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Diseased Rhododendrons ? fungal #871849
Asked June 06, 2024, 9:35 AM EDT
Chittenden County Vermont
Expert Response
Lois: As discussed over the call-in helpline phone this morning, I am sending this ticket back to you so you may attach a photograph of both sides of the leaf, and also the bark. This will help us to identify the problem.
Dear Lois,
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YOUR QUESTION #0141768:
Diseased Rhododendrons ? fungal
I would like to know if I need to remove our mature Rhododendrons or can remediation save some of them. they are 30 years old and a foundational plant in our front garden facing the road. Home visit might be needed.Ask Extension offers one-to-one expert answers from Cooperative Extension/University staff and volunteers within participating Land-Grant institutions across the United States.
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Lois: The photos were a great help in identifying the problem.
I was able to identify the white spots as Bark Scale. You should remove these by crushing them and rubbing them off the stem. Below is a link to a leaflet from the University of Maryland Extension on the identification and control of bark scale.
https://extension.umd.edu/resource/azaleas-and-rhododendrons-identify-and-manage-problems/
I asked the UVM Plant Pathologist for help in identifying the problem with the black spots on the leaves and stems, and she responded with the following information, along with links to control recommendations.
Don't remove the plants!!
The black sooty growth is a fungus called
sooty mold and it is not harmful to the plant as a pathogen but is
growing on the excrement of a sucking pest, such as aphids, scale,
whiteflies, etc. https://chemung.cce.cornell.edu/resources/sooty-mold
Once the pest is controlled, the sooty mold will go away. The scale
maybe be the culprit since that pest produces a lot of sweet sticky
excrement for the sooty mold. https://hortsense.cahnrs.wsu.edu/fact-sheet/rhododendron-azalea-bark-scale/
I would try to control the vulnerable stage right now (crawlers) with
one of the organic sprays listed above. I would keep up the vigor of the
plant through fertilization with an acid-loving plant fertilizer in the
spring after flowering. You may have to spray for a couple of seasons
to get the infestation under control.
I hope you find this information helpful. Thank you for contacting the UVM Extension Master Gardener Helpline!
