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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!):
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
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:
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:
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:
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.
Spraying best practices include:
References
Revised: 18-Oct-2003
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