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Plant Answer Line Question

Keywords: Plant diseases--Control, Fungicides, Powdery mildew diseases, Integrated pest management, Dahlia

What can I do about powdery mildew on my dahlias? Should I throw the bulbs away, or does it only contaminate the plant above the ground? I have heard both too much water and not enough water cause this problem. Is either true?


The main thing you will need to do is destroy all the foliage affected by the mildew. The mildew can survive the winter on infected foliage, and then spread to new foliage.

Powdery mildew thrives where plants are crowded and there isn't enough air circulation, so give your plants space, a sunny site, and try watering in the morning, and watering from beneath the plants (not over the leaves) so they are able to dry off during the course of the day. As you indicated, too little water can also be a problem.

Here are two websites with additional information:
Univ. of California IPM Online Guide
Washington State University Extension

I did not come across any information specifically saying that powdery mildew will affect bulbs or tubers. I spoke to an experienced dahlia and begonia grower here who said that it should be all right to store and replant your tubers, as long as you thoroughly get rid of all the diseased foliage aboveground.

Some sources (such as The Organic Gardener's Handbook of Natural Insect and Disease Control, edited by Barbara Ellis, Rodale Press, 1996) suggest that a baking soda spray (1 tsp. per 1 quart of warm water, with a bit of dish soap) is protective or preventive, but Washington State University Extension professor Linda Chalker Scott disputes the efficacy of this method. She says that other methods work better:
"Other treatments have been more successful in powdery mildew control, including horticultural oils, potassium bicarbonate, potassium phosphate, sulfur, milk, and even water sprays. Probably the most field success has been found in combining SBC [sodium bicarbonate] with horticultural oils, including mineral and vegetable oils (see the Fall 2008 MasterGardener magazine). The mixtures are so effective that they've been successful even on serious powdery mildew epidemics."

Date 2019-10-04
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Plant Answer Line Question

Keywords: Cornus kousa, Cornus florida, Powdery mildew diseases, Trees--Diseases and pests

Where can I find information about dogwood hybrids, especially crosses between Cornus kousa and C. florida? Won't these trees be more resistant to the mildew affecting many dogwoods?


In addition to powdery mildew, many dogwoods can suffer with anthracnose. In his book Dirr's Hardy Trees and Shrubs (Timber Press, 1997), Michael Dirr mentions Rutgers Hybrids (which are a cross of the kousa and florida species of Cornus). These trees were developed at Rutgers University by Elwin Orton, and are resistant to dogwood anthracnose. This article from North Carolina State University Extension discusses powdery mildew resistance. Scroll to the second table at the end which charts cultivars and their resistance or susceptibility to powdery mildew.

Oregon State University provides information about each of the six hybrids of C. florida x C. kousa. Two of the trees on this list are resistant to powdery mildew.

Clemson University Extension offers further information about the insects and diseases affecting dogwoods.

Date 2019-05-10
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Plant Answer Line Question

Keywords: Rhododendrons--Diseases and pests, Powdery mildew diseases, Plant diseases--Diagnosis, Master gardeners

My rhododendrons have a problem. What appears to be a white powder covers the buds and spreads up the leaves. What is it, and what can I do to stop it?


I cannot be absolutely certain without seeing the plants, but it sounds as if your rhododendrons could have powdery mildew.

Here is an article from the Washington State University Cooperative Extension which describes this disease. One preventive measure you should certainly take is to clean up all the fallen leaves and twigs under your rhododendrons, because the fungus which causes powdery mildew can overwinter there.

You could bring in a sample to a Master Gardener Clinic, and ask if they can diagnose the disease as well (they are at the Center for Urban Horticulture and other locations--see their website.

Date 2019-10-30
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Plant Answer Line Question

Keywords: Vegetables--Diseases and pests, Powdery mildew diseases, Perovskia

Last year I planted 25 gorgeous Russian Sage in 1 1/2 gal pots. They were fabulous last year, but barely came back this year. We had a mild winter here in New Jersey, but even so, I know that is not an issue with that plant as I've grown it for years. This year I planted an additional 40 in the same hillside and they are doing phenomenally well. On last year's plants, some have flowered a tiny bit, but none have come back to the size they were and most are so small, they look like they came from a 6-inch pot! Do you have any thoughts as to what I can do to improve the situation?

