Ask Mr. Smarty Plants

That is strange,because Ilex vomitoria (yaupon) is ordinarily a most dependable plant, able to adapt to a lot of different conditions.  In terms of the "white stuff" on the Rosemarinus officianlis, that plant is native to the Meditteranean, and outside of our area of expertise at the Lady Bird Johnson Wildflower Center. We did learn that this member of the Lamiaceae (mint) sometimes developed powdery mildew if the plant gets too moist. If you are watering extensively and using overhead sprinklers to do so, that might be the source of the problem with the rosemary. However, if you have had your yaupons for several years, under the same conditions, it hardly seems likely the mildew would have spread from the rosemary and done that kind of damage.

Since we are not plant pathologists, we are at a loss to determine what would cause such a tough plant to die back so severely. About all we can do is ask some questions that might lead you to answers. Are you sure the shrubs are dead? Have you scratched the bark and found no living green material under the bark? If there are signs of life, you could try pruning them back, mulching the roots with compost or shredded hardwood mulch, and sticking the hose into the soil to let a slow dribble soak into the root area, and hope they leaf back out. Beyond that, have you seen any evidence of insect damage? Yaupons can be hosts to scale, leaf miners, mites, and aphids, but none of them are considered a serious threat to the vitality of the plant, and you would have seen some indication that the insects were there-holes or tunnels in leaves, honeydew (excretions from aphids and other insects) on which a black sooty mold can grow, or tiny red specks (spider mites) on the bottom of the leaves.

Once you have eliminated any of these possibilities, you need to consider that something external may have caused the problem. Has there been any spraying or spreading of weed and feed fertilizers, herbicides or insecticides?  Could someone have dumped paint or cleaning products into that area without your knowledge? We suggest you go to the website for Texas A&M AgriLIFE Extension for Bexar County, to the Horticulture section, where you will find contact information and, hopefully, someone who can tell you what happened to your yaupons.