I have a small cottonwood approx. 10' tall that is growing well but some lower leaves are pale green to yellow and some have a burnt appearance around the edges. I was wondering if this is a disease or a water and fertilization problem.
|Cottonwood are extremely thirsty trees, which is why they are often seen growing near water sources in the wild. Often times home watering systems are not timed to run long enough to provide a slow, deep watering. When leaves brown or burn around the edges, the problem is often salt burn. This is common in the Southwest with low rainfall, alkaline soil and water high in salts. Browning usually occurs on the old leaves first. This excess salt accumulates in the leaf edges, where it kills the tissue and the leaf dries out and turns brown. It's important to water deeply, slowly and usually infrequently. With a young tree, you should water once a week, and make sure the water penetrates 2-3 feet deep. Use a pointed stick or piece of metal to push in the soil. It will move easily through moist soil and stop at dry soil. At least once a month, water deeply enough to "leach" or push salts well below the root zone. Frequent, light "sprinklings" or short time spans with drip irrigation, allow salts to accumulate in the top layers of soil, where the roots are, which is bad news.
Similar symptoms occur when too much fertilizer has been applied. It's generally not advisable to fertilize landscape plants in our summer heat. Plant roots often "burn." I hope this information helps!