Powdery Mildew

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.

PM Basics: Life Cycle, Symptoms, and Effects

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

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.

PM on Variegated Euonymous. Note classic circular spots in this early infection 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.

Mary Louise Flint, University of California at Davis, Director of UC Integrated Pest Management Education & Publications says, “A simple fungicide can be made at home by combining 2-1/2 tablespoons of horticultural oil (Sunspray Ultra-Fine, Saf-T-Side, etc.) in a gallon of water and adding 4 teaspoons baking soda. This solution is sprayed on plants to prevent powdery mildew infections. Sprays of both potassium bicarbonate and baking soda can injure the plant, so use these materials with caution. Also, baking soda sprays can have deleterious effects on soil structure and should be used sparingly.”

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:

  • Horticultural oils such as JMS Stylet Oil, Saf-T-Side Spray or Sunspray Ultra-Fine Spray Oil.
  • 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.

PM on Zepherine Drouhin climbing rose, note leaf curl caused by the disease.

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

By: Allen Buchinski, 2003