Stephen Nameth 

Jim Chatfield

Almost all landscapes have plants that become diseased with one of the powdery mildew fungi. Although the fungi that cause powdery mildew are usually different on different plants, all of the powdery mildew diseases are similar in appearance. In most cases, prompt recognition and control actions can prevent severe damage to plants from powdery mildew diseases.

Symptoms

Powdery mildews, as the name implies, often appear as a superficial white or gray powdery growth of fungus over the surface of leaves, stems, flowers, or fruit of affected plants. These patches may enlarge until they cover the entire leaf on one or both sides. Young foliage and shoots may be particularly susceptible. Leaf curling and twisting may be noted before the fungus is noticed. Severe powdery mildew infection will result in yellowed leaves, dried and brown leaves, and disfigured shoots and flowers. Although it usually is not a fatal disease, powdery mildew may hasten plant defoliation and fall dormancy, and the infected plant may become extremely unsightly. On roses, uncontrolled powdery mildew will prevent normal flowering on highly susceptible cultivars.

Hosts

Powdery mildew fungi infect almost all ornamental plants. They are commonly seen only on those plants more naturally susceptible to the disease. Susceptible woody plants include some deciduous azaleas, buckeye, catalpa, cherry, a few of the flowering crabapples, dogwood, English oaks, euonymus, honeysuckle, horse chestnut, lilac, privet, roses, serviceberry, silver maple, sycamore, tulip tree, some viburnums, walnut, willow and wintercreeper. Powdery mildews are also common on certain herbaceous plants, such as chrysanthemums, dahlias, delphiniums, kalanchoes, phlox, Reiger begonias, snapdragons and zinnias. Remember that each species of powdery mildew has a very limited host range. Infection of one plant type does not necessarily mean that others are threatened. For example, the fungus that causes powdery mildew on lilac does not spread to roses and vice versa.

Environment Favoring Powdery Mildews

Most powdery mildew fungi produce airborne spores and infect plants when temperatures are moderate (60 to 80 degrees F) and will not be present during the hottest days of the summer. Unlike most other fungi that infect plants, powdery mildew fungi do not require free water on the plant surface in order to germinate and infect. Some powdery mildew fungi, especially those on rose, apple, and cherry are favored by high humidities. Overcrowding and shading will keep plants cool and promote higher humidity. These conditions are highly conducive to powdery mildew development.

Control of Powdery Mildews

Before using fungicides you should attempt to limit powdery mildews by other means. The following cultural practices should be beneficial for controlling powdery mildews.

• Purchase only top-quality, disease-free plants of resistant cultivars and species from a reputable nursery, greenhouse or garden center. Horticulturists in the green industry and Extension offices should be consulted concerning the availability and performance of resistant varieties.
• Prune out diseased terminals of woody plants, such as rose and crabapple, during the normal pruning period. All dead wood should be removed and destroyed (preferably by burning). Rake up and destroy all dead leaves that might harbor the fungus.
• Maintain plants in a high vigor.
  • Plant properly in well-prepared and well-drained soil where the plants will obtain all-day sun (or a minimum of 6 hours of sunlight daily).
  • Space plants for good air circulation. DO NOT plant highly susceptible plants–such as phlox, rose, and zinnia–in damp, shady locations.
  • Do not handle or work among the plants when the foliage is wet.
  • Water thoroughly at weekly intervals during periods of drought. The soil should be moist 8 to 12 inches deep. Avoid overhead watering and sprinkling the foliage, especially in late afternoon or evening. Use a soil soaker hose or root feeder so the foliage is not wetted.

Chemical Control of Powdery Mildews

In many cases, powdery mildew diseases do little damage to overall plant health, and yearly infections can be ignored if unsightliness is not a major concern. For example, lilacs can have powdery mildew each year, with little or no apparent effect on plant health. On some plants, powdery mildews can result in significant damage. Thus, fungicides must be used to achieve acceptable control. For best results with fungicides, spray programs must begin as soon as mildews are detected. Spray on a regular schedule, more often during cool, damp weather. Use a good spreader-sticker with the fungicides. Be sure and cover both surfaces of all leaves with the spray. Fungicides generally recommended for powdery mildew control include: Triadimefon (Bayleton, Strike); Triforine (Funginex), Thiophanate-methyl (Cleary’s 3336, Domain) and Propiconazole (Banner).

