It looks like a slight magnesium deficiency, Sdavis. There may also be too much sodium in the soil and/or water--the two problems often go together. To solve the latter, apply organic matter (compost, steer manure, "forest mulch", etc.) regularly, and try to water more at a time, but less often. In containers, water enough that a good deal of water comes out the drainage hole afterwards. Also be sure to empty any saucers out, 15 minutes or less after watering.
If that doesn't fix the magnesium deficiency, then add a little epsom salts to your fertilizer schedule. In the ground, one tablespoon per foot of plant diameter, every 6 months. In containers, about 1/4 teaspoon per gallon of water, once a month. Hope this helps!