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

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

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!