Springtime, pines, and Western gall rust…

western gall rust

Newly emerged galls on a lodgepole branch.

‘Tis the season for Western gall rust. Just when the earth is emerging from its blanket of snow and little green shoots are appearing, so, too, do galls. Right now, take a look at your ponderosa and lodgepole pines; see any pale-colored, bulbous growths on the branches? If so, you’re looking at the emergence of galls, full of spores that are ready to set sail on the winds. Is this something to worry about? Well, that depends. First, let’s learn a little about Western gall rust.

Western gall rust is caused by the rust fungus Endocronartium harknessii. It is unique in that it doesn’t require an alternate host to complete its life cycle (with other rust fungi, like white pine blister rust, an alternate host – like gooseberry – is required to pass the fungus along). This means Western gall rust can spread easily from tree to tree via airborne spores without the assistance of an insect or another plant. Thus, if you see galls in some of your trees, it’s likely it will spread to others nearby.

Western gall rust affects trees of all ages. The younger the tree, though, the more damage it does.

You can identify galls most easily in the spring when they take on a pale yellow to orange color. They appear as small, pear-like objects growing on the limbs or sometimes the trunk of the trees. Galls on the trunks of larger trees create what’s called a stem canker, which looks like an ugly, deformed crack. It rarely kills the trees directly, but it can weaken the structure of the tree and make it susceptible to breaking in a wind event or heavy snow. Galls growing on limbs will eventually girdle the limb and kill it. A tree with too many galls in the crown will have a difficult time photosynthesizing as all the live branches begin to die back. Without sufficient photosynthesis, the tree’s health spirals into decline. In saplings, galls on the trunk are usually fatal as they girdle the tree and prevent the flow of water and nutrients.

Pruning limbs with galls before spore generation – ideally in the fall and winter months – will help reduce the rate of spread throughout the tree and to other trees. In addition to lodgepole and ponderosa, keep an eye on your ornamentals like Mugo, Austrian, and Scotts pine. They are also susceptible.

Ultimately, Western gall rust is a part of life in the forest here. It’s been around just as long as its host trees have been, and our ponderosas and lodgepoles still thrive. However, your yard is a managed landscape, and with that management comes the responsibility to keep an eye on the health of your trees. If you’re concerned about Western gall rust, give us a call. We’re happy to assess the vitality of your trees.

For more information about Western gall rust, check out this page from Washington State University, “Garden Friends & Foes.”