Answer: It is difficult to diagnose a problem with your tree without a little background information. Have you been fertilizing the tree? Did you happen to use a fertilizer containing an herbicide? Some herbicides can cause leaves to yellow and thicken. If you used a broadleaf herbicide in the root zone of your apple tree, that could affect the leaf color.
If you didn't use an herbicide in the root zone of the tree, how extensive is the yellowing? Is every leaf or the majority of leaves yellowing? This may indicate a pH problem and iron deficiency. Apply a fertilizer containing chelated iron to alleviate this problem. Sulfur can also be applied to the soil outside the dripline of the tree. The sulfur will help acidify the soil and make nutrients more available to the tree.
Have you been irrigating the tree regularly? We have recently changed from a rather wet spring to dry weather. Sometimes we don't begin irrigating as soon as necessary. A brief period of drought stress will also cause many leaves to turn yellow and fall from the tree. Water every two weeks to moisten the soil to a depth of 2 to 3 feet. Apply the water from the dripline of the tree outward. This distance outward depends on the size of the tree and the landscape around the tree. Wetting the soil deeply from the dripline outward 5 to 10 feet should be sufficient.
Finally, we have entered into a period when natural leaf yellowing and leaf drop occurs. As the weather becomes hot and dry, many trees dispose of surplus leaves to conserve water. You will find some of the first leaves formed this year are those that have yellowed and fallen. If this is the case, there is no cause for worry. You will also see some fruit drop as the tree naturally thins some of its fruit with the changing weather.
Hope this information helps you determine just what is going on with your apple tree.
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