help troubleshoot problem - see pic

johndoug(z6 Philly)May 27, 2006

these are leaves from my rhododendron. it kinda has buds but is not really doing much at all. it's neighbors are doing great. is this one a goner?

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rhodyman(SE PA, USDA Z6)

Those photos aren' t much help. We need to see the fronts and backs of leaves closeup. Here are some possibilities.

1) 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.

2) Small bright yellow spots is a symptom of azalea rust, a fungal disease. 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.

3) 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.

4) 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.

5) 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.

6) Yellowing of leaves surfaces, often with brownish burned areas, occurring on leaves that are more exposed to sun, is caused by more sun exposure than the plant is able to tolerate. Some varieties need shade, while all plants that have been protected from direct sun will be tender until hardened off by gradual exposure to sun light. Possible solutions are to give the plant more shade or move it to a more protected site.

    Bookmark   May 28, 2006 at 8:32AM
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