Boardman Australian Willows Experiencing Leaf Yellowing and branch die off
Here in northeast Oregon we have a person with a number of Australian willow trees that have started to have yellow leaves on specific branches only this past year. The trees are about 15 years old and are irrigated near a lawn from an irrigation ditch next to the house. The owner is very concerned about the trees, she has been pruning off the affected branches but is worried about whatever is causing it spreading to the other trees. She hasn't observed any webs or obvious insect differences on the tree this year, but did note that this was the first year she had a lawn care service come out to spray the lawn (she didn't know what with) near the trees and wondered if that might affect them.
I have been looking into it and I'm pretty stumped by the information out there about Australian willows specifically, though I did notice they are classified USDA zone 9-11 (we are about zone 6 here) and wonder if this past winter did not do them any help.
Admittedly, I am not a tree expert and this has me very confused. The owner has a sample in to the OSU plant pathology lab, but is very anxious to prune or spray something to save her trees. Any help would be much appreciated! Thanks!
Morrow County Oregon
Thanks for your question. The symptoms you describe could be caused by a number of issues, including low winter temperatures of last winter and spray drift from herbicides. It is good to get the OSU Plant Lab involved to see if the damage is caused by an insect or disease affecting the leaves or twigs. It there are no problems with the leaves/twigs then it might be environmental/abiotic or there might be something going on with the roots or trunk or large branches. there are insects and diseases (such as cankers) that can affect the needles/branches higher in the tree. Gophers can feed on roots for example. Without seeing this up close and being able to examine the root crown, roots, and trunk my hunch is that this damage is likely as you say, low temperatures last winter (maybe with drying winds) and/or herbicide drift or herbicides that were taken up by the trees from the lawn application.
Pruning out the dead branches and keeping the plants vigorous with good deep watering will help the trees recover. If the lab results indicate an insect or disease pest then follow their recommendations for applying pest control.