Answer:rosemary and lavender are evergreen plants, they do tend to lose their oldest leaves. These are at the bottom and towards the center of the plant. Regularly pruning these plants will force new growth both at the center and at the ends of the stems and they won't become leggy or bare at the bottom. On the other hand, the symptoms you describe might also be attributed to root rot or crown rot which is usually the result of overwatering or planting in soil that does not drain well. If this is the case, there's not much you can do to save your plants. If you replace them, be sure to amend the soil so it drains quickly. The white powdery substance sounds suspiciously like powdery mildew, a fungal disease. Once it attacks a plant, it is difficult to control. You might try pruning off the affected plant parts. I hope your plants recover!
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