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Reporting
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. 

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