Various species of mesquite (Prosopis spp.), including the honey mesquite (Prosopis glandulosa), grown in U.S. Department of Agriculture plant hardiness zones 6 to 9, are enjoyed for their low water requirements and survivability in harsh arid environments. A handful of different factors could contribute to a worrisome yellowing of mesquite leaves. Although mesquite is not known to be highly toxic, seeds may cause digestive problems if eaten. In some cases, flowers are allergenic and the sap can cause irritation. The sharp thorns on some mesquites can also irritate or cause pain, so use caution around these plants.
Although mesquite trees are very drought tolerant, they still benefit from deep, infrequent irrigation during extended dry spells. Drought stress typically causes leaf yellowing or drop on a mesquite. Excessive or improperly applied water can be just as stressful to a mesquite as inadequate water, causing leaf yellowing or drop, dieback and other problems. Water the mesquite slowly and deeply only when soil 2 to 4 inches below the surface feels completely dry to the touch. When watering, allow the water to penetrate at least the top 24 inches of soil and distribute the water evenly over an area that extends at least to the edge of the mesquite's canopy. In general, an established mesquite benefits from a watering every seven to 21 days in summer and every 30 to 60 days in winter when rainfall is insufficient.
Mesquite trees are generally cold deciduous, meaning that they drop their leaves when cold temperatures or other stresses occur. If the leaves become paler and drop following the onset of cold weather or a freeze, it is not necessarily cause for alarm. However, in mesquites that are under drought stress, the discoloration is typically more pronounced.
Armored scales look like immobile bumps less than 1/8 inch in diameter on mesquite bark or leaves where they settle to feed. A small amount of scale feeding may go unnoticed, but heavy activity can cause leaf wilting, yellowing and premature drop and bark cracking. Severely affected mesquites may experience dieback and decline. Pruning off heavily infested branches and preserving the populations of natural scale enemies is the most feasible control option, so avoid the use of broad-spectrum, persistent pesticides and alleviate dusty conditions, which interfere with scale predators and parasites. Multiple species of roundheaded, or longhorned, borers, which generally only attack damaged or dying mesquites, bore into trees, leaving holes in the bark and stains or oozing liquid. Leaves on affected mesquite limbs turn yellow and wilt and entire limbs may die. Avoid injuring trees to prevent infestation and promptly prune off and destroy infested parts to prevent this pest's spread.
Nutrient and Mineral Considerations
With repeated shallow watering, in certain soils or following improper or several fertilizer applications, salts from fertilizers, soil and water can accumulate in the top several inches of soil. Excessive salts and nutrients and chemicals cause a yellowing or browning along leaf margins, leaf tip dieback and even branch dieback. Avoid salt burn and other toxicity problems by properly applying fertilizers and pesticides around the mesquite and watering the mesquite deeply and slowly to flush accumulated salts out of the tree's root zone.