July 1997 - White Pine


White pine : How to prune for blister rust

Bright red branches against the green-as-grass background of the rest of the tree. That's what many people are noticing this summer as blister rust infections show up on their white pines. White pine blister rust is a fungal disease that infects only white pines. Needles are infected then the fungus grows into the twig and towards the stem of the tree. White pine blister rust causes branch death, topkill, stem cankers and can be fatal. The disease becomes more common and more serious as you go north and east in Minnesota, where the late summer weather is cool and moist.

A blister rust canker is the result of an infection that has been growing inside the branch or stem for several years and is just now cutting off the supply of food and water to any part beyond or above that infection. This is how blister rust "girdles" and kills infected branches and stems. As the needles die and dry out, they begin to turn red and eventually are a brilliant red color that you just can't miss seeing. This is a "flagged" branch.

Old and large trees can live a long time with white pine blister rust. The real problem is establishing and growing white pine seedlings because most of the tree is needle covered and therefore completely vulnerable to infection. Once the infection reaches the stem, it always kills seedlings and small trees.

Fortunately, there are some steps people can take to reduce losses from blister rust. One of the best steps for already established seedlings and even very large trees is pruning. Pruning removes the branches that are most likely to become infected and also removes infections that already exist.

Here's how to prune your white pines:

  • Pruning of live branches is best done during the dormant season.
  • Begin pruning the lowest branches off the trees when they are 4 or 5 years old and 2 feet or more tall.
  • Don't remove too much crown. Try to leave ½ to 2/3 of the tree height with branches.
  • Be sure to find and prune off the lowest branches. These often lay on the ground and are hidden in the grass.
  • Pruning should continue to a minimum height of 9 feet but you may want to continue up to 17 feet.
  • You don't have to prune every tree on your site - only the most desirable ones.
  • Pruned branches can be left on site. They won't spread the infection.
  • Prune off any branches higher up in the tree that show blister rust infections.
  • Trees with blister rust cankers on the main stem or on a branch but within 4 inches of the main stem cannot be saved. Do not waste your time pruning these trees.
  • Pruning tools do not have to be sanitized between pruning cuts. They won't transmit blister rust infections.
  • Do not flush cut the branches. Do not leave long stubs, either. Pruning cuts should be made just outside the branch collar. Use "natural target" pruning methods.
  • Use a pruning or lopping shears on small trees or a sharp pruning saw on larger trees. Shears should have bi-pass blades (like scissors) rather than the anvil type.
  • Do not use chain saws, Sandviks, hatchets, bow saws or clubs; this is delicate surgery.
  • Do not paint or treat the pruning cuts.
  • Graphic of how to prune a tree

    Colored drawing of a pruned white pine