With the spring finally here, we’re constantly thinking fungus. A wet spring brings about new growth coming on the trees, so we have to think about treating these trees for fungus. The trees that have struggled the last few years are evergreens, mainly spruce trees. Many of the trees we are seeing decline are the older, more mature trees. New research is showing that there is another type of fungal issue to watch for other than needle cast.
Phomopsis is a canker causing fungal pathogen known to cause branch death. We now know that a group of Phomopsis strains of unknown species are at the center of the current landscape spruce problems that we are now calling “Phomopsis spruce decline”. Normally, Phomopsis, a fungal pathogen, is only found on young trees in nurseries and on tree farms, including Christmas tree farms. For some unknown reason, this pathogenic fungus has moved out of the nurseries and tree farms and is now causing mature tree defoliation, branch death, and, in some rare cases, tree death. Phomopsis appears to cause these symptoms by establishing cankers (bark infections) on older branches, usually on the lower half of the tree. The cankers can be found somewhere on the large branches near the dying small branches extending from the branch. We have also found trees with severe defoliation throughout the trees but without too much branch death (terminal buds are still alive). This would look very similar to a true needle cast disease. Keep in mind that on spruce there are other cankers caused by other pathogenic fungi, such as Cytospora and Diplodia, but the predominant canker-causing pathogen currently appears to be Phomopsis. It may appear as a needle cast problem, but it is a canker disease and finding the canker without skinning all of the thin bark from the branch is difficult.
There is not much outward appearance to the canker infection. That is, there is little in the way of a sunken canker that can be observed without removing the bark. What we believe is occurring is a fungal infection that expands around the branch, girdling the current year’s sap-conducting vessels or phloem. As the fungus grows deeper into the resinous branch, the branch begins losing connections with the main stem and the needles begin to drop from the older portions of the branch outward. It is similar to cutting a branch off a tree and putting it in a vase of water. It will stay fresh for a while but, sooner or later, the nutrients and water resources are lost and the branch begins to fail and finally dies. This is why we are seeing so many spruce trees dropping needles which is followed by branch death. The progression of the symptoms will depend on how much of the branch is girdled by the canker caused by the Phomopsis infection and how long the infection has been in present on the tree.