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More on Mildew

Fred Minch
Puyallup, Washington

During the past few years there has been a lot of concern over powdery mildew. Powdery mildews have been known for a long time with 150 species on 7,700 host plants. As far back as the 1800s there have been articles written about powdery mildew. In a book published October 1875 there was an article written by Peter Henderson on powdery mildew on roses. He wrote that as a preventative using the fumes of sulfur, applied to and diffused by heated pipes in a greenhouse, was a never failing means of destroying the germs of mildew or any other fungoid growth. He also suggested dusting the foliage lightly with a mixture of sulfur and tobacco dust once a week, after the leaves had been wet by syringing or watering, to prevent mildew. The tobacco was for insect control.

The harmful species have caused much concern for the rhododendron growers, but the bright side is that it can and will be controlled by proper care. The program entails cleaning up all debris (leaves) and using a fungicide in the fall (sulfur, Funginex, etc.). When new growth appears use a systemic fungicide. Keep the plants healthy and free of stress with adequate water and fertilizer.

As mentioned above, there are many forms of mildew. However, rhododendron growers have been most concerned with the powdery type, although downy mildew has been suspected. (1) The downy mildew and powdery mildew can be confused with each other. The main difference between the two classes of mildew is that downy mildews are deep seated parasites and penetrate deeply into the cell tissues of leaf or stem, whereas the powdery mildews are superficial, living on the surface of leaves and stems, sending down only small suckers into the epidermal cells to obtain nourishment. The downy mildews by their deep-seated growth kill the internal tissues and are far more serious parasites than the powdery mildews, although the latter, if unchecked, are able to do a great deal of injury to plants by the smothering effect of their copious growth as well as by taking food from their hosts.

Powdery mildew has a whitish powdery, felted growth on the surface of the leaves. Downy mildew is indicated by yellowish patches on the surface of affected leaves, and on the under surface of the leaves it will appear as a whitish gray or even a bluish-gray mealy growth - symptoms similar to those of rust. The only true way to determine the source of the disease is with a magnifying glass or by consulting a pathologist.

Nigel Price, head gardener at Brodick Castle and Inverewe Gardens in northwest Scotland, wrote an article on powdery mildew for the Royal Horticultural Society's Rhododendron, Camellia & Magnolia Group Yearbook. He noted that the most susceptible were plants with root balls which suffered periods of drought and nutritional deficiencies, for instance, plants near tree roots. He first noticed it on 'Lady Chamberlain' in 1987. By 1988 there were approximately 200 showing signs of infection. He started feedings of a 4-ounce slow release fertilizer and a foliar feeding with a 2% solution of magnesium sulfate. He also tried to keep the plants stress free with adequate water and good routine care. Any factor that puts the rhody under stress will make it more susceptible to mildew infection, he said.

Root balls that are near tree roots and that suffer periods of drought and high humidity will tend to have nutritional deficiencies. Price notes that phosphorus and magnesium deficiencies can cause purple blotching and premature leaf fall - the same symptoms as powdery mildew.

Price established a program of using a systemic fungicide spray while the plant is growing, especially when buds are starting to open, and a contact fungicide spray when the plant is dormant to combat powdery mildew. When spraying it is necessary to have at least 20- to 90-pound pressure to turn the leaves and get the spray to the underside of the leaves, he said.

Cornell University is doing studies of bicarbonates to reduce the use of fungicides to combat fungal rose diseases. A study using anti-transparents, bicarbonates, sulfur and lime for control of powdery mildew are being tried. A 2% sunspray oil with sodium, potassium and ammonium bicarbonates ( NAHCO3, KHCO3 and NH4HCO3) are being used along with Funginex on dormant plants and systemic spray on new growth. I believe the successful use of bicarbonates against powdery mildew on roses should encourage the experimental use of bicarbonates on rhododendrons. I suggest that although bicarbonates are not effective as a preventative by themselves, applying the fungicides benomyl, thiophanate-methyl, or triadimeton on an alternating schedule with bicarbonates and oil could provide the necessary systemic activity for good mildew control.

In brief, I recommend that the best control is to remove infected plants or parts of them and preferably burn. Keep the plants as stress free as possible with proper use of fertilizer, watering and free circulation of air. Spray or dust with a suitable fungicide and an ultrafine spray oil (2%) combined with bicarbonate.

Resources
1.  Margaret Daughtrey, Dept. of Plant Pathology, Cornell University.
2.  D. L. Strider, Prof., Dept. of Plant Pathology, North Carolina State University.
3.  Ken Gibson, Tofino, B.C., Canada.
4.  Kenneth Cox, Perth, Scotland.
5.  Dictionary of Gardening, Royal Horticultural Society, pp. 1300-1301; 1992.

¹ Pathologists at the Clackamus Experiment Station, Clackamus, Wash., suspected downy mildew or rust on plants belonging to Ken Gibson of Tofino, B.C. The plants were viewed on slides for diagnosis.

Fred Minch, a member of the Tacoma Chapter, is a nurseryman in Puyallup, Washington.