How To Get Rid Of Downy Mildew: Prevention And Treatment

There are a lot of diseases out there that can attack your garden or indoor plants, but some have a habit of sneaking up on you.

One such fungal disease is downy mildew, which is often mistaken for powdery mildew early on but is far more destructive. Many misconceptions about this disease can lead to it spreading rapidly through your vegetable garden.

grape leaves covered with mildew (downy)Pin

Let’s take a closer look at what downy mildew is, how to get rid of it, and how to protect your garden from future infected plants.

Tips On Getting Rid of Downy Mildew: Prevention and Treatment

Prevention is the best medicine for this nasty plant disease. However, treating it as soon as possible once a plant is infected is important to prevent further spread or destruction to your garden.

What is Downy Mildew?

Downy mildew is a collection of water molds that can cause major damage and invade healthy plants.

Four different genera of water mold can cause downy mildew:

  • Basidiophora
  • Bremia
  • Peronospora
  • Plasmopara

These molds aren’t fungi but oomycetes that display fungal-like traits in the way they spread.

This infection can affect a wide variety of crop and ornamental plants, especially in warm, dry weather.

Some of these plants include:

  • Brassicas
  • Carrots
  • Columbine
  • Cucurbits
  • Foxglove
  • Geum
  • Grapes
  • Hops
  • Impatiens including New Guinea impatiens
  • Lettuce
  • Onions
  • Pansy
  • Parsnips
  • Peas
  • Poppy
  • Rhubarb
  • Roses – even Knock Out Roses
  • Soybean
  • Spinach
  • Sunflower
  • Tobacco
  • Veronica

It feeds on the entire plant like a parasite, draining the water from infected leaves.

Unlike many other pathogens, downy mildew doesn’t bother trying to survive the winter. Instead, the mold continues to thrive in the Gulf states, where winters are mild. They then migrate north each spring on infected crops.

Temperatures under 65° degrees Fahrenheit alongside high humidity or other wet conditions are a perfect breeding ground for the mold, which can spread rapidly. 

Its fungal spores travel effectively in water droplets and are carried by air currents and insects. They then germinate on the upper surface of leaves and stems, spreading rapidly and making this a difficult disease to prevent completely.

Because early symptoms resemble powdery mildew, many gardeners fail to identify it before the disease becomes widespread.

Symptoms of Downy Mildew

There are several symptoms to look out for; the earlier you identify the signs of infection, the easier it will be to treat.

The first signs of downy mildew will be on the undersides of the leaves (leaf spots) of the host plant.

These tiny sporangia will look like fine speckling along the leaf and (depending on the species of downy mildew) be blue, gray, purple, or white in coloration.

Once these spores begin growing into the leaf and draining it, you will notice discoloration forming on the upper side of the leaf directly opposite the sporangia. 

In severe cases, you will also notice the entire leaf turning yellow or dark brown.

Depending on your plant, these splotches will usually appear to be yellow but may also be brown, light green, or purple.

The splotches may even form straight edges along the leaf veins. Meanwhile, the sporata produce spores and expand into a fuzzy mold that lends to this disease’s name.

It takes 7 to 10 days for the sporata to produce new spores, at which point they burst, spreading the spores to other nearby leaves.

High humidity levels of 85% percent or higher can lead to spores forming in as little as 4 days.

Severely infected leaves may shrivel, curl, or die. The plant’s growth can also become stunted due to the infection and, if not treated, could eventually be killed.

Note that it’s possible for other parts of some plants also to become infected as the oomycete spreads.

Treating Downy Mildew

A few effective treatments are available to tackle downy mildew, although none of them is 100% percent guaranteed to work.

Broad-Spectrum Systemic Fungicides

These are usually a last-resort measure against downy mildew.

  • When used properly, the fungicide soaks into the plant and will attack any fungal infection.
  • Depending on the downy mildew’s resistance, it may work quickly or take a long time.
  • It’s usually best to rotate applications between different brands to keep the infection from developing a higher resistance to any single product.
  • If even a broad-spectrum fungicide fails, you will probably have to destroy the plant.

