Walnut trees are notoriously tough, tall and long-lived. From the towering black walnut (Juglans nigra) to the English walnut (Juglans regia), these trees can grow to heights of more than 100 feet and live for more than 200 years, according to the University of Minnesota Extension. Though they are generally drought and pest-resistant, there are a few things that can cause a walnut tree to die, including girdling of the trunk's bark, insect borers, fungus, root disturbance and simple old age.
Notice the tree from year to year, and the size of the walnut crop. Some years will be highly productive, followed by a year of few or no nuts at all. That's normal. If you have several years of low-to-no harvest, the tree may be in serious decline.
Watch for the walnut tree to bud and produce leaves in mid-spring. Walnut leaves break a little later than many other deciduous trees, and lose their leaves earlier in fall. If the tree doesn't leaf at all, that's a sure sign something's very wrong. The problem could be a fungus called thousand cankers disease, which causes tree death in two to five years. The first symptom is failure in parts of the canopy to leaf out in spring.
Look for peeling bark on the trunk. Walnut bark is normally shaggy and rough, but if you can pull it away with your fingers, the tree is dying or dead. If the bark is peeled all the way around the trunk, it's definitely dead. If the tree's cambium layer is exposed, it cannot pull water and nutrients from its root system to the canopy, and the tree dies.
Check the exposed cambium under areas of peeled bark. If it's soft and flexible, you may have a chance to save the tree by cutting away unhealthy bark and allowing the wound to heal. If it's dry and gray, the tree is dead.
Test the flexibility of side branches (if you can reach one). If they flex and bend, they're alive. If the tips snap, the branches are dead.