Organic Ways to Kill and Prevent White Powdery Mildew
What Is This Powdery Mildew on My Plants' Leaves?
As a former landscaper, I have seen a lot of plant diseases. One of the easiest ones to recognize is powdery mildew; it looks just like its name. The mildew will form a white or gray powdery film on a plant's leaves, stems, and fruit.
Powdery mildew is a fungal disease caused by the many varieties of the fungi that belong to the order Erysiphales. The fungus thrives in warm, humid environments, and overwinters in the soil. The mildew forms spores that spread through wind, insects, and water run-off, which carries the disease to other plants. According to the Arizona Cooperative Extension, "Unlike most fungi, spores germinate on the surface of plant parts without the presence of free water."
•When treating your plants, make sure the leaves are coated liberally with the solution.
•Reapply weekly unless otherwise specified on a product's label.
8 Organic Treatments to Get Rid of Powdery Mildew
- Potassium bicarbonate
- Neem oil
- Baking soda
- Copper fungicides
1. Potassium Bicarbonate
is a safe, effective fungicide that kills spores on contact. Like baking soda, it is also a great preventative treatment because it raises the pH level above 8.3—an alkaline environment that is not ideal for fungal growth. Potassium bicarbonate
How to Use:
Mix 3 tbsp. of potassium bicarbonate, 3 tbsp. vegetable oil, and 1/2 tsp. soap into a gallon of water. Spray onto affected plants.
Numerous studies have shown milk and/or whey to be even more effective at killing powdery mildew than chemical fungicides. In a 2009 study by the University of Connecticut, which tested a milk treatment of 40% milk and 60% water on plants infected with powdery mildew, "the milk treatment provided significantly less disease than the untreated control, and the chemical treatment had equal or significantly less disease than the milk." Scientists are not sure why milk is so effective, but they believe that when milk interacts with the sun, it produces free radicals that are toxic to the fungus.
2 Ways to Use:
- Mix 60 parts water with 40 parts milk or whey, and spray onto the affected plants bi-weekly. You can even use whole milk without dilution for a strong effect.
- Mix 1 oz. powdered milk to 2 liters of water, and spray onto affected plants bi-weekly.
3. Neem Oil
Neem oil is made from the seeds and fruit of the evergreen neem tree, and it is powerful enough to kill powdery mildew in less than 24 hours. The oil works by disrupting the plant's metabolism and stopping spore production. Neem oil is also a great insecticide and since spores can be carried by bugs, this oil is a great preventative treatment as well.
How to Use:
Mix 3 tbsp. of neem oil to one gallon of water, and spray onto affected plants every 7-14 days. Take precautions to avoid sunburning the leaves, and avoid spraying the plant's buds and flowers.
Vinegar is very acidic. When sprayed on powdery mildew it raises the fungi's pH level above 8.3, effectively killing it.
How to Use:
Mix 4 tbsp. of vinegar (5% solution) with 1 gallon of water. Reapply every three days.
5. Baking Soda
Baking soda has a pH of 9, which is very high! Treating with baking soda raises the pH level on the plants and creates a very alkaline environment that kills fungus. There have been mixed reports of success when using baking soda to treat severe cases, so it may be better as a preventative treatment than a fungicide.
How to Use:
- Mix 1 tbsp. of baking soda and 1/2 tsp. liquid hand soap with one gallon of water.
- Spray solution on affected leaves, and dispose of any remaining solution.
- Do not apply during daylight hours. It may be best to test one or two leaves to see if the solution will cause the plant to suffer sunburn.
Garlic has a high sulfur content and is an effective anti-fungicide. Garlic oil can be bought commercially if you do not wish to make the solution at home. It works best when added to organic oil mixtures.
How to Use:
- Crush six cloves of garlic and add to one ounce of an organic oil such as neem oil and one ounce of rubbing alcohol. Let set for two days
- Strain and retain the liquid and crushed garlic.
- Soak the garlic again (this time in one cup of water for a day). Strain out and dispose of the crushed garlic.
