Borers may be source of roses’ dieback

Recently, I noticed that I did not entirely answer a few questions that were sent to me by e-mail. I receive quite a few e-mail questions. My goal is to answer all of them but sometimes a few slip through the cracks and go unanswered or some of the questions are missed. I tend to read them quickly and may miss a few points.

If that is the case with your question or I have misunderstood it, then e-mail it back to me and clarify. It does help me if you group your questions together so I can answer them point by point.

Q: I have three tree roses that bloom regularly each year. This year was no exception. Suddenly, in the middle of the summer, one of them started to lose its leaves. I assumed it was just the normal Las Vegas heat stressing the rose. It appears to be dead now. I scraped a small area on the trunk and it shows no green. There are suddenly five or six suckers growing from its roots. Is there hope for this rose? Attached are pictures of the one that has died.

A: These are great pictures. They tell a very good story. My newsletter readers will see them.

So we do know that the top has died back and the roots or rootstock has survived. Because we know this, we can assume it did not die because of root rots that can be a common problem with roses that are overwatered.

Because I don't get a lot of questions on tree roses I will stick my neck out and guess that the trunk has borers in it. I did a quick check online to confirm that borers can be a problem on tree roses and they are in other parts of the country. The size of your trunk is large enough to accommodate the laying of eggs by borers.

This is the correct time of year when borers cause the dieback of limbs and stems. I would guess that this is one of the flatheaded borers.

I'm also guessing that if you begin removing the trunk at soil level and put pressure on the trunk over your knee so that it snaps, it will snap where the borer has done the most structural damage. At this time of year they will normally begin to tunnel toward the center of the trunk to pupate and turn into an adult. The adult form, if it is a flatheaded borer, would most likely be a beetle. The larva or borer would be a cream colored "worm" about an inch long with a flattened head. If you dig through the trunk, you will most likely find one toward the center.

Bob Morris is an associate professor with the University of Nevada Cooperative Extension. Direct gardening questions to the master gardener hot line at 257-5555 or contact Morris by e-mail at morrisr@unce.unr.edu.