Powdery Mildew

Quick Facts…

• Powdery Mildew outbreaks are increasingly more difficult to treat on indoor hydroponic gardens
• Powdery mildew is one of the most widespread and easily recognized plant diseases.
• Powdery mildews are characterized by spots or patches of white to grayish, talcum-powder-like growth.
• Powdery mildews are severe in warm, dry climates.
• Many plants have been developed to be resistant or tolerant to powdery mildew.
• Succulent tissue is more susceptible to infection. Once the disease is a problem, avoid late summer applications of nitrogen fertilizer.

Powdery mildews are one of the most widespread and easily recognized plant diseases. They affect virtually all kinds of plants: cereals and grasses, vegetables, ornamentals, weeds, shrubs, fruit trees, and broad-leaved shade and forest trees. In Colorado, powdery mildews are common on ash, lilac, grape, roses, turfgrass, vegetables (such as cucurbits and peas), euonymus, cherry, apple, crabapple, pear and Virginia creeper, among others.

Symptoms

Even though there are several types of powdery mildew fungi, they all produce similar symptoms on plant parts. Powdery mildews are characterized by spots or patches of white to grayish, talcum-powder-like growth. Tiny, pinhead-sized, spherical fruiting structures that are first white, later yellow-brown and finally black, may be present singly or in a group. These are the cleistothecia or overwintering bodies of the fungus.

The disease is most commonly observed on the upper sides of the leaves. It also affects the bottom sides of leaves, young stems, buds, flowers and young fruit. Infected leaves may become distorted, turn yellow with small patches of green, and fall prematurely. Infected buds may fail to open.

Conditions That Favor the Disease

The severity of the disease depends on many factors: variety of the host plant, age and condition of the plant, and weather conditions during the growing season.

Powdery mildews are severe in warm, dry climates. This is because the fungus does not need the presence of water on the leaf surface for infection to occur. However, the relative humidity of the air does need to be high for spore germination. Therefore, the disease is common in crowded plantings where air circulation is poor and in damp, shaded areas. Incidence of infection increases as relative humidity rises to 90 percent, but it does not occur when leaf surfaces are wet (e.g., in a rain shower). Young, succulent growth usually is more susceptible than older plant tissues.

About the Fungi

Powdery mildews are host specific — they cannot survive without the proper host plant. For example, the species Uncinula necator, which causes powdery mildew on grape and linden, does not attack lilac. Similarly, Microsphaea alni affects elm, catalpa, lilac and oak but not turfgrass.

Powdery mildews produce mycelium (fungal threads) that grow only on the surface of the plant. They never invade the tissues themselves. The fungi feed by sending haustoria, or root-like structures, into the epidermal (top) cells of the plant. The fungi overwinter on plant debris as cleistothecia or mycelium. In the spring, the cleistothecia produce spores that are moved to susceptible host tissue by splashing raindrops, wind or Insects.

Control

Organic

There are many choices when it comes to treating and curing a powdery mildew outbreak. There are several that are organic and work relatively well. SNS 244 is a rosemary based extract formulation made by SNS, applied as a foliar spray works effectively on powdery mildew outbreaks. There is also PM Wash which is frequency water based treatment with promising results. Then there is No Powdery Mildew™ which is by far the best application in my opinion on the market today. No Powdery Mildew™ works immediately and on contact. Made from natural plant oils and extracts, No Powdery Mildew™ has a dual lysis action that attacks powdery mildew spores on contact while penetrating your plants cell walls giving them added strength and vigor.

Cultural

Several practices will reduce or prevent powdery mildews. Many plants, such as roses, vegetables and Kentucky bluegrass, are developed to be resistant or tolerant to powdery mildew. Inquire about resistant varieties before a purchase. If resistant varieties are unavailable, do not plant in low, shady locations.

Once the disease becomes a problem:

• Avoid late-summer applications of nitrogen fertilizer to limit the production of succulent tissue, which is more susceptible to infection.
• Avoid overhead watering to help reduce the relative humidity.
• Remove and destroy all infected plant parts (leaves, etc.). For infected vegetables and other annuals, remove as much of the plant and its debris in the fall as possible. This decreases the ability of the fungus to survive the winter. Do not compost infected plant debris. Temperatures often are not hot enough to kill the fungus.
• Selectively prune overcrowded plant material to help increase air circulation. This helps reduce relative humidity and infection.

