Disease in two rhododendrons - Ask Extension
Last year, the new buds/leaves/small branches on two of the rhodies began to die. The early signs were black spots/patches on leaves.
The problem is ...
Knowledgebase
Disease in two rhododendrons #864755
Asked April 16, 2024, 2:31 PM EDT
Last year, the new buds/leaves/small branches on two of the rhodies began to die. The early signs were black spots/patches on leaves.
The problem is much worse now.
Have been advised that they are inflected with an unknown pathogen and lace bugs. Advised also spread worm castings around their bases and mulch with hemlock out to the drip line.
I'd like to save the plants if possible and also prevent these diseases from spreading to my three other rhododendrons.
I'd also like a recommendation for a Benton County rhododendron expert who could help me in person.
Benton County Oregon
Expert Response
You have a very nice large Rhododendron there. The azalea lace bugs are a problem causing stress but will not the cause of the dieback. I suspect you were advised about mulching to help manage and/or prevent sunburn. Some aspects of your bushes tend to indicate sunburn but it is hard to see in these low resolution images. Some leaves have irregular spots on them that could be either sunburn or fungal leaf spotters due to any number of different fungi. See here for more details on these diseases: https://pnwhandbooks.org/plantdisease/host-disease/rhododendron-leaf-spots
But the dieback is more of the main issue where the shoots wilt, leaves die and entire branches seen to die back. I do not think this due to a root rot but it is possible. I suspect it is due to a fungal infection of the branch further down toward the trunks. There are a few fungi that can cause this especially in older bushes that have old pruning wounds on them. The images do not allow me to see something like this but you can investigate on your own. Check the cambium starting with the dead shoot tips and track it back to healthy tissue. You find a transition zone if this is the problem. Removing the branch below that point may be necessary to remove the problem. The following may be what the issue is all about: https://pnwhandbooks.org/plantdisease/host-disease/rhododendron-botryosphaeria-dieback
As for someone in your area to help, please contact the local master gardeners as they should be able to help: https://extension.oregonstate.edu/mg/linn-benton
But the dieback is more of the main issue where the shoots wilt, leaves die and entire branches seen to die back. I do not think this due to a root rot but it is possible. I suspect it is due to a fungal infection of the branch further down toward the trunks. There are a few fungi that can cause this especially in older bushes that have old pruning wounds on them. The images do not allow me to see something like this but you can investigate on your own. Check the cambium starting with the dead shoot tips and track it back to healthy tissue. You find a transition zone if this is the problem. Removing the branch below that point may be necessary to remove the problem. The following may be what the issue is all about: https://pnwhandbooks.org/plantdisease/host-disease/rhododendron-botryosphaeria-dieback
As for someone in your area to help, please contact the local master gardeners as they should be able to help: https://extension.oregonstate.edu/mg/linn-benton
