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Susan, are you sure it was Susan, are you sure it was the pine-needle mulch that killed your rhododendron?  If you haven't already, I'd suggest consulting a horticultural expert or a plant-disease specialist through your county (or state) Cooperative Extension office. Most states have labs that will diagnose plant diseases from samples and suggest remedies (if any), or at least help you track down the cause. 

2015 Special Edition Garden GuideCooking Fresh with The Old Farmer's AlmanacThe Almanac Monthly Digital MagazineWhat the heck is a Garden Hod?