Sunburn/stress or Disease?

whaas_5a(5A SE WI)March 10, 2013

I've had this plant two years now and this is the second year that the old growth slowly defoliates and the new growth gets these dark spots and get worse as the season progresses.

With a combination of late afternoon sun (shaded up until 2pm) and neutral soil (well drained) it could be stress/sunburn but would like to know if this plant shows any signs of a particular disease.

Thank you!

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Winter burn from the effects of sun and wind is generally seen in the outer margins and tips of the leaves rather than along the midrib. The leaf to the lower left especially seems to show evidence of one of the many leaf spot diseases which can effect rhododendrons. This also the likely cause of the discoloration at the midrib in the leaf on the upper left and the dark brown necrotic spot on the leaf to that one's right.

These diseases are very rarely fatal and don't require any special treatment even if you could identify the specific causal agent. Cleaning up fallen leaves may help, but good care and nutrition for the plant is really all you can do.

    Bookmark   March 11, 2013 at 5:28AM
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rhodyman(SE PA, USDA Z6)

Sunscald is what you have: Large leaved rhododendrons are susceptible to sunscald if the plants do not receive enough moisture before the soil freezes. The leaves curl in cold winter weather, exposing the central part of the leaf but not the edges, resulting in the area near the main vein becomes desiccated and turns brown. To prevent this, water plants thoroughly before the first hard frost, protect from drying winds, apply mulch, and locate in partial shade.

Windburn: Wind and cold damage is indicated when the edges of the leaves become distorted and turn brown. This occurs frequently in plants that may be in a colder climate than recommended or in a location with severe exposure. Provide protection from winter winds and winter sun. Note, boron poisoning or fertilizer burn will create the same symptoms except uniformly over the plant rather than just on areas exposed to the sun and wind.

Here is a link that might be useful: Rhododendron Problems and Solutions

    Bookmark   March 11, 2013 at 2:58PM
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