The cedar deodar (Cedrus deodara) is a 70-foot tall tree that grows best within zones 7 through 9 on the U.S. Department of Agriculture Plant Hardiness Zone Map. This native of India bears somewhat drooping branches with light green needles. It's a "finicky tree to grow," according to Gerald Klingaman, extension horticulturist with the University of Arkansas. When the needles turn yellow it's time to do some detective work to find out what is troubling the deodar.
The cedar leafminer is the larvae of several types of moths. They tunnel into the foliage where they feed and overwinter. The caterpillars start on the outer portion of the cedar tree and work their way to the interior. Check the tips of branches for small caterpillars in early spring and prune off and burn or otherwise destroy infested branches.
All cedars, including deodara, are susceptible to fungal pathogens. The pathogens are most active during wet weather and periods of high humidity. Foliage may turn yellow, brownish-purple or brown. It may eventually fall, or remain on the tree. There's not much you can do about an existing infection except stop it from spreading. Spray the deodara with copper fungicide spray or a fungicide containing chlorothalonil.
When the deodara turns yellow and there is no indication of a pest infestation or fungal infection, it may be suffering from root rot, a condition to which this tree is particularly susceptible. The deodara must be planted in well-drained soil to avoid root rot. Plant pathologists with North Carolina State University suggest applying mefenoxam, a fungicide soil drench, to help prevent root rot. Once the tree shows symptoms of the disease there is nothing that can be done for it.
Some of the reasons a deodara turns yellow may be traced back to the soil. If the tree isn't getting enough water it may go into drought stress. Symptoms of this disorder include yellowing that begins at the top of the tree on the outer parts of the branches and works its way down and inside the tree. Sometimes yellowing is an indication of chlorosis, or iron deficiency. If the soil's pH is too high, the tree's roots can't absorb iron from the soil and requires chelated iron supplements to correct the deficiency.