Also, although we haven't had much rain, I am seeing what I'm assuming is mold on several of my plants: trumpet vine, roses, honeysuckle. The zucchini and cucumbers have been totally decimated so there is no fruit. White is covering the leaves and with the veggies, the leaves are crumbling and disintegrating.


There are a few possibilities for the poor showing of your Russian Sage (Perovskia atriplicifolia) plants. However, without knowing more details about the growing conditions, I may be offering you advice that does not help. If the pots you selected to grow your plants in were made of a thin material, there is a chance you did not provide them with enough protection from the winter temperatures. Planting them directly into the ground can help to avoid this, if it's possible for you to do so in their location.

Depending on the amount of sun and moisture your plants are receiving, poor growth can result. Russian Sage plants like "a well-drained soil and need to have a warm to hot, sunny position" in the garden. (The Cultivation of Hardy Perennials by Richard Bird published in London by B.T. Batsford Ltd in 1994) If the soil is too wet, root rot can occur.

If they are in the proper cultural environment (lots of sun and well-drained soil), then perhaps they are lacking nourishment. The Plant Care Manual by Stefan Buczacki (published by Crown Publishers in New York in 1993), suggests feeding your plants with a general purpose fertilizer in mid-spring and in mid-summer.

As for your second question, I believe what you are describing is a case of powdery mildew. It is a fungus that shows itself in times of dry weather. The main thing you will need to do is destroy all the foliage affected by the mildew. The mildew on infected foliage will spread to new foliage.

Powdery mildew thrives where plants are crowded and there isn't enough air circulation, so give your plants space, a sunny site, and try watering in the morning, and watering from beneath the plants (not over the leaves) so they are able to dry off during the course of the day.

Here is a link to the University of California-Davis, Integrated Pest Management website. You can learn more about this fungus, including host plants, life cycle and management.

Colorado State University Cooperative Extension has some information about powdery mildew as well, including preventative measures and a recipe for making your own baking soda fungicide.

Date 2019-12-27
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Plant Answer Line Question

Keywords: Powdery mildew diseases, Nandina domestica

Can you give me some suggestions on how to treat a Nandina infected with powdery mildew?


Apparently, powdery mildew on Nandina is becoming a common problem in our area, as the article linked here indicates.

While this fungal disease is unsightly, it generally does not kill affected plants. Sometimes improving air circulation around the plant (by pruning congested growth) can help, and making sure to practice good sanitation by picking up fallen leaves affected by the mildew is also important.

There is an interesting idea in this Science News article on using milk powder in water as a spray to control the disease.

Several organic gardening sources recommend a baking soda spray. The Organic Gardener's Handbook of Natural Insect and Disease Control by Barbara Ellis (Rodale Press, 1996) recommends dissolving 1 teaspoon of baking soda in 1 quart of warm water. You can add up to a teaspoon of dish soap to make the solution stick to the leaves more effectively. Here is another source with slightly different recommendations, from the National Sustainable Agriculture Information Service.

More from University of California, Davis. Excerpt:

Shade and moderate temperatures favor most powdery mildews. Locate plants in sunny areas as much as possible, provide good air circulation, and avoid excess fertilizer. A good alternative is to use slow-release fertilizer. Overhead sprinkling may actually reduce the spread of powdery mildew because it washes spores off the plant; also, if spores land in water, they die. The best time to irrigate is in mid-morning so that the plants dry rapidly, reducing the likelihood of infections by other fungi, such as the ones that cause rust or black spot infections on roses. As new shoots begin to develop on perennial plants, watch closely for the appearance of powdery mildew.

Date 2019-05-18
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Garden Tip

Keywords: Powdery mildew diseases, Organic gardening

Powdery mildew season has begun (May), so act now to prevent or slow the development of this disfiguring (though not usually lethal) disease. Research published in Crop Protection demonstrated good results for preventing and managing mildew by spraying once a week with a 20%-40% solution of non-fat milk diluted in water. The down side to this organic remedy is the white residue left behind by the milk, which resembles the mildew we're trying to cure in the first place! Other less-toxic sprays that are new to the market are:

  • Eco E-Rase (also sold as Detur), a jojoba oil spray that smothers mildew spores;
  • Citrall Organic Lawn and Garden Fungicide derived from Backhousia citriodora (Lemon Myrtle), native to Australia;
  • Rose Defense made with Neem oil, which is not so new, but is still unfamiliar to many gardeners.

Go online to Colorado State University for a fact sheet on powdery mildew.

Date: 2007-04-03
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