Chemical

If cultural controls fail to prevent disease buildup or if the disease pressure is too great, an application of a fungicide may be necessary.

These include:

• sulfur,
• neem oil (Rose Defense, Shield-All, Triact),
• triforine (Ortho Funginex), ornamental use only, or
• potassium bicarbonate (Kaligreen, First Step)

Chemicals are most effective when combined with cultural controls. Apply fungicides at seven to 14-day intervals to provide continuous protection throughout the growing season. Follow the instructions on the fungicide label for use on specific plant species, varieties, rates to be used, timing of applications, and waiting periods before harvest.

An alternative nontoxic control for mildew is baking soda (similar to the potassium bicarbonate listed above) combined with a lightweight horticultural oil (Sunspray). Researchers at Cornell University have discovered the fungicidal properties of this combination against powdery mildew on roses. Applications of 1 tablespoon baking soda plus 2.5 tablespoons of Sunspray oil in 1 gallon of water are still experimental. Use it at your own risk.

It’s the rare gardener who hasn’t had to deal with powdery mildew (PM). With many different species, each one “tailored” to attack different kinds of plants, it’s one of the most common garden ailments. How common you might ask? Well, a little research on the internet yields the following (although most of us don’t need statistics to believe it!):

• A search for “powdery mildew” on the UC IPM site returns 151 results including 18 pest notes dedicated to PM on specific plants (including vegetables, grapes, ornamentals, fruits and berries, apples, turf grass, lettuce, cucurbits, and tomato. And that’s only half the list!) Plus there’s additional PM info in their on-line tomato and grape disease databases.
• The same search on Google returns 89,000 results including bulletins many university cooperative extension, articles in gardening related sites, discussion forums, and even one with a URL of www.powdery-mildew.com.

Of course, all that proves is it’s common, and we already knew that. So let’s dig a little deeper to learn more about what it is and what we can do about it.

Powdery mildew Basics: Life Cycle, Symptoms, and Effects

Powdery Mildew on McClellan Ranch squash. The disease has almost completely covered the leaf.

Powdery mildew is a disease caused by many different species of fungi. Each species attacks specific plants, usually only one or two, but others attack a wide range of plants. The fungi require live tissue to grow and reproduce, and affect all types of plants, from trees to vegetables, even houseplants.

The most common reproduction is asexual production of spores, with the time from germination to spore production being as little as 48 hours. The spores move from plant to plant by wind as well as through direct contact by insects, animals and sometimes gardeners! Some species of PM require year-round availability of plant hosts to survive, other species switch to sexual reproduction in the fall, producing small black dots, called cleistothecia, which overwinter and eject spores in the spring.

Infection is favored by high humidity (50 to 90%) during twilight hours, and temperatures between 60° and 80° F. While there’s some ambiguity in the UC IPM literature, most sources state that not only does PM not require moist conditions to germinate and grow but that the spores can be killed or inhibited by water on the plant. Shady conditions (which are most conducive to retaining humidity) and vigorous plant growth favor disease development while high temperatures (above 90°F) have an inhibitive effect.

Symptoms most often appear in the form of circular white spots; the spots enlarge as the white mycelium (threadlike filaments) grows over plant surfaces and produces spores, giving a powdery appearance that covers the leaves (although one species that affects onions, peppers and tomatoes has a yellow appearance with little powdery growth). The infection forms on either side of the leaves, spreading rapidly over the entire surface with the mycelium sending a root like structure (called haustoria) into the leaves, taking nutrient from its host.

Affected leaves will not manufacture as much food as they should, and may turn yellowish or brown and drop from the plant, thereby increasing problems with sunburn on some plants. Other injury may appear as stunting and distortion of leaves and buds, sometimes but less often, affecting fruit. Since nutrients are removed from the plant by the fungus, the result may be a plant with reduced vigor in growth, bloom and fruit. The extent of damage depends on the species of PM and the plant variety affected.

A side note: don’t confuse PM with downey mildew, they’re different diseases, with different symptoms and control mechanisms. While downy mildew can also produce a powdery growth, it grows primarily on the underside of leaves, and when viewed with a hand lens, has a tree like structure instead of the threads of PM. Downey mildew infections are favored by moist humid summers (high humidity with moderate temperatures), making it more of a problem along the coast where it can cause serious problems for plants including roses.

