Iron Chlorosis- Yellow leaves
Trees that commonly suffer from iron chlorosis are pin oak, aspen, apple, elm, linden, silver maple and spruce as well as other trees growing in alkaline soils. Leaves have yellow margins with green veins. This can be localized to one area or branch, or throughout the entire tree. This condition is most prevalent on new growth.
Manganese deficiencies show similar symptoms as iron chlorosis. In some instances, a soil analysis needs to be performed in order to confidently diagnose the issue.
In Colorado, where the soils are typically alkaline, iron chlorosis can be a common issue. Roots have a hard time uptaking iron in alkaline soils and the trees become deficient in this micronutrient.
Iron plays an important role in synthesizing chlorophyll, the green pigment, which functions as the photosynthesizing mechanism in plants. This can lead to premature leaf drop and branch dieback, and can also result in death if severe enough.
Iron chlorosis is a subsurface issue. Sometimes, managing the root zone will green the tree back up. There are also iron solutions that can be injected directly into the trunk.
For more information on iron chlorosis see the Colorado State University extension bulletin #0.545: http://www.ext.colostate.edu/pubs/crops/00545.html