Leaf spots have several origins:

* Brown, reddish-brown or purplish 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.

* Some leaf spots are caused by a virus thought to be a potexvirus, the most common ailment being called necrotic ring spot [ http://plant-disease.ippc.orst.edu/disease.cfm?RecordID=1241 ]. 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.

* Leaf spotting can also be caused by chemical 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.

* 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 times 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.

Phyllosticta is a fungus found on wild as well as cultivated rhododendron. Spots are dark brown, zonate, and frequently cover half the leaf area. The spot's upper surface may feel bumpy due to the fungal fruiting bodies (pycnidia). Control: Kocide DF or the Mancozeb-based products: Fore 80 WP, Pentathlon DF, Protect T/O, Microcop, Spectro 90 WDG (chlorothalonil plus thiophanate methyl) or Zyban WSB (Not to be confused with the smoking cessation drug)

Phomopsis rhododendri symptoms vary from leaf spots to chlorosis and then browning of leaves which then wilt. Browning streaks extend down the stem to a wound. Fungicides such as metalaxyl (Subdue) should control an outbreak. Sanitation and applying a fungicide after pruning may provide control.

Small bright yellow spots is a symptom of azalea rust, Puccininastrum vaccinii, a fungal disease. Orange-red pustules containing spores form on the lower surface of the leaf. This rust is only sporadically severe and typically infects deciduous azaleas. 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.

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. Rhododendron 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.

* A number of insects cause leaf spots:

Yellow mottling on the upper surface of leaves and black sooty mold and transparent insects on the bottom are symptoms of Azalea whitefly (Pealius spp.) and Rhododendron whitefly (Dialeurodes spp.). They are more prevalent on certain varieties and on plants grown in protected areas. These small white flying insects look like an aphid with wings and suck on the underside of foliage, leaving white spots where it has been. Heavy infestations cause the margins of terminal leaves to cup. These infested leaves will eventually turn yellowish and appear wilted. The lower leaves become covered with honeydew, followed by sooty mold (a black coating). To check for the presence of whiteflies, shake the terminals of white azaleas to flush out adult whiteflies which look like tiny white moths. Examine the lower surfaces of leaves for the presence of nymphs, which are flat, yellowish green, and resemble scale insects. All stages occur on the under sides of leaves. This whitefly is usually limited to varieties of the snow azalea, Rhododendron mucronatum. If the infestation is light, little or no plant symptoms are evident, and if beneficial insects are present, spray the undersides of leaves with insecticidal soap or a horticultural oil at the 2%summer rate. If the infestation is heavy use a registered residual insecticide such as Malathion, Diazinon or Orthene.

Whitish specks on the upper surface of leaves and dark spots varnish-like on the bottom are symptoms of rhododendron lace bugs, Stephanitis rhododendri, and azalea lace bugs, Stephanitis pyrioides, small insects with transparent wings on under-surface of leaves.. This insect hatches early in spring as the new foliage begins to mature and its numbers may build to damaging levels with successive generations. Lace bugs reach their peak in late summer and do their worst in sunny, exposed sites. Spiders are important predators of lace bugs and since they shy away from sunny, hot places, plant your azaleas where there is some shade. Insecticidal soap, horticultural oil, or a systemic insecticide may spare your azaleas the damage if applied in spring when the first hatchlings are noticed. Care must be taken to spray the lower surfaces of the leaves where the lace bugs live. Moving a plant to an area with more shade may alleviate the problem. Lace bugs are more prevalent on certain varieties. The following azalea cultivars have resistance to azalea lace bug: Dawn, ÂPink Star, ÂEreka, ÂCavalier, ÂPink Fancy, ÂDram, ÂSeigei, ÂMacrantha, ÂSalmon Pink, ÂElsie Lee, ÂRed Wing, Sunglow and ÂMarilee.Â

Small white spots on the underside of leaves and small white flies on under-surface of leaves may indicate in infestation of Azalea whitefly (Pealius spp.) and Rhododendron whitefly (Dialeurodes spp.). They are more prevalent on certain varieties and on plants grown in protected areas. These small white flying insects look like an aphid with wings and suck on the underside of foliage, leaving white spots where it has been. Heavy infestations cause the margins of terminal leaves to cup. These infested leaves will eventually turn yellowish and appear wilted. The lower leaves become covered with honeydew, followed by sooty mold (a black coating). To check for the presence of whiteflies, shake the terminals of white azaleas to flush out adult whiteflies which look like tiny white moths. Examine the lower surfaces of leaves for the presence of nymphs, which are flat, yellowish green, and resemble scale insects. All stages occur on the under sides of leaves. This whitefly is usually limited to varieties of the snow azalea, Rhododendron mucronatum. If the infestation is light, little or no plant symptoms are evident, and if beneficial insects are present, spray the undersides of leaves with insecticidal soap or a horticultural oil at the 2%summer rate. If the infestation is heavy use a registered residual insecticide such as Malathion, Diazinon or Orthene.

helpful

PJMs go through a lot of color changes. Here's a picture of one of mine taken a little over a week after you first posted. Perhaps yours is just going through Fall color changes before the old leaves drop. It's one of my favorites.
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