Answer: It's difficult to diagnose a plant problem without actually seeing it, but here are a few thoughts:
Yellow mottling on the upper surface of leaves and black sooty mold and transparent insects on the bottom are symptoms of whitefly. They are more prevalent on certain varieties and on plants grown in protected areas. When damage first appears, it may be controlled by any of a number of contact insecticides. Care must be taken to spray the lower surfaces of the leaves where the whitefly live.
Yellowing of leaves surfaces, often with brownish burned areas, occurring on leaves that are more exposed to sun, is caused by more sun exposure than the plant is able to tolerate. Some varieties need shade, while all plants that have been protected from direct sun will be tender until hardened off by gradual exposure to sun light. Possible solutions are to give the plant more shade or move it to a more protected site.
Yellowing and dropping of leaves is normal toward the end of the second summer. Your azaleas should have dense enough habit that this doesn't matter.
Uniformly yellowish-green leaves is often just the need for more nitrogen. This will be more noticeable in the full sun. Some less sun tolerant varieties will always be light green in full sun.
Yellowing of leaf edges has been noted in gardens where sandy soil conditions or root competition with other plants caused insufficient soil moisture and nutrients. Usually incorporating organic material in the soil and removing the plants with the competing roots solved the problem. Care must be taken not to disturb the roots of the azaleas, and it is therefore best to prepare the soil adequately before planting.
Hope this information helps you determine the cause of the problem.
Spring is usually the best time to fertilize azaleas. Use a special acidified rhododendron and azalea fertilizer for best growth and apply in amounts as recommended on the fertilizer label.
Best wishes with your garden!
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