Copper-Based Fungicides

While copper is an essential micronutrient, too much can be deadly.

Copper-based fungicides take advantage of this fact by using copper as the main active ingredient.

When downy mildew and other fungal or bacterial infections come in contact with the spray, the copper penetrates their cell walls and disrupts the enzymes within, causing the infection to die.

These fungicides can stunt or damage new growth and should not be used when the weather is cold or wet, as this can cause further damage to the plant.

Be sure to follow all instructions on the container carefully to get the best results.

Neem Anti-Fungal Foliar Spray

Neem oil is one of the best natural defenses you have against pests, but did you know it also works on some strains of bacteria and fungi?

A neem foliar spray can help kill the surface layers of downy mildew, although it won’t affect parts of the oomycete that have penetrated the plant.

For this reason, the foliar spray works best as an early treatment or to provide a one-two punch with a systemic fungicide.

To make your own spray against fungal infections, add 1 to 2 teaspoons of Dawn dish soap or pure castile soap to a gallon of warm water and blend.

Then add 2 tablespoons of .5 to 1% percent clarified hydrophobic neem oil and pour this mixture into a spray bottle.

If the infection has advanced, you may need to use a stronger concentration, such as 2% percent neem.

Test a small portion of the plant 24 hours before applying to ensure it isn’t sensitive to the neem.

When applying, spray the plant thoroughly, including the undersides of leaves and crevasses.

You will need to repeat the application every 2 days for 14 days or until the infection is gone.

Neem Soil Soak

This neem recipe is a natural remedy that uses 100% percent cold-pressed pure neem oil instead of clarified neem.

The recipe is otherwise the same as the foliar spray. However, instead of spraying this version, you’ll need to evenly pour 2 to 4 cups around the base of your plant.

As the roots soak up the neem, it becomes a systemic pesticide, attacking anything that pierces the plant for up to 22 days.

This includes downy mildew. You will need to reapply every 14 days until the infection is gone.

Baking Soda and Hydrogen Peroxide

Another home remedy to treat downy mildew is spraying the diseased plant with a baking soda solution. This changes the plant leaves’ pH level, making it less hospitable for the fungus to thrive there.

You must thoroughly mix 1 to 2 tablespoons of sodium bicarbonate (baking soda) with 1 gallon of water.

You can also try 3/4 cups of 3% hydrogen peroxide to a gallon of water. Then pour the solution into a sprayer, evenly coating the plant areas, especially the affected leaf surfaces. It’s also best to spray the solution on any susceptible plants nearby.

How to Prevent Downy Mildew

Prevention is the best medicine, and there are a few options available.

You can use neem foliar sprays (formulated for either fungi or pests), or neem soil soaks as an effective preventative measure that is safe and non-toxic.

Apply foliar sprays every 14 days, and soil soaks every 21 days.

You will also want to prune your plants to have decent airflow, especially near ground level. Using adequate spacing between plants is also an excellent way to prevent downy mildew, as it allows good air circulation and stops the spread of the disease.

Try only to water your outdoor plants early in the morning so they can properly dry, and water them early in the day at ground level to prevent splashing or wet leaves.

Irrigating at the base of plants is also a good gardening practice (soaker hoses) to prevent downy mildew.

It’s also important to remove plant debris dropped by infected plants where diseases can survive. This will also help stop the spread of downy mildew further.

Isolate any infected (or suspected) indoor plants during treatment and try to avoid high humidity levels when possible.

Always use sharp, sterile tools and resterilize between each cut to avoid spreading any possible infections.

Burn or securely dispose of any infected leaves you prune away.

Practice crop rotation so that the same type of plant isn’t in the same spot two years in a row, especially if there was a downy mildew outbreak the previous year.

This reduces the risk of any soil-borne spores infecting plants the next year.

Moreover, if a plant is severely infected, it may need to be destroyed to prevent the disease from spreading to other plants.

Finally, one of the best proactive steps in preventing downy mildew is planting resistant varieties, as they can avoid serious infection and are less likely to be infected.

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