My rhododendrons have brown patches around the leaf edges with some white, powdery coating this spring.
Can you tell me what this disease is and how I can address it?
I am happy to say that this does not sound like a disease, though you have some fungal growth in spots.
Your rhododendrons are showing classic symptoms of winter damage from wind and sun during ground freeze. It is most common in plants that are in sunny, exposed positions during winter and less common on plants in shadier areas. It is normal to see rhododendron leaves curl under as a strategy to retain moisture in the winter. Crisping of edges is often the next step as moisture is lost first along the leaf's tip and edges. Fungi may grow and bear spores on the scorched area, but they are not causing the damage you observed. This issue becomes most visible in spring.
It is not possible to reverse the damage so you can remove the scorched leaves. If your rhododendrons are in an area with wind exposure and little overstory protection from sun in winter, consider adding a windbreak as it's the combination of sun and wind that causes the worst problems. Water the plants well in the months leading up to winter. Mulch the roots after ground freeze to stabilize soil temperature and reduce ground water evaporation.
Courtesy of NYBG Plant Information Service
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