- Add the oil and alcohol mixture and garlic water to one gallon of water.
- Spray your plants, coating only the leaves.
Sulfur is a natural product that is very effective at preventing and controlling powdery mildew. Sulfur can be bought as a dust or as a liquid and can be added to sulfur vaporizers.
How to Use:
Follow the dosing instructions closely and wear gloves, eye protection, and a face mask. Avoid inhaling or coming into contact with the sulfur.
8. Copper Fungicides
Copper is a very effective fungicide, but it is very important to follow label directions closely. Too much copper will be detrimental to the plant and the soil.
Some ingredients, such as vinegar and baking soda, can cause sunburn to your plants. Ensure that plants are well-watered before applying and don't apply during daylight hours.
How Can I Prevent Powdery Mildew?
Preventing the spread and/or severity of powdery mildew is the most cost-effective way of dealing with the fungus. Powdery mildew thrives in temperatures 50-65 degrees Fahrenheit with humidity levels of 80-90 percent. To prevent powdery mildew from forming in the first place, avoid low-temperature, high-humidity environments.
Do Not Crowd Plants
Good air circulation ensures lower humidity levels, inhibiting the growth of powdery mildew. Crowded plants also provides too much shade for the lower leaves, which encourages fungi growth.
Do Not Grow Susceptible Plants in the Shade
Powdery mildew does not tolerate high temperatures. Direct sunlight helps stem the growth of mildew because the sun's strong rays kill spores before they can spread. Plants that are shaded much of the day will stay cooler, thus encouraging the growth of mildew.
Dispose of Infected Leaves and Stems
Never use infected plant leaves or fruit as mulch or compost. Trim off infected leaves and stems and dispose of them properly. If your municipality allows backyard fires, then burn the debris. If not, dispose of the debris according to your local plant disposal regulations.
Water the Soil, Not the Plants
While water itself will not encourage mildew growth, splashing the leaves with water will spread the spores. Run a hose to the base of your plants instead of using a sprinkler system.
Buy Mildew-Resistant Varieties
There are a large variety of hybrid plants that are resistant or tolerant to the growth of powdery mildew. The resistant plants will be less likely to develop the mildew. The tolerant plants will show fewer ill-effects of an infestation of the fungi.
Plants and Vegetables Most Susceptible to Powdery Mildew
Are All Forms of This Fungus the Same?
There are many forms of powdery mildew, and each is species specific. Grapes will suffer from the powdery mildew that affects only grapes, roses suffer from rose powdery mildew, and so on.
Although all plants can get the fungus, certain species are more susceptible to it. If your plant has a black sooty substance on its leaves, it may be sooty mold.
Where Does Powdery Mildew Start?
The mildew usually starts on a plant's lower leaves, and if the fungus is not treated it will spread over the entire plant. When the leaves become severely covered with the fungus, photosynthesis will be affected and leaves will yellow and drop off. As a result, the plant may become so stressed it will not flower and/or fruit with any vigor.
For More Information
- Powdery Mildew
Most powdery mildews are recognized by the white to gray, powdery spots or large blotches on the surface of leaves, stems and fruits of host plants. T
Questions & Answers
Can I treat the soil prior to planting to prohibit the mildew from growing?
Yes, you can. There are several ways to do this.
Terraclor, Iprodione, and Mefenoxam are examples of chemicals that can treat the soil. Check the labels to see what fungi it controls and how to apply.
Some organic gardeners pretreat large gardens areas by heavily soaking the ground with water, then covering the garden area for 4-6 months. The heat and humidity kill spores as well as weeds.
My mango tree is a sapling and has powdery mold. How should I treat it?
Cut off the affected leaves and remove any diseased fruit or blossoms. Remove the infected plant parts as the fungus can still be carried to healthy parts of the plant.
Spray with any of the products mentioned in my article, concentrating on the leaves and their undersides; repeat every two weeks. In the spring, a preventive spraying will reduce fruit loss.