Disease resistant Rhododendron.

IpmMan(5)April 15, 2011

A customer of mine has two problem Rhododendron that always plagued with a variety leaf diseases. They are planted, watered and fertilized properly, receive the correct light, are in good soil and the PH is correct. Other Rhododendron in the same development look fine, and are not receiving fungicide treatments. I don't know what variety they are. I told her I could treat them every few weeks, but it would make more sense to replace them with a more disease resistant variety. Which Rhododendron, preferably white, are more disease resistant to the general leaf diseases? It is not important to know what the problems are at this point, I would ID them if I thought it were worth the trouble, these are just a problem variety. Oh it is Zone 5.


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morz8(Washington Coast Z8b)

You may find something helpful here:

Here is a link that might be useful: ARS Proven Performers by Zone

    Bookmark   April 15, 2011 at 11:06PM
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mainegrower(Z5b ME)

For Z5 four white rhodendrons: Boule de Neige, Cunningham's White, Wynterset White, Crete.

Cunningham's White is recognized as having excellent resistance to various root problems, not sure about leaf diseases. Boule de Neige is somewhat prone to lacebug damage in sunny locations. Crete has the thick indumented leaves of its yak parent. All four are vigorous growers and very hardy.

    Bookmark   April 16, 2011 at 5:35AM
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rhodyman(SE PA, USDA Z6)

IpmMan. Each rhododendron variety has different needs.

If the leaf problems are lace bug, then the plants need more shade.
If the leaf problems are weevils, then a more resistant variety is in order.
If the leaves are chlorotic, then soil testing is in order.
If the leaves are uniformly pale green, then spring fertilizing is in order.
If the leaves have brown edges, the wind and cold damage is indicated or fertilizer burn.
If the leaves are yellow or brown near the main vein, then it is sun scald.
If some leaves are yellow and stunted, then you probably had a drought.
If the leaves have crescent shaped holes, cranberry rootworm is the culprit.
If the leaves are wilting, then it more likely a stem or root problem.
If new foliage wilts in the heat of the day, that is normal.
Of course leafminer, aphid, gall midge, caterpillar, leaf roller, sawfly, cut worm, looper, whitefly, scale, and thrips all have their characteristic damage.

Resistance to which problem are you concerned about?

There is no one plant fits all solution. Yaks probably come the closest. Or you could find out what variety the plants that are not diseased are and chose those varieties. If you don't know the variety, you can send cuttings to Van Veen nursery in Portland, Oregon, and get them cloned. Kathy Van Veen could possibly identify the variety also.

Here is a link that might be useful: Rhododendron problems and solutions.

    Bookmark   April 16, 2011 at 9:15AM
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I did a Google search for leaf disease resistance and found nothing. Plenty of info on root disease resistance. As I said these Rhodies. are growing in the right environment. Yes there are lace bug, but no other pests. I know this customer and she follows a good fertilization and watering schedule. All Rhodies. get a wide variety of leaf diseases but they are usually minor and don't require attention. I thought there might be information on general resistance to these, but I guess not. This is a Condo type development, so I am sure the developer just planted what was cheap and available and she probably ended up with either a variety not suited to the zone, or just disease prone. I will do a bit more research to see if there is anything that I am missing, or she is doing that she has not told me about, before we dump them.

Thanks anyway.

    Bookmark   April 16, 2011 at 3:03PM
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rhodyman(SE PA, USDA Z6)

Rhododendron leaf diseases have names and the resistant varieties vary depending in the disease. Leaf diseases are primarily:

1) Powdery Mildew on Rhododendrons

Not too much is known about this disease. It seems to affect rhododendrons in the coastal areas, such as the Pacific Northwest, the UK and New Zealand, but apparently has little affect in areas where the winter temperatures are quite cold. Some varieties are damaged much worse than others and can be defoliated by the disease. Can be controlled with Benlate or Bayleton.

Powdery Mildew resistant varieties include: R. yakushimanum group; R. macrophyllum, R. ‘Nova Zembla,’ R. ‘Palestrina,’ R. ‘Vulcan.’

2) Rust

Orange-red pustules containing spores form on the lower surface of the leaf. Control by avoiding those hybrids and species which are very susceptible to rust. Good air circulation is helpful. Triadimefon seems to help, but may not be registered for use on rhododendrons.

Rust is fairly rare in rhododendrons and more common in deciduous azaleas. It involves an alternate host such as cedar or spruce.

3) Leaf Spotting - Fungi

There are a number of leaf spots or burns caused by fungi such as Botryis, Pestalotia, Phyllosticta, Septoria and others. Many are secondary infections happening after mechanical damage or environmental stress, such as sunburn, drought, winter damage or windburn. They generally occur during wet weather and many tlmes are self limiting with drier weather. Good sanitation is helpful, so remove brown and fallen leaves. Also provide good air circulation. Spraying with Benomyl or similar fungicide can be useful, but is frequently not necessary.

Resistant varieties include: ‘Boursalt,’ ‘Chionoides White,’ ‘Cunningham’s White,’ ‘English Roseum, ‘Le Barr’s Red,’ ‘Roseum Two’ and ‘Wissahickon.’

4) Leaf Spotting - Genetic

The vast majority of leaf spots that occur on many cultivars, including R. 'Blue Ensign' and R. 'Mrs. G. W. Leak', are physiological and not disease caused. These spots are generally purplish and are inherent in the cultivar. Environmental stress may increase their appearance. They do no harm to the plant.

Avoid plants with known genetic leaf spotting.

