Answer: Sounds like powdery mildew and you can use a fungicide to help control the disease. Whether or not you spray, be sure to remove and destroy any fallen leaves now and in the fall to try to limit reinfection. These trees prefer an evenly moist yet well drained soil (meaning not soggy) and have fairly shallow root systems so they are very susceptible to drought damage. You might try to water it throughout the root zone (out as far as the branches and often extending further) and then mulch the area with several inches of an organic mulch such as shredded bark to try to keep the soil moist and cool. Dig down a bit and see how effective your watering actually is; it can take an amazing amount of water to do a thorough job. Then, periodically check and see if the soil is dry an inch or two beneath the surface. If so, water again. Water slowly and deeply rather than daily light sprinkling. The frequency will depend on the soil type, the precipitation and the drainage pattern but a rule of thumb is an inch of water a week from the sky or the hose. These trees are not heavy feeders and any fertilizing would be done in the late fall or early spring. Hope your tree recovers soon!
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