Depending on the time of year, the rolling and wilting of your rhododendron leaves could be caused by cold temperatures or a disease called rhododendron wilt. During the winter, rhododendron leaves often droop and curl in response to cold temperatures. It's their way of protecting themselves from dehydration. If you see drooping and rolling of foliage during the growing season, it's likely that your rhododendron has a wilt disease caused by the soilborne fungus phytophthora. Phytophthora is most often a problem in poorly drained, wet soils (such as that near a downspout). 

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Symptoms include stunted growth, leaf yellowing, and drooping leaves. Infected roots are dark and mushy instead of light tan and firm. Plants may be killed by rhododendron wilt. Fungicide treatment is ineffective. A better solution is to change the growing conditions. Improve soil drainage and aeration if you want to continue growing the plant in the same location. Incorporate compost or other organic matter to loosen heavy soils. Consider installing a raised bed to improve drainage, and transplant your rhododendron into the amended raised bed.