Can you help me with my rhododendron? This spring’s bloom was the best in a long time. Now the leaves are turning a rust color, curling upward, and falling off. It seems to be happening in sections of the plant. Once a section becomes afflicted the progress of the affliction spreads rapidly. The plant is about 7 feet tall and 6 feet in diameter. It sits against the north facing wall of my house. Our summer has been more humid than usual. I have been using a chemical herbicide for a number of years to control weeds around the base of the plant.
A rhododendron in full bloom is a glorious sight and in many circles it is known as the king of evergreen flowering shrubs. I have a large rhodie myself and can understand your anxiety about losing such a magnificent shrub to pests or disease.
There are several environmental stress factors that can cause the symptoms your shrub is displaying. These include late freezes, drought, virus and disease, mites and insects. While we can’t control the weather, we can protect evergreen shrubs from late freezes by covering the plants over night during cold snaps. Just be sure to remove the covering before the sun gets intense in the morning.
Give them plenty of water during periods of drought.
The use of an insecticide may be necessary if there is evidence of mites or insects. Look for very fine webbing amongst the leaves for mites or in rolled twisted leaves from the larval feeding of the Gall Midge fly, a common pest.
The best way to determine if the problem is a virus or disease is to take fresh samples to your local extension service along with photos and they will be able to advise you.
The browning, cupping and sectioning also suggests possible herbicide poisoning. When the soil around the base of a shrub is treated with an herbicide, it can be absorbed by the plant roots. This is especially a problem if too much is applied at one time or if the amount applied is compounded by residue from the previous year. Also, if an herbicide is sprayed when temperatures are above 80 degrees F, it can evaporate into a vapor cloud and float up through your shrub causing severe damage or death. The safest way to apply an herbicide is to apply it in the morning when temperatures are cool and there is no breeze to cause it to drift into your valued landscape plants. Always follow the package directions to the letter.
I hope this information helps. It sounds like you have a spectacular shrub.