Powdery Mildew

Powdery Mildew is common garden problem. There are over one thousand closely related species of  fungi that cause powdery mildew. They belong to the genera: Erysiphe, Microsphaera, Phyllactinia, Podosphaera, Sphaerotheca, and Uncinula. Most are quite host specific. Powdery mildew on roses is typically caused by Sphaerotheca pannosa var. rosae while powdery mildew on phlox is caused by Microsphaera penicullata and that on lilacs by  Microsphaera syringae.  While the fungi may be unique, the symptoms and general treatment of the problems they cause are similar.

As the name suggests, powdery mildew is characterized by patches of white, grayish or brownish, powder-like growth comprised of a mycelial mat (a felt-like web of fungal filaments) and chains of colorless spores (conidia) borne on upright stalks (conidiophores). It affects both sides of leaves and may cause leaves to curl or twist and fall prematurely. New leaves are most susceptible and it may also prevent flower buds from opening. It looks similar to downy mildew (caused by other groups of fungi), but downy mildew occurs mostly on lower leaf surfaces and looks distinctly fuzzy or downy. Late in the season, powdery mildew can take on a speckled appearance as the fungi begin to fruit and small black structures called cleistothecia become apparent.

Like most fungal pathogens, disease is spread by spores borne on air currents or by spores physically transferred by insects, animals, unclean tools, etc. Although humidity needs vary, powdery mildew species can germinate and infect plants in the absence of water. Spores overwinter in bark and leafbuds and begin to germinate when temperatures exceed 5°C. Powdery mildew flourishes when days are warm (20° to 35°C) and nights cool. Alternatively, downy mildew is favoured by cooler temperatures (15° to 25°C) and higher humidity.

How do you prevent and manage powdery mildew?

1. Avoid susceptible plants. Among the most susceptible in our area are:  chrysanthemum, dahlia, delphinium, columbine, crabapple, flowering dogwood, euonymus, honeysuckle, hydrangea, lilac, phlox, rose, snapdragon, and zinnia.

2. Plant resistant varieties. For instance, resistant Phlox paniculata cultivars like David are rarely severely affected by mildew. Seed catalogues frequently specify disease resistance. Also, ask nursery personnel or check my webpage.

3. When planting, think about the soil, water, light and air.

   • Your soil should be rich in organic material. Beneficial microbes in the soil will help to suppress the growth of many pathogens. For instance, strains of fungi in the genus Trichoderma colonize and penetrate plant root tissues and induce systemic resistance in the entire plant to fungal pathogens.

   • While as a general practice, it is best to water the base of plants, spraying the leaves of plants infected with powdery mildew helps reduce the severity of the disease. Water inhibits the growth powdery mildew. However, as wetting foliage can increase the spread of such things as rust, black spot and downy mildew, spray early in the day so that leaves can dry.

   • Powdery mildew is sensitive to sunlight. Space and prune plants so that the foliage can receive maximum sun exposure and good air circulation. While plants like phlox and columbine can tolerate shady conditions, keep their foliage open and do not overcrowd them.

   • If mildew is a problem, avoid mass plantings. Interplant a variety of species to limit plant to plant transfers.

4. Avoid using fertilizer when powdery mildew is present, particularly mid summer. Fertilizer promotes new growth which is the most susceptible to infection.

5. Prune out diseased materials. Clean your tools and be careful when working amongst your plants so that you do not inadvertently spread the spores.

6. Unlike many fungi, powdery mildews only colonize the surface of plants making control much easier.

The least-toxic fungicides available include oils, soaps, potato starch and milk sprays, sodium and potassium bicarbonate sprays, sulfur and biological fungicides (e.g. compost teas, AQ10 and Serenade). These can all negatively affect beneficial insects and microorganisms so use them only when needed.

• Oils, soaps, starches and milk-based sprays flatten the conidia and prevent them from releasing spores into the air. In addition, the mycelium and conidia of powdery mildew fungi are waxy and these "wetting" agents promote the spreading of water on fungal surfaces thereby diminishing growth rates.
  

  • Use horticultural oil or plant based oils such as Neem, jojoba, sesame, sunflower oil etc. To prevent phytotoxicity keep oil concentrations low (below 1%). Do not apply oils when temperatures are above 32°C or to drought-stressed plants or if sulfur has been used. Some oils also appear to increase host plant resistance response.

  • "Plant safe" soaps solutions can be purchased or made at home using dish detergents, such as Ivory, Palmolive and Sunlight. Apply the detergent, diluted with water to a concentration of about 2 to 3% (about 1 tablespoon per liter of water), using a spray bottle. Wash off after two to four hours.

  • A potato starch spray can be made from 1 liter of water, 3 T (45ml) of potato flour, 1/2 tsp (2ml) liquid, non-detergent soap.

  • A 10% milk solution(1 part milk to 9 parts water) has been found to reduce the severity of powdery mildew infection on the plants by 90%.

• More effective control of powdery mildew can attained by adding sodium bicarbonate to an oil and soap solution. Try mixing 1Tbsp (15ml) baking soda, 1Tbsp (15ml) oil, 1/2 tsp (2ml) soap with 4liters of water.  The baking soda disrupts the ion balance within the fungal cells, causing cell walls to collapse. If mildew has been a problem in previous years, treat every 14 days in spring to prevent outbreak. (Potassium bicarbonate has the same effect - MilStop® is the only registered potassium bicarbonate based fungicide sold in Canada).

• Sulfur (dry or wettable) is an age-old preventative, but should not be used in conjunction with oils. Use only when temperatures are below 32°C. Safer® sells a premixed 40% sulfur product. Read instructions and cautions carefully.

• A foliar spray of bacteria-dominated compost tea is extremely useful to prevent foliar diseases in general and powdery mildew in particular. (For bacteria to dominate, compost should be made from a preponderance of green materials. Read, Elaine R. Ingham, October/November 2000, Brewing Compost Tea, Kitchen Garden Magazine). The microorganisms in the tea can compete for nutrients, induce systemic resistance and in some cases actively destroy fungi.

• Some beneficial microorganisms are commercially available. For instance, Ampelomyces quisqualis, (sold as AQ10) is parasitic fungus that feeds on powdery mildew and Bacillus subtilis (sold as Serenade) a bacteria that reduces colonies presumably by competing for nutrients and or by attacking the pathogen.

For links to more information and lists of resistant ornamentals, go to Tidbits and Resources.

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May toads nestle in your flower beds and help you tend your beauties in the spring.

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