Powdery Mildew

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Ethical Farming & Ferts Guru
Jan 2, 2011
Here are steps you can take to prevent powdery mildew from blemishing your garden.

powdery mildew.jpg
(Above) PM on Cannabis


A gray, talcum powder-like coating that covers the leaves, flowers, and even fruit of some of your vegetables, perennials, and shrubs.
Where it's a problem
Powedery mildew is found througout North America.
Fungal spores are spread by wind and overwinter on plants and in plant debris. Unlike mildews that appear in bathrooms or basements, powdery mildew does not need direct contact with water in order to grow. The warm days and cool nights of late summer create an ideal climate for spore growth and dispersal.
Plants it attacks
Powdery mildew is the blanket name for a few different species of fungi that infect many ornamentals, such as beebalm (Monarda), lilacs (Syringa), zinnias, roses, and garden phlox (P. paniculata). It also affects vegetables, including beans, cucumbers, grapes, melons, and squash.
Why it's a problem
Powdery mildew is unattractive and it can affect the flavor and reduce yields of some fruits and vegetables. Although plants are unsightly and can be weakened by an infection, they do not usually die. Powdery mildew on ornamentals is an aesthetic issue, and not usually worth treating. Prevention and control is more important for vegetables.
Organic damage control
Powdery mildew can be prevented, and it can be controlled once it appears, but it can't be cured. The key to preventing it is planting mildew-resistant or mildew- tolerant varieties. Resistant varieties get less mildew than susceptible varieties; tolerant varieties may get some mildew, but it shouldn't affect the performance of the plant. Prevention also includes siting plants where they will have good air circulation, and exposing as much leaf surface as possible to direct sunlight, which inhibits spore germination.
To control minor infestations, pick off affected plant parts and either compost them in a hot compost pile or bag them tightly and put them in the trash.
Homemade Sprays
Research studies in 1999 and 2003 on infected zucchini and winter wheat (respectively) indicated that spraying cow's milk slowed the spread of the disease.
To try this at home, mix 1 part milk with 9 parts water and spray the stems and tops of leaves with the solution. Reapply after rain.
Spraying leaves with baking soda (1 teaspoon in 1 quart water) raises the pH, creating an inhospitable environment for powdery mildew. (JM reccomends a 20% solution of skim milk and water, if its all you have)

Other Details:

Powdery mildew occurs on many different flowers, woody ornamentals and trees including roses, snapdragons, African violets, kalanchöe, English ivy, zinnias, photinia, oak, lilac, and begonias. Several different genera of fungi cause powdery mildew. Although usually one genus specifically attacks one or two different plants, some species of powdery mildew (such as Erysiphe cichoracearum) attack a wide range of plants. All the powdery mildew fungi are obligate parasites, requiring live tissue to grow and reproduce. In greenhouses, the fungus survives by spreading from the diseased plants to the new plants of that same crop. If that crop is not grown for several weeks, the fungus dies out and diseased plants must be brought into the greenhouse to establish the fungus again. Outdoors, fungal structures form on leaves and twigs that allow the fungus to survive winter conditions.


White powdery fungus grows on the upper leaf surface of the lower leaves and flower parts.
Leaves may be twisted, distorted, then wilt and die.
On some plants such as kalanchoe, infected leaves have dry, corky, scab-like spots and fungal growth is not obvious.

Conditions favoring powdery mildew:

High relative humidity at night
Low relative humidity during day
70-80 F (22-27 C) temperatures (These conditions prevail in spring and fall)
The spores are carried by air currents and germinate on the leaf surface. Liquid water on leaves inhibits spore germination. The fungus grows on the leaf surface but sends fine threads (haustoria) into the cells to obtain nutrients. From the time a spore germinates to the time new spores form may require only 48 hr. High humidity favors spore formation while low humidity favors spore dispersal.

Some powdery mildew are inhibited by free moisture on leaves while others are favored by wetness on leaf surfaces.

