Jane Magnolia with black leaves

I have a Jane Magnolia that has a problem with it's leaves turning black and drying up. The new growth seems to be alright. It has new growth at the end of one of the branches and some from the bottom of the plant as well. Can anyone tell me what the problem is? It's awfully hot down here in Texas at this time of the year. Can it be something as simple as the need for more water? If so, I'll feel awfully silly.

 

can you rub off the black? if so, it could be black sooty mold. Maybe you can post a picture?

 

Lilac blight is somewhat prevalent on deciduous magnolias up here.