What to do About Dieback on Mesquite

Q. Can you help me identify what is causing Mesquite trees to stress? I have attached photos here on this email. There’s sap releasing from previous cuttings, but there has been more die back from the top (side of the south) at a faster rate, any clue what is happening?

Mesquite tree dieback mentioned in the question. See how helpful good pics can be?

A. Thanks for the detailed pictures. That helps. The second picture you sent clearly shows where the mesquite tree was pruned (actually it was a very bad pruning job called “topping”). The tree grew vigorously from these cuts and then this new growth died.

Nothing Common

Nothing I could find has been observed in mesquite like this in the past. We are going to have to start making some educated guesses. Because it is in the new growth and seems to be very closely associated with growth from previous pruning cuts, I would guess it is what we call “a vascular disease” that plugs up the water carrying vessels from the roots that travel up the trunk and through the limbs. 

Maybe Unsanitized Pruning Tools

My second guess is that it MIGHT have been carried to this tree through the pruning tools. This could be particularly true if the pruning tools were not sanitized before pruning. The most common reasons trees are pruned is to remove dead or dying limbs. If these limbs were infected with a disease, it is possible to take them from an infected tree and pass them off to an uninfected tree. Development of the disease takes time. But if it was an aggressive disease this could happen in a season. If not terribly aggressive, two or more seasons.

This kind of problem sometimes causes sap “bubbles” to form on limbs. I call it "stress related".

What to do? 

Cut the infected plant parts out of the tree using a sanitized chainsaw. Remove at least 12 inches of wood below where the infection started. Disinfect after every cut so that it is not spread to other locations.

Another possibility is borer damage. Less likely but possible. If this is the case, do the same and disinfect just in case.