Managing powdery mildew in greenhouses:

When conditions are favorable for 3-6 consecutive days, heat and ventilate in late afternoon to reduce night humidity.
Antitranspirant materials such as Vapor Gard* or Wilt Pruf* applied to coat the leaf can prevent infection. In the landscape, application remains effective up to 30 days. Treat plants such as lilac on June 15, July 15, and August 15 (*Trade name).
Apply a fungicide on a regular schedule until conditions change. Be certain the crop is on the label.

Don't use any fungicide that's not approved for use on something that is consumed by humans. Many of the fungicides that treat powdery mildew are only approved for use on ornamentals.

I personally use Serenade:

Serenade.... organic bacteria extract... working for me AtM. also handles blight, and many other fungal diseases...


Serenade Garden Disease Control - How to Use

Serenade Garden Disease Control is a broad spectrum, preventative biofungicide recommended for the control or suppression of many important plant diseases (see section below). It may be used on vegetables, fruits and nuts including tomatoes, peppers, melons, carrots, broccoli, lettuce, onions, apples, pears and walnuts in addition to annual and perennial bedding plants and flowers, roses, potted flowers, foliage plants, trees and shrubs located in residential greenhouses and residential and commercial landscapes and interiorscapes.

Diseases Controlled

Following is a list of vegetable, fruit, nut and ornamental diseases controlled or suppressed by Serenade:

Anthracnose (Colletotrichum spp.)
Bacteria (Erwinia, Pseudomonas, Xanthomonas spp.)
Bacterial Leaf Blight (Xanthomonas campestris)
Bacterial Speck (Pseudomonas syringae pv. Tomato)
Bacterial Spot (Xanthomonas spp.) - suppression
Black Mold (Alternaria alternata)
Black Root Rot / Black Crown Rot (Alternaria spp.)
Black Spot of Rose (Diplocarpon rosea)
Botrytis (Botrytis spp.)
Downy Mildew (Bremia lactucae, Personospora spp.) and (Plasmopara viticola - suppression)
Early Blight (Alternaria solani)
Fire Blight (Erwinia amylovora) - suppression
Gray Mold (Botrytis cinerea)
Late Blight (Phtytophthora infestans)
Leaf Spots and Rusts - (Alternaria, Cercospora, Entomosporium, Helminthsporium, Myrothecium, Septoria, Puccinia, and Phragmidium spp.)
Pin Rot (Alternaria / Xanthomonas complex) - suppression
Powdery Mildew (Uncinula necator, Erysiphe spp., Sphaerotheca spp., Oidiopsis taurica, Leveillula taurica, Podosphaera leucotricha)
Scab (Venturia spp.) - suppression
Walnut Blight (Xanthomonas campestris)

View attachment serenade-rtu-msds.pdf

OTHER Comments by other Professional growers:

Immunox plus by spectracide. Ask for it by name and accept no substitute. I have dealt with PM and mites on new clones with thia in one dip. Got a plant from a guy that would drop pm like snow when you tapped it, dip, and with one week isolation no issue.

Sulfur burners are easily replaced by a sulfur based spray like safer with less of a learning curve and easier application.

I use the immunox in veg and then switch to a three part mix in flower till day 21 of safer fungicide, safer insecticide soap, and neem. ECh at one tablespoon per gallon.

Always isolate new clones and plants. Always spray every 7-10 days until day 21 of flower. Never let PM or mites get even a chance to gain a toe hold. Spray everything. Top of plants, bottom leaves, sides of canopy, top of soil, the stalk, rim of the pot, if outside the surrounding vegetation....
I choose Serenade cuz I'm a purist on organics... and I know what Serenade is (Bacillus Subtillus Strain QST 713)
see also: http://en.wikipedia.org/wiki/Bacillus_subtilis

see also discussion of: https://www.autoflower.net/forums/f23/powdery-mildew-12890.html#post248331


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