Management and Control

All sources seem to agree on this: the best way to avoid PM problems is to plant varieties that are resistant to the disease. If you have a problem plant, seriously consider removing it and finding something else. Of course, first decide if it’s really a “problem”; just because the plant has PM (for all or parts of the year) doesn’t mean you have to worry about it. If the plant is growing “well enough” for you, and it’s purpose isn’t significantly affected, you may do well enough by leaving it alone.

But if you have your heart set on a susceptible plant, remember your IPM practices: cultural control first! The most basic cultural practices include:

• Plant in a sunny location wherever possible
• Make sure there’s good air circulation (through plant spacing and pruning)
• Promote steady growth with moderate application of nitrogen fertilizer (or use a time release fertilizer)
• In a sunny dry location, an occasional spray of water can be used to wash the PM from the plant
• Remove and discard infected material, take special care in the fall to reduce the amount material available for over wintering
• If you’re planting annuals, consider different plants from year to year since the mildew species from the previous may not affect a different plant
• An intriguing note: there’s one kind of lady beetle (Psyllobora spp) that actually eats powdery mildew. Unfortunately, this isn’t the variety you can purchase for use in the garden, but it does live in California.

For example, I have a Variegated Euonymous that has white spots almost the entire year. It’s in a sunny location, well enough established to need almost no water during the summer months, and sends out new growth the entire year. This is the perfect case for letting things be. While I might wash the plant occasionally for appearance’s sake (although I never seem to make time for this), there’s no real “problem”. On the other hand, my Zepherine Drouhin climbing rose, which sits in a partially shaded location along the side of the house, is often affected on new shoots where the leaves curl severely. In this case, other than a spraying regimen, the best idea seems to be to try moving it to a sunnier location where there’s more air circulation. I could consider a spraying regimen, but need to think about the environment, not to mention the time required. Let’s talk more about those options next.

As always, if you consider spraying, know your options. One of the most important things to know about controlling PM is that some sprays act to ward off the disease (preventatives) while others will kill it once it’s established (eradicants). Some sprays will do both, but many serve only one purpose or the other. Spraying an eradicant before you see symptoms, therefore, is typically a waste of time, as is the use of a preventative once you have an infection.

Preventatives: if you know you have a susceptible plant, you can consider regular sprays to avoid infections. Preventatives include:

• Sulfur: used for centuries, sulfur is most effective when applied in wettable form (such as Safer Garden Fungicide). Note that sulfur can damage ornamental plants; do not apply above 90°F or within two weeks of an oil spray.
• Bicarbonates: available in the form of potassium bicarbonate (Kaligreen) and home-made solutions using baking soda. Be careful to avoid injury to the plants, and note that baking soda can have adverse effects on the soil when over used. Baking soda solutions can use salad oil as a spreader-sticker, or horticultural oil (which adds eradicant capabilities).
• The biological fungicide Serenade contains a bacterium which helps prevent infections from occurring.

Eradicants: Once you have active symptoms, you may need an eradicant to kill the infection to avoid further damage. Since the established PM is a parasite to the host plant, using a preventative to avoiding spreading will not stop damage to the plant. Eradicants include:

• No Powdery Mildew™ has been proven to be an excellent eradicator as well as preventative.
• Plant based oils including neem oil or jojoba oil.
• The biological fungicide AQ10 contains a parasitic fungicide that attacks the PM fungus.

Combination: Unfortunately, sprays combining both preventative and eradicant qualities fall mostly into the synthetic fungicide category and are least recommended from an IPM viewpoint. The least toxic combination seems to be Potassium Bicarbonate, which, while primarily preventative, has some eradicant capability. There has been a new product that has been rapidly gaining traction and shown tremendous results for many gardeners with a powdery mildew outbreak. No Powdery Mildew™ is an all new natural plant oil based extract formulation that goes to work immediately on your powdery mildew outbreaks. It has a dual lysis action that attacks mildew spores on contact while penetrating your plants cell walls giving them added strength and vigor.

Powdery Mildew on Zepherine drouhin climbing rose, note leaf curl caused by the disease.