Brown and black spots on rhododendron leaves - Ask Extension
What is going on with my rhododendrons? I have many different rhododendrons and most have these large brown dead spots on their leaves that get bigger...
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Brown and black spots on rhododendron leaves #785624
Asked March 31, 2022, 1:24 AM EDT
What is going on with my rhododendrons? I have many different rhododendrons and most have these large brown dead spots on their leaves that get bigger over the season. There is also another problem (not sure if it's the same or a different cause) where the leaves have tiny brown/black spots on the backside which, on the front/tops of the leaves, look like whiteish dots (so close together the leaf looks white/pale). Do you have ideas what these problems might be and what if anything I should do about it?
Thank you!
Washington County Oregon
Expert Response
Thank you for your question, Kristine. The black spots indicate that the leaf is dying a bit at a time, probably from the high temperatures during the heat dome and inadequate water. Once temperatures begin rising again, give your plants plenty of water.
The stippling is caused by azalea lace bugs that live and feast on the undersides of the leaves. Here is an Extension article about this pest: http://pnwhandbooks.stage.extension.oregonstate.edu/insect/hort/landscape/hosts-pests-landscape-plants/azalea-rhododendron-azalea-rhododendron-lace-bug There is not much you can do, as the article explains.
I hope this is helpful. Good luck!
The stippling is caused by azalea lace bugs that live and feast on the undersides of the leaves. Here is an Extension article about this pest: http://pnwhandbooks.stage.extension.oregonstate.edu/insect/hort/landscape/hosts-pests-landscape-plants/azalea-rhododendron-azalea-rhododendron-lace-bug There is not much you can do, as the article explains.
I hope this is helpful. Good luck!
BTW, if you want to remove these leaves with brown spots (or any other damaged foliage), wait until the blooms on the plant have faded, and prune the leaves as well as the spent flowers. You can't safely do that with all of the leaves with the azalea lace bug, because you'd be removing too many leaves to sustain the plant. And they are likely to return. Here is another article: https://catalog.extension.oregonstate.edu/fs12/html
Thanks.
Good to know about the azalea lace bug.
I'm still wondering about the dead spots. These photos were actually taken last April (2021), well before the heat wave in July and August, so the dead spots can't be from that. They're on an automatic drip irrigation system. They have been in the yard, shaded under maple trees, on a slight slope, for over 20 years--maybe even 30 years. I'm attaching more photos hoping you might have other ideas about these large dead/dying spots.
Thank you,
Kristine
Thank you, Kristine. The more information, the better. Since there are 32 fungal, bacterial and environmental causes of rhododendron problems, whichever one(s) yours had can't be diagnosed through photos. Your plant may have too high a pH (not acidic enough), which could account for the interveinal yellowing. You could have the soil tested for that and nutrient content . https://catalog.extension.oregonstate.edu/em8677
If it is bacterial or fungal, you'd need to have a lab analysis, and the OSU Plant Disease Clinic, which can be accessed here: https://bpp.oregonstate.edu/plant-clinic I hope they can assist.
If it is bacterial or fungal, you'd need to have a lab analysis, and the OSU Plant Disease Clinic, which can be accessed here: https://bpp.oregonstate.edu/plant-clinic I hope they can assist.
I didn't know this existed. Thanks for letting me know about the OSU Plant Disease Clinic.
Kristine
You bet!
