Black Walnut trees dying
Our Black Walnut trees are dying as described in the link below:
Is there anything we can do to save our trees? Cut off dead braches? spray with insectiside? More water/ less water?
Jefferson County Colorado trees and shrubs
The problem with your walnut certainly could be Thousand Cankers disease; an exam of the damaged limbs will confirm that. You can have these branches examined by an arborist, look for the holes yourself or bring a sample to the Jefferson County Plant Diagnostic Clinic in Golden. We are located on the Jefferson County Fairgrounds, just west of the intersection of the 6th Avenue Service Drive and Indiana. Office hours are 8 am to 5 pm. If a clinician is unavailable when you come in, you can drop it off after completing some paperwork and paying a $7 fee.
If you have thousand cankers (and it is probable), you'll face some difficult decisions as the disease is often fatal. Removal of the dead branches is a good idea for a couple of reasons. You'll remove whatever insects might be in the branches. You'll also reduce the likelihood of injury to people or property. However, removing dead branches (and insects in them) may not make much difference in slowing down the disease if the fungus Geosmithia is already in the tree (but not showing symptoms yet.)
Insecticides have had limited effectiveness in managing this pest, unfortunately. Because black walnut is classified as a nut crop, it is difficult to find pesticides that can be utilized on this plant.
Sufficient water/good cultural practices can be helpful in slowing the disease down.
Here is some additional information on the disease and insect by our CSU Extension entomologist.