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Rhododendrum Disease?

Gardening Reference » Gardening in 2006
by Danno on June 18, 2006 12:41 PM
hey guys, heres some pics. and heres the story. . .

I baught these rhododendrums from home depot. one has been 3 years now, and the other i put in this year alone. Both of them, after planting, has grown a brown-rust color spot on alot of leaves. . and sometimes even a blackend with outer white ring (looks alot like a ciggarette burn) within this browned area.

NOW. . . I did NOT remove the flowering part of the first rhodo for 3 years. . and it never got very big, and the stem parts of the flowers were still there the year after. This last year however, i was reading on this forum and learned proper care of rhodo's by removing the flowering part after the flowers die off.

My new guy is doing the same thing, and this is newly planted just this year. Any suggestions? Soil has an acidic PH, i use Miracid on these plants, but am switching to organic acid material this year. I have TRIED daconil now for this last years fall & this years spring applications. Doesnt seem to have helped.

newly planted on this year. . .

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this is the 3rd year for this one here. . .
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by obywan59 on June 20, 2006 04:36 PM
I used to have that problem with rhododendrons. I think they need fertilizer. I use Holly-tone now. I fertilize in the fall after growth starts, and then again in early spring. Amounts depend on the size of the plant. I just follow the directions on the bag. Last year I didn't dead-head any of the flowers, and I had the most blooms ever this year. I'm thinking, with sufficient fertilization, deadheading is a waste of time. We'll see how it goes next year. One of my rhodies (about four foot tall and wide) had about 100 bloom clusters on it.

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Terry

May the force be with you
by RugbyHukr on June 20, 2006 04:40 PM
it could be a fungus OR lack of water before winter to keep the plant healthy into the spring

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I love the sweet scents wafting in the breeze. I stop to admire the vibrant colors of all living things. And people think me odd. Then ODD I am!!!

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by luis_pr on June 21, 2006 09:25 AM
Hello, Danno. I was puzzling about your pictures. Below are some thoughts/suggestions that I thought of.

Do you know what variety of rhodies you have? Brown, reddish-brown or purplish leaf spots that occur on many cultivars (like R. 'Blue Ensign' and R. 'Mrs. G. W. Leak') are physiological and not disease caused. Stress may increase their appearance.

I also thought about the possibility of fungal infections. Then I started wondering how much sun/water do they get? Is there any other type of shrubs around them that prevents having good air circulation around the rhodies? Are all of your rhodies having this problem or just some? Do they tend to have winter injuries? These things can promote development of fungal diseases.

You mentioned you were changing the type of fertilizer that you use and that started me thinking... how many times do you apply it? where do you apply it? What other chemicals do you use around the rhodies and on the rhodies? For example, aluminum Sulfate? Aluminum Sulfate should not be used around rhododendrons as aluminum becomes toxic to them in large quantities.

Also, be aware that rhododendrons are not heavy feeders and will flourish with just the food furnished by decomposing mulch. If the plant looks sickly, feed a small amount of cottonseed meal or Holly-tone in April and June. Spread it around the drip line. Do not feed any fertilizers after June as this will promote tender growth surely to be killed by your cold winters.

Also, note that your current choice of Miracid has too much nitrogen (the NPK Ratio is 30-10-10); a fertilizer with a ratio around 10-6-4 would be ideal per Oregon State University studies. Cottonseed Meal varies but is around a NPK Ratio of 6-2-1; Holly-tone is 4-6-4.

Last item... is your soil naturally acidic or alkaline? Have you ever done soil tests? If so, what were the results? Any mineral deficiencies or PH Problems? The reason for asking all that is, if your soil is too acidic, rhodies may have difficulty absorbing some minerals. And also, a shortage of the mineral Magnesium shows up in the loss of healthy green leaf color between leaf veins. The color gradually changes to yellow, then to a reddish or purplish color. Epsom Salts is a good source of supplemental magnesium; so is potassium-magnesium-sulfate (also known as Sul-Po-Mag), a product for sale in many organic nurseries. However, Sul-Po-Mag contains sulphur and should not be used in cases where the soil Ph is already very acidic (use Epsom Salts instead).

Hope that helps you,
Luis
by Danno on June 21, 2006 09:57 AM
great info guys!!! Thanks a ton! Yes i'm switching to holly tone as you said obywan. Great stuff! Espoma is my love of life now [Smile]

Rugby, i usually do lay off watering in the fall. maybe i'll water those a lil more for winter purposes.

Luis, great info bud! Very thoughtful post here. . . I DID use miracid, and not that much. maybe 3-4x a summer. But i will be feeding it like you say now, once @ fall & onceagain at spring. Soil is acidic slightly, it comes in roughly 4.7 on a mini tester i baught. Magnesium will NOT be a problem anymore, holly-tone is great supplement for plants. I'm sure i'll be happy after a year of using this stuff! And fungal like you all said was my first thing i thaught of, as i've been using Daconil now twice. (last fall and this spring). Thnx for the input guys, i'll keep pluggin away at er [thumb]
by luis_pr on June 21, 2006 10:18 AM
PS - I forgot to tell you something else. If a leaf has brown areas with white spots, it probably has a local fungal infection of Pestalotia leaf spot. This is seldom controlled with fungicides and "the" solution is to make sure you do not have excess moisture. For example, make sure the plant has good air circulation, water the soil under the plant instead of watering the leaves, water only in the mornings, properly dispose of plant debris under or near the plant, etc etc etc

Have a great day!

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