This is the second year in a row that my rhododendron Blue Peter has flower buds but they are dry and somewhat dark and have no flowers at all. These buds are easy to deadhead. Can you help me salvage this rhododendron, which is very old, and beautiful when it blooms?
In order to get an accurate diagnosis you will need to take a sample of your plant (including both healthy and affected parts if possible) to a Master Gardener clinic. If they do not know what it is, ask them to send the sample to the pathology laboratory in Puyallup. It is best to go through Master Gardeners first so you will not be charged. If you send the sample yourself there will be a fee.
Meanwhile, several sources mention frost, drought, and “bud-blast” (unlikely in the Pacific Northwest) as potential causes of bud failure. Damaged flower buds and poor bud set: It is always most disappointing when fat, healthy looking flower buds either fail to open at all or only open a percentage of their buds, the rest being black and dead. Some rhododendrons regularly abort some or even all of their buds for no apparent reason. This may be due in some cases to a deficiency, perhaps magnesium, or to drought reports from various places give mixed results from applying magnesium (usually as Epsom salts)… By far the most usual cause of bud damage is frost. Flower buds are invariably less hardy than the rest of the plant so a really hard winter is sure to cause losses to flower buds. Early autumn frosts can damage buds that are not fully hardened off. This is a very annoying type of damage that may be overlooked and may not be noticed until the buds attempt to open in spring. Rhododendrons vary greatly in their ability to harden up enough to withstand early frost. In areas very prone to spring frosts, it is better to avoid growing plants that always burst into growth at the first sign of spring. Plants that frequently loose their first growth flush (and sometimes even their second) are liable to become stunted and rarely flower.
Source: The Cultivation of Rhododendrons, by P. Cox, 1993, p. 244.