Question
I have a pair of 'important' southern magnolias which are believed to be at least 150 years old. They have become incredibly stressed and i believe both have become infected by a borer which are leaving dark spots on the lower to middle trunks. i want desperately to save these trees and i have contacted arborists and university arborists in south carolina where these trees are located. I believe the problems started when we changed the driveway and installed pea stone on what was grass and probably have stressed/damaged the roots. Possibly the new driveway has deprived the trees of water.

I suspect that the treatment will consist of treating the borer and finding ways to insure proper watering and feeding.

How concerned should i be about the possibility of my other trees becoming infested if i attempt to treat these trees and it fails? What is the prognosis for my trees based on the facts as i described them? Any other info would be appreciated!!

Answer
Borers attack trees that are under stress. yes the recent digging would put the tree under stress. The added gravel and driveway damaged the root system of the trees making them more easily attacked by borer insects. I would treat the tree with a  product called Bayer Advanced tree and Shrub Insect control. This is a systemic insecticide that is applied to the soil around the tree. he roots transport the insecticide to the trunk and branches killing the borers in the wood. This product will protect the tree for the season. here is a web link to more information on this product. You can find this at your local nursery.

i would also fertilize the tree with 10-10-10 fertilizer at the rate of 1 lb per inch of trunk diameter scattered around the tree and watered in good. I would water with 1 inch of water if it does not rain for a couple of weeks--place a pan under the tree and turn the sprinkler on and when the pan has 1 inch of water in it stop. Do this once a week during dry periods.

Borers on one species will not usually infest other tree species. If the trees are kept healthy the borers will not attack the trees.
http://www.bayeradvanced.com/product/Tree-Shrub-Insect-Control/concentrate.html