Q. I have a 20-year-old sumac tree that has developed yellow leaves in several areas. There has been no change in watering or the soil. What is causing this and how can we treat it. Branches dying in the bottom of the canopy of sumac. This may be due to too much shade. If not, it definitely contributed to the problem. A. If the yellowing or browning of the leaves is occurring in shaded areas it might be because there is not enough light reaching the leaves. If the canopy is dense and creates too much shade then leaves and stems in these heavily shaded areas will die. When the leaves are first dying they turn yellow and finally brown when they are dead.             Try removing some limbs on the tree to allow more light to penetrate inside the canopy. The problem is that African sumac responds very well to pruning with new growth and limb removal might have to be done regularly. Regrowth or watersprouts coming from a large African sumac limb after removal.             Do not remove too many limbs but allow the entry of filtered light inside the canopy.             An easy way to see if enough light is entering the canopy is to look at the ground. If the ground beneath the tree is a solid shadow, not enough light is entering the canopy. It should be pixelated. Some limbs should be removed until the light on the ground is “speckled”.             You can do this pruning any time of the year and do not have to wait for winter. I would focus on removing limbs around 1 inch in diameter and no larger unless the tree needs major pruning work done. Do major pruning only in the winter months.             African sumac does not have very many diseases so I have ruled out the possibility of disease. At least in our desert climate.. It has a few insect problems but nothing serious except aphids which leaves a sticky or shiny appearance on the leaf surface.             Aphids would be a problem in the spring and fall months. Heavy feeding by aphids could cause yellowing of leaves as well but they will be sticky. They can usually be removed with soap and water sprays.