5-6 weeks ago I bought a few rhodos in containers at a local nursery: Horizon Monarch, Anna Rose Whitney, Percy Wiseman. When I planted them I noticed that some leaf tips were cut off. I did not think much of it at the time. :-(
Quite soon I saw that some of the leaves were developing brown spots. The brown area always progresses from the edge (often the tip) of the leaf inward. Eventually the all leaf turns brown. More affected by this disease are the leaves at the base of the plant, and particulalry the ones directly attached to the woody (older) stems.
At first I kept removing only the leaves that were in the worst shape, with scarce results. A couple of day ago I decided to remove and dispose of all the diseased leaves.
Could phytophthora die back be the cause? Considering I live on the wet coast (Vancouver), do these rhodos stand a chance? Should I apply a fungicide? I have other established (and healhty) rhodos next to these new plants, may they be at risk?
I doubt if it is phytophera. If it was, there is nothing you could do.
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 is best mitigated by good sanitation and avoiding excessive moisture
Another possibility is rhododendron mildew. Light green or yellowish patches on leaves sometimes accompanies by brown spots on the back side of leaves is a sign of rhododendron powdery mildew (Microsphaera azaleae) . One of the puzzling aspects of this fungal problem is the fact that two different affected rhododendrons vary in appearance. Rhodendron cultivar 'Unique,' for instance, shows almost no upper leaf changes, other than occasional very faint lighter yellowish areas, while the underside of the leaves will be completely covered in brown spots. A deep green leaf may begin to show lighter green patches, and these areas will gradually become more yellow. Another cultivar, 'Virginia Richards,' gets brownish purple spots on both tops and bottoms of leaves. This common disease is named rhododendron powdery mildew despite how little the symptoms resemble the familiar fungal disease often seen on roses and azaleas. Usually the disease doesn't produce the familiar white powder-like spores, although late in the summer some may become visible. The disease manifests instead as color changes in the leaves, followed by defoliation toward the end of the growing season. Many rhododendrons, if basically healthy, will coexist with the disease and seem to outgrow or at least survive the symptoms. Last year's leaves, once they have been hit by the disease, will always have it, with symptoms persisting from year to year until the leaves drop off. High relative humidity at night and low relative humidity during day with 70-80 F (22-27 C) temperatures is ideal for the disease to flourish.
Keep rhododendrons healthy to help them manage this problem. If you notice symptoms on last year's leaves, consider protecting the new growth with a fungicide . Apply it now to the new growth as it expands, before symptoms appear on this spring's leaves. Fungicides won't get rid of the existing problem on old leaves. A new fungicide, 'Remedy,' which is a potassium bicarbonate (made by Bonide Company), is registered for the problem. Thorough leaf coverage is necessary with all fungicides. Fungicides containing sulfur (such as Safer Garden Fungicide RTU) are also registered. Others are Funginex (sold as Ortho RosePride Funginex Rose and Shrub Disease Control Concentrate.) Be very careful to read all label instructions, and wear protective goggles and gloves. Funginex can be corrosive to eyes. For more information see the section above on 'azalea powdery mildew'. The symptoms are different, but the organism and control are the same.
Rhododendrons that have been hybridized with Rhododendron cinnabarinum as one of the parent plants do get the disease severely. Two of those are 'Elizabeth' and 'Lady Chamberlain.' The Cornish Cross hybrids, including 'Virginia Richards,' 'Seta' and 'Mrs. G.W. Leak' seem vulnerable also.
"Stephen M. Henning" wrote in news: snipped-for-privacy@news3.west.earthlink.net:
I have 2 rhododendrons with very similar symptoms to what the original poster described. I posted some pics here.
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There were three of these plants that were planted a year ago by the landscaper. I lost one already last summer to root strangulation, because the lanscaper did not losen the rootball. I fear I am about to lose the other 2. These plants are north facing and I thought it might be winter burn too, but as both shrubs growing profusely, so do the brown spots. The other plant, not in these pics, looks the same.
Do you have any idea what's wrong with these plants and is it terminal?
2 out of 3 English Spreading yews he planted look sickly too and show no new growth, but that's another story.
You have a whole host of problems. I would guess that the plants are stressed. If they were doing better they would be more resistant to these problems. Some of the problems are:
Pestalotia leaf spot Wind and cold damage Black Vine Weevil and/or Strawberry Root Weevil Rhododendron gall midge
I wouldn't worry too much about the individual problems, but would worry more about the general health of the plants. They need good drainage. A wet root zone is a real killer. The need good acidic soil. They are sodium sensative, so salt will set them way back. They like a spring application of a good rhody fertilizer like Holytone. Residual herbicides or lawn fertilizer will set them back.
"Stephen M. Henning" wrote in news: snipped-for-privacy@news4.west.earthlink.net:
Stephen, thank you for the reply. I have removed the mulch (and in intend to destroy it). The crown fine roots look to be in good shape for the most part, though one side is pretty brown the 9/10's of the crown shows white fine shoots. The soil is pretty sandy but I know the guy who planted them was pretty clueless and they could be in a puddle below for all I know. If I remove it to look and perhaps remedy a problem, do you think it will take it?
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