Answer: There are a number of potential reasons for premature leaf drop and I think the best way to determine just what the problem with your trees might be is to go through each potential problem and see if the symptoms match what is happening with your trees. Begin with leaf blight, a fungal disease which causes summer leaf drop. The leaves develop spots before they turn yellow so check the leaves closely for spotting. Insect feeding can cause similar symptoms and if you carefully check the leaves for a sticky substance (honeydew) or cast skins, you can either confirm or eliminate insect problems. Drought stress can also cause yellowing dropping leaves. River birches should be watered deeply once or twice a week during the growing season. Heat stress usually affects only young trees but if yours have not been in the ground for more than 10 years, they can be considered young trees. Eventually the trees will outgrow this when the root systems become better established. Or, it could be a problem called chlorosis. This is an iron difficiency. The tree does not take up enough iron or the soil in the area is too alkaline for iron to be available to the tree. Leaves on the tree turn yellow with green veins. Applications of iron sulphite can help. There's nothing you can do about heat stress, but you should be able to address the other common causes of leaf drop in your birches. Best wishes with them!
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