5) Leaf Spotting - Viruses

Some leaf spots are caused by viruses, the most common ailment being called necrotic ring spot. The symptoms are reddish-brown rings or spots on the leaves. It generally occurs only on the two year leaves of a few rhododendron cultivars such as R. 'Unique', or on Kalmia latifolia. It also appears on the first year foliage of some R. 'Loderi' clones. Little is known about the disease and a does not seem to spread from one cultivar to another. No control is known or generally necessary.

Avoid cultivars known to be susceptible to virus leaf spots such as Unique and the Loderi group.

6)Leaf Spotting - Chemical

Leaf spotting can also be caused by chemlcal injury, such as drift from cleaners, paints, or chemicals used to kill moss on roofs. Sometimes the results of such injury may not show up for weeks or months.

Avoid chemical injury in the first place.

    Bookmark   April 17, 2011 at 10:37AM
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rhodyman(SE PA, USDA Z6)

I apologize for using characters Garden Web doesn't support:

Powdery Mildew resistant varieties include: R. yakushimanum group; R. macrophyllum, R. 'Nova Zembla', R. 'Palestrina', R. 'Vulcan'

Fungal Leaf Spot resistant varieties include: 'Boursalt', 'Chionoides White', 'Cunningham's White', 'English Roseum', 'Le Barr's Red', 'Roseum Two' and 'Wissahickon'

    Bookmark   April 17, 2011 at 10:47AM
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Embothrium(USDA 8 Sunset 5 WA)

I'm currently losing two long-established 'Vulcan' to powdery mildew. Notice that the similar 'Vulcan's Flame' was used to illustrate powdery mildew infestation on this page.

Here is a link that might be useful: Rhododendron -- Powdery Mildew

    Bookmark   April 20, 2011 at 12:09PM
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davidrt28 (zone 7)

Bboy, isn't powdery mildew treatable by the most innocuous fungicide available - potassium bicarbonate? You can even use the easier to obtain sodium salt.

Here is a link that might be useful: Use of Baking Soda as a Fungicide

    Bookmark   April 23, 2011 at 9:29AM
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rhodyman(SE PA, USDA Z6)

Sodium kills rhododendrons, so avoid sodium salts.

Potassium bicarbonate was used on roses in a solution sprayed every 3 to 4 days. People who raise rhododendrons usually raise them because they don't want a plant like a rose that has to be sprayed all the time. I even don't like spraying deer repellant every 3 to 4 weeks. It was always teamed up with another product such as insecticidal soap or spray oil. On some plants it damaged the leaves.

An article in the January 2008 Rhododendron & Azalea News reported:

Prevention and treatment of powdery mildew can involve both cultural controls and chemical sprays. Fungicides will not remove the damage to infected leaves. However, if there are symptoms on last year's leaves, you might consider spraying the new Spring growth to protect against further infection.

Trifloriine (Funginex) and sulphur sprays, such as Safer's Garden Fungicide are registered for use by homeowners. Newer, potassium bicarbonate baking soda) based sprays show some promise. In all cases, full leaf coverage with the spray is essential. Follow label instructions closely and wear protective goggles and clothing.

Maintain healthy plants by providing good soil drainage and adequate water and nutrients. Stressed rhododendrons are more susceptible to disease. However, avoid over fertilizing, especially late in the season, as this practice encourages soft new growth that is a prime target for Microsphaera spores.

Inspect your rhododendrons regularly to identify infected plants and problematic areas on the garden.

Move infected plants to areas that have less favorable conditions for the disease. Be particularly careful to have plenty of space for R. cinnabarinum, R. thomsonii, and their hybrids. Try to plant them in areas with a fair amount of sunshine and good air circulation.

Prune to increase air circulation. At the University of British Columbia Botanical Garden, they do nothing to rhododendrons to control powdery mildew other than occasional thinning of both the surrounding material and the rhododendrons themselves.

Remove old, heavily infected leaves from rhododendron in Fall or Winter. Rake up diseased leaves that have fallen under the plant and burn them or dispose of them in the garbage.

If all else fails, discard heavily infected plants and replace them with rhodies that are more resistant to powdery mildew.

Here is a link that might be useful: Rhododendron & Azalea News

    Bookmark   April 23, 2011 at 9:52AM
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Thanks again. Now at least I have a few varieties to choose from. I know that we can treat most diseases, but in this case the person is not a Rhododendron lover, (beg forgiveness in this forum,) but just wants something evergreen and flowering, she is not interested high maintenance. I always stress the importance of planting resistant plants, then found myself stumped on this one when I realized that I was not aware of any Rhododendron that were resistant to leaf diseases.

    Bookmark   April 23, 2011 at 10:36AM
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rhodyman(SE PA, USDA Z6)

Yakushimanum hybrids and other plants with indumentum are the most resistant to leaf problems. One yakushimanum hybrid is very hardy and also very root rot resistant. It may be what you are looking for.

Ingrid Mehlquist (Besse Howells x yakushimanum), hardy to -25F
White frilly flowers emerging from lavender buds in small but very full and abundant trusses on a rock-hardy yak hybrid. This low, compact plant has great foliage and buds heavily from a young age. Resistant to root rot disease. Exceptional!

The downside is that Yaks need some shade to prevent sun scald. I am not sure if Ingrid Mehlquist has this problem but would assume it does.

    Bookmark   April 23, 2011 at 3:46PM
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Thanks Rhodyman. Interesting that you should mention the Ingred Mehlquist. I knew Gustav Mehlquist in passing, and have not thought about him in years. He was a character to say the least, and really loved his Rhododendrons.

    Bookmark   April 23, 2011 at 7:32PM
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