Downy mildew can grow on any type of plant but is most often found on grapes (Vitis spp.), roses (Rosa spp.), pansies (Viola spp.) and impatiens (Impatiens spp.).[1]
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Missouri Botanical Garden
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You should check plants susceptible to downy mildew once or twice a week when weather conditions are humid and take steps to remove the infection before it spreads to the rest of the plant and kills it.
Removing Downy Mildew with Baking Soda
Create a safe, effective solution using baking soda, cooking oil, and shampoo. As the main active ingredient, baking soda is a good alternative to chemicals when trying to get rid of downy mildew. Mix together 2 tablespoons of baby shampoo, 2 tablespoons of any type of cooking oil and 2 tablespoons of baking soda in 1 gallon (3.8 L) of water in a container. Put a cap on the container and shake it until all ingredients are thoroughly mixed. Pour a small amount of the solution into a spray bottle. Spray a few hidden leaves on plants infected with downy mildew. Wait 24 to 36 hours and then check the leaves for brown or yellow spots, brown tips or pale scorched leaves. These are all symptoms of phytotoxicity, which can occur in some plants when they are treated with this remedy.
Spray the plant thoroughly with the solution if there are no symptoms of phytotoxicity. Shake the rest of the solution up, pour it into the spray bottle and spray the plants until the solution drips from the leaves. Be sure to coat the tops and bottoms of the leaves. This baking power solution will also kill pests like aphids, lacebugs, mealybugs, scale, spider mites, thrips and whiteflies.
Treat the plants once a week until the weather conditions no longer support the growth of downy mildew. Downy mildew flourishes when temperatures are between 58 and 72 degrees Fahrenheit and the relative humidity is 85% or higher. So once the humidity and temperatures outside drop, you can stop applying this remedy to your plants.
Removing Downy Mildew with Chemicals
Call your local County Extension Office to find out which chemicals are permitted for use in your area. Though there are several possible chemicals that can be used to get rid of and prevent downy mildew, they are tightly regulated and some of them are restricted or not permitted for use in the United States.
Use copper fungicides. Copper fungicide is one of the more traditionally used chemicals to remove downy mildew. However, these fungicides are controversial and are regulated for use. Though copper is an essential plant nutrient in small amounts, fungicidal levels of copper are toxic to beneficial organisms like earthworms and social microbes like blue-green algae. Excessive use of this chemical can cause a toxic level of build up in the soil.
Use neem oil. This botanical pesticide is derived from the Azadirachta indica tree species and is labeled for use on downy mildew. Though it is organic, neem oil kills beneficial insects like lady beetles and bees.
Use these chemicals cautiously and only when absolutely necessary. Due to the toxicity of these chemicals, only use them on downy mildew as a last resort. Apply them just before the plant blooms, one week later, and then again 10 days to two weeks later. Always wear protective eyeglasses and protective clothing when applying these chemicals as they can cause permanent eye damage as well as severe skin irritation. These chemicals also kill beneficial organisms called Mycorrhiza, which are essential for healthy plant growth. Some of these chemicals can also be hazardous to people and wildlife.
Cleaning Up Debris and Maintaining a Dry Environment
Always prune off any infected leaves before spraying the plants with a remedy. The damaged, infected leaves will never recover and removing them will make it easier for the baking soda solution or the chemicals to kill the mildew organisms.
Use a rake to remove any plant debris on the ground around the plants. If this debris remains on the ground, it will become a hiding place for downy mildew and can re-infect the plants when the weather becomes humid and moist. Put all of the debris in the garbage to ensure the area around your plants is kept clean and free of any potential downy mildew.
Water your plants under their leaves. Doing this will prevent the growth of downy mildew as no water will be on the plant's leaves. Wet leaves or foliage makes it easier for downy mildew to thrive, especially when weather conditions are humid and moist. Use a soaker hose or water the plants individually by hand with a watering can until temperatures warm.
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