Hello, Mesto. Oak trees are good trees to grow rhododendrons under; other good trees are magnolias, some pines and dogwoods. On the other hand, you have "bad" trees like walnut, butternut, ash, beech, elm, some maples, poplar, pin oak, sycamore, black walnut.

Walnut and butternut trees have roots that produce a toxin that can kill rhododendrons and other plants. The others trees do not produce toxins but have characteristics that make it difficult to grow rhododendrons under them. For example, some of the trees are surface rooted like rhododendrons so there is intense root competition for moisture and nutrients. Others attract insects that cause problems.

I suspect your recent weather may have done them in. Do you see any evidence of bark split as a result of sudden cold weather following a spring thaw? Late spring freezes can cause the sap to re-freeze and rupture/break the bark. Note that, sometimes, the damage is not visible until later in the spring/summer.

Other things that I was wondering while considering other causes.... How long have the rhododendrons been in this location? How much sun do they get? If you insert a finger in the surrounding soil to a depth of 3 inches, does it feel dry, moist or wet? Do you see any indication of insects in the top or bottom of the leaves? Can you describe how the leaves are dying?

Luis

Have you had reports of Sudden Oak Death in your area?

"The most common symptoms on shrubs such as rhododendron are twig dieback and leaf spotting. Blackened shoots with or without foliage attached are typical of twig dieback caused by P. ramorum. Leaf spots are brown in color with diffuse, fuzzy margins."

There are other explanations though....it's not uncommon for rhododendrons that are planted with an intact compacted rootball to fail in about their third year (when the roots have circled each other to the point of strangulation)...You don't say how mature your plants are.

Drought can cause parts of rhododendrons or entire rhododendrons to die. Bark split as was mentioned above...After some severe winter temperatures here I would expect to see symptoms of bark split but I don't know what type of winter you had.

Insects that could cause the damage you describe are stem borer, and weevil larvae feeding on roots.

Here is a link that might be useful: Sudden Oak Death