Ask Mr. Smarty Plants

According to this USDA Plant Profile, Prunus mexicana (Mexican plum) grows naturally in Harris County, so the soil, climate and rainfall should be appropriate for it there. The problem sounds to us like chlorosis, a condition usually resulting from some imbalance in the soil.

Please read this article from the University of Illinois Extension on chlorosis. Note the comment that the presence of chlorosis is often due to high alkalinity in the soil. We could not find out if your soil in  Harris County is particularly alkaline, but we usually expect East Texas soils to be more acidic.

Among the steps we would recommend are to use some sort of iron supplement, not too much, as native plants do not ordinarily care for fertilizer. Water less, because it's possible the main problem is lack of drainage in soil where the plants are growing. Watering once a week should be adequate.  Finally, using a good quality organic mulch, spread the mulch over the root area without allowing it to touch the trunk area. This will protect the roots from heat and cold and, as the mulch decomposes, will add some material to the soil to assist in drainage.

In answer to your question on whether this is a watering issue or a disease issue, here are the growing conditions for this plant from our webpage on Prunus mexicana (Mexican plum):

"Growing Conditions

Water Use: Low
Light Requirement: Sun , Part Shade
Soil Moisture: Dry , Moist
Cold Tolerant: yes
Soil Description: Dry to moist, well-drained soils. Sandy, Sandy Loam, Medium Loam, Clay Loam, Clay, Limestone-based"

As you can see, this plant is adaptable as far as soils are concerned, but does not like wet feet. You get more rain in Harris County than we do in Central Texas and if you are also watering, the roots could be standing in water, which they definitely do not like.

Next, on the subject of mites. Please read this article from the University of Missouri Extension on Aphids, Scale and Mites on Home Garden and Landscape Plants. In particular, note that these pests often will attack plants that are already under stress; for instance, if the soil in which the tree is planted does not have good drainage. Also, pay attention to the reminder to carefully read and follow instructions on the use of pesticides. The pesticide you are spraying could very well be killing instead the natural predators of the pests on your plant- ladybugs, for instance.