pruned from the trees, or sanitize pruning tools (they won't transmit infection) between uses. Trees with blister rust cankers on the main stem or on a branch within 4 inches of the main stem cannot be saved, so do not bother pruning these trees. It is not necessary to remove infected trees from the area since blister rust in not transmitted from tree to tree.
Blister rust and Ribes
Shrub species that are part of the Ribes family (primarily currant and gooseberry) play a critical role in the life cycle of the blister rust fungus. The spores that infect white pine needles are produced on Ribes leaves. Making sure not to plant these species in and around your white pines helps break the life cycle. If time and energy permits, eradicating existing Ribes plants in the immediate area can be beneficial to the health of your white pine.
White pine weevils
White pine weevils tunnel into the terminal leader of white pines, causing terminal death, growth loss, and crooked or forked stems. As mentioned earlier, this happens more when trees are grown in open, unshaded conditions. It usually occurs when the trees reach a height of 5 feet or more.
Corrective pruning for weevil infestations
If white pine become infested, clipping currently wilted terminals in July will help prevent damage in the following year by killing the new generation of weevils. Be sure to remove enough of last year's growth to get all of the larvae feeding under the bark. Collect and destroy the clipped terminals.