Rhododendrons diseased; treatment suggestions?

Discussion in 'Ericaceae (rhododendrons, arbutus, etc.)' started by Gwen Miller, Apr 8, 2020.

  1. Gwen Miller

    Gwen Miller Active Member

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    North Vancouver
    My large old Rhododendrons are unsightly with disease. ( And yes, I have read previous posts appearing under 'Rhododendron bud blast.') My plants still grow fairly well, and grow vigorously in response to pruning.

    They appear to suffer from bud blast (guessing Pycnostysanus azaleae fungus with leaf hopper as the vector,) leaf spot fungal disease(s), weevils, and possibly more. I’d appreciate identification of the problems, the vectors, and treatment suggestions.

    The two plants are between 50 and 70 years old. They were healthy when I bought the home 23 years ago, though leaf hoppers were significant even then. They’re trees, about 13 feet tall and of a similar width. There is roughly 8’ between their canopies. They’re situated in the open with a southern exposure. (One Rhodo was formerly sheltered by the neighbor’s 40’ cedar, which was removed roughly 15 years ago. Laurel shrubs have taken over the cedar’s place, and now provide some later afternoon shade to the one plant.) I gave very little care to the bushes for a number of years. They were rarely watered, and I’ve been wary of fertilizing them, only using fertilizer spikes on two occasions. They’ve been lightly limed a couple of times. The fallen leaves were often left on the ground below the trees. The old dead buds have black fruiting bodies, and as the high growth is out of reach, the dead buds stay on the tree for years. I understand the fungus remains viable in the diseased buds for three years. I pull off as much of the unsightly foliage as is practicable, but the problem has worsened over the years. I did the first significant pruning to improve airflow last summer, and am now cleaning as much of the leaf litter as I can. Watering has been better over the past few years, but I’ve been using an oscillating sprinkler, which I now understand spreads disease. In fact, the leaf spot has dramatically increased over the past few years.

    One of the trees looks better than last year, due to pruning and cosmetic leaf removal. Starting work on the second tree today, I doubt that my efforts can solve the problems. The size and situation of the trees makes them virtually impossible to manage myself. They’re on a heavily-junipered slope leading down to a 5’ drop to the boulevard. I had considered buying or renting a large 3-leg ladder, but trying to perch on and between ancient junipers on that slope is lunacy.

    Perhaps hiring someone with a cherry-picker to groom the trees and apply fungicide, or even pesticide, is the only solution. A drastic step would be a massive prune, removing all foliage and diseased buds, but I’d want the trees to recover their bloom and substantial appearance within a couple of years. I’m not sure that’s a realistic recovery time.

    I’d appreciate diagnoses and treatment recommendations. And if a professional service seems warranted, can you recommend a firm? I’m in North Vancouver.

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  2. Margot

    Margot Generous Contributor 10 Years

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    Nanoose Bay, BC Canada
    Those must be beautiful old rhodos and I can understand why you'd want to do everything you can to ensure many more years of health for them.

    I am very familiar with bud blight from the many years I lived with that problem on some of my rhodos in Burnaby. I never found any solution then but hopefully someone can offer practical suggestions. Certainly, discarding the diseased buds is great but problematic for you - not worth risking your life, that's for sure.

    Here are a few thoughts (if you're not doing these things already):
    - Apply a thick mulch under the plants. Something like arborists' wood chips that allows water to permeate but keeps moisture in the soil would improve the overall health of the trees and thereby allow them to fight off infections. Remember to keep the mulch a few inches away from the trunks.
    - Water regularly; even on the North Shore, rainfall is not as regular or plentiful as it used to be.
    - Fertilizing is a controversial subject when it comes to rhodos. Some say to leave them, well-mulched, and allow mycorrhizal fungus to establish and provide phosphorus and other nutrients naturally. Others advise twice-yearly applications of special rhodo fertilizer. Some members of the rhodo society I belong to, do both.
    - Consider hiring an arborist to selectively prune the plants and allow more air circulation - this might reduce the fungal problems on buds and leaves.
    - Join the Vancouver Rhododendron Society to access the knowledge of experienced rhodo growers in your area. Even if you cannot attend many meetings, you can still learn a lot from an organization like that.
    Gwen Miller likes this.

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