The symptom called chlorosis is a yellowish coloration of the foliage. The leaves may turn pale yellow-green to bright yellow, depending on the tree species and the severity of the problem. The leaves may be uniformly yellowish or, more often, the veins are green while the area between is yellow. Severe chlorosis may lead to scorch on leaf margins.
Chlorosis is usually caused by lack of some nutrient, especially iron or manganese, in the leaves. This deficiency may be due to lack of fertility, but may also result because the element, while present in the soil, is unavailable to the roots. This condition is especially common in alkaline soils such as occur over most of North Dakota.
Birches and maples seem especially prone to chlorosis, and should be avoided if high soil pH problems are known to occur. Chlorosis is extremely common on silver maples. Some trees may show severe chlorosis year after year and yet make good growth otherwise. Individual trees vary in appearance of chlorosis even when grown under identical conditions. No reason for this variability is known, other than genetic variation.
Control: Foliar applications of iron chelate or other iron-containing compounds will sometimes temporarily correct the chlorosis. Where practical, adjustment of the soil pH and proper application of micronutrient fertilizer may offer the best long-term solution. Micronutrient capsules implanted in the trunk may provide a remedy lasting for several seasons but mixed results have been reported from this therapy.