My walnut tree is showing signs of dieback as are several others in the area. Could Thousand Cankers disease be the cause?
It's always hard to answer this sort of question without visiting the trees that are affected since there are often clues to the problem which can't be seen in photographs like the ones that were submitted with your question.
That being said, the photographs show what appears to be a fairly straightforward case of cold-induced dieback on some upper branches of an otherwise healthy Persian (or English) walnut.
It isn't all that unusual for members of the walnut family to exhibit signs of dieback following a winter like the one we just had. It's not really a hardiness issue since walnuts are very hardy plants that often withstand winter temperatures as low -30C, but what they have a hard time dealing with is atypical cold temperatures.
For this reason, the ideal winter for a walnut is one that slowly descends into a deep freeze, hits rock bottom, then slowly rises back out again and in years when that's the case, walnuts tend to fair well.
Last winter, however, was kicked off by a severe, unseasonable deep freeze in the autumn, followed by an unusually mild winter and a long, cool spring during which frosts came and went as they pleased. That's exactly the kind of weather that walnuts object to and they have a tendency to express those objections by dying back, sometimes quite severely.
Gardeners usually become aware that dieback has occurred in the late spring when twigs - and in some cases entire branches - fail to sprout leaves.
Thousand Cankers disease, on the other hand, is usually noticed by gardeners not because their walnut is adorned with bare twigs, but because it's adorned with yellow or dead leaves on the end of very obviously dead branches, a condition that's known as “flagging”. It's not particularly subtle and the flagging tends to make the tree look like it's going to croak in the very near future, which is almost always the case.
There are several other reasons why Thousand Cankers disease isn't likely in the case of your walnut, the primary ones being that the disease strongly prefers black walnuts to all other kinds of walnuts and the beetle that piggybacks the disease into susceptible trees has a preference for walnuts with a thick phloem layer, which is found in trees much older than yours.
If your walnut continues to show signs of decline later in the summer then it would be worthwhile getting a tree specialist in to take a look at it, but chances are there is nothing for you to worry about.
If you have any questions or comments, please send them to me at vanessa@gardenmuse.ca.
