Q: I got through winter relatively unscathed, but one problem is a large, 35-year-old rhododendron that has wilted, curling leaves on about a third of the branches. It’s a gorgeous thing with huge blooms every year. The leaves are still green, but is it doomed? Cutting them out will ruin the shape. Is this winter related? Is recovery possible?
A: I’d first suspect phytophthora root rot -- a common soil-borne fungal disease that kills rhodie roots and causes those saggy leaves.
I’m afraid you’ll lose the green color shortly and end up with brown leaves hanging from dead branches. About all you can do is prune off the dead branches and hope the disease doesn’t progress enough to gradually kill off the rest of the plant.
You’ll know if the branches are dead by scratching off a little bark to check for green or brown color. Or you could just wait until later in spring when everything has leafed out to see if you’re getting any new growth.
There is a fungicide soil drench called Alliette that’s supposed to be effective at controlling phytophthora rot. That might help limit further spread.
Sometimes as plants age, they lose some of the disease resistance they had in their youth (kind of like people). Poor soil drainage also is a contributing factor, so the wet weather we had last summer and the long, slow snow melt we had in the last few weeks only help fuel the spread.
Some of the worst outbreaks are when a drought year follows a wet year. It’s like a one-two punch where the wetness rots out the roots, then drought finishes off the weakened remnants.
Another possibility is the rhododendron borer. The larvae of this insect tunnels into stems and branches and feeds on the wood, often killing tissue above the feeding area. Inspect the stems of the wilting branches to see if you any slits or openings.
Again, there’s not much you can do at this point about any dead stems, but control involves spraying a borer-killing insecticide (permethrin is a common one) on the plants when the adults are active. That’s normally later May into early June. Adults look a bit like yellowjackets with clear wings.
If you ever lose the plant altogether, I’d switch species instead of replanting with another rhododendron or azalea. The pathogen or bug is already present and could reinfect/re-attack a new rhodie-family plant.
Plant a replacement rhodie in a new spot (morning sun, afternoon shade is ideal) with super-improved soil of 50 percent native soil, 25 percent sand and 25 peat moss or compost. Then plant an inch or two above grade and mulch only lightly (2 inches tops).