Downy Mildew: How To Stop This Plant Disease Before It Starts

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Downy mildew and powdery mildew are often confused. If you’re looking for how to get rid of powdery mildew, check here. But downy mildew is a completely separate problem, even if it might look similar. Both are common issues in the garden.

Today, we’ll go over everything there is to know about downy mildew. I’ll tell you how to treat it, but more importantly, I’ll help you determine how to prevent it so you don’t get it again!

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Downy Mildew Overview

Common Name(s) Downy mildew, downy mold
Scientific Name(s) Peronospora, Bremia, Plasmopara, and Basidiophora genuses
Family Peronosporaceae
Origin Worldwide depending on species
Plants Affected Extremely wide range of plants, including ornamentals and food crops.
Common Remedies Neem oil as a preventative or light-infection treatment. Copper fungicides or phosphorus acid fungicides can treat larger scale infections. Prevention is the best cure.

What Is Downy Mildew?

Downy mildew on cucurbits
Downy mildew spotting on the upper surface of cucurbit leaves. Source: NYSIPM Image Gallery

Anyone who lives in a humid climate is likely experienced with treating downy mildew. With a heavy preference for wet surroundings, this fungal mold lives in the soil and later colonizes the leaves of plants, causing severe damage.

This downy mold is a member of the Peronosporaceae family. There are multiple variations of Peronosporaceae that make up the water mold family, each with slightly different plant preferences.

Peronospora and Plasmopara species are particularly dangerous for most gardeners. Bremia and Basidiophora can also become problematic, especially on ornamental plants.

Technically, Peronosporaceae are not a true fungus. Research has shown that downy mold has similar traits to fungal development, but they are oomycete microbes, not fungi. For gardening purposes, we often treat it as if it were a fungal disease because of how they spread.

Downy mildew grows on and into the leaves of their preferred plants, living off the plant’s water supply. While small amounts of downy mildew aren’t likely to do major harm to your garden, it’s a symptom of a larger water-related issue.

Unfortunately, many food crops and some flowers and shrubs are susceptible to this fungal infection.

While it rarely causes damage to more than 25% of a given crop field in agricultural surroundings, in the home garden it can become quite destructive. It’s important to take it out before it can spread too widely!

Life Cycle of Downy Mildew

Downy mildew closeup on cucurbits
A closeup of downy mildew on the upper part of a cucurbit leaf. Source: NYSIPM Image Gallery

In most gardens, downy mildew begins as a fungal spore in the soil. These spores are windblown, so can appear almost anywhere.

Any moisture which splashes soil onto the leaves of plants can help move the spore onto your leaves. Once there, it will latch into the underside of the leaf. It penetrates the leaf’s surface with its mycelia and begins to grow.

As the downy mildew grows, it causes spotting on the upper side of leaves. Underneath the leaf, a mat of sporangia forms, ranging from white through grey and even purplish in color. These release more spores into your yard.

Spores which land on leaves will begin to colonize the leaves if the conditions are right. However, they are only viable for a short period of time.

Others will be dispersed into the soil via plant debris rotting away, just waiting for the chance to get into a juicy plant. Spores can overwinter in the soil and reappear the moment infected soil gets onto plant leaves. Soil may be infected for up to five years.

The mildew’s mycelia, or fungus-like root system, can spread throughout your plant’s stems. New growth may already be infected by downy mildew if the plant’s had issues with it before.

Plants which are fully infected by Peronospora’s mycelia may produce damaged fruit or have visible problems with newly-formed growth.

In hops, the cones themselves can become colonized, essentially destroying them before they can fully form. Cucumbers can have a similar problem with young fruit, especially if they aren’t put on a trellis.

Needless to say, it’s essential to remain vigilant and watch for damage, especially in places that had issues with downy mildew in the past!

Evolution of Downy Mildew

Downy mildew on basil
The underside of a basil leaf with downy mildew. Source: Starr Environmental

Unfortunately, like so many other plant diseases, this fungus-like growth can adapt and change over time.

In 2007, basil downy mildew (Peronospora belbahrii) was first identified in the U.S. By 2012, it was reported throughout the United States and into Canada.

It’s believed that this form was an evolution of an old strain which was only reported in Uganda in 1933, and that it evolved into a new and more aggressive variation.

As there are many other causes for yellowing of basil leaves, it often isn’t properly treated, which can spell doom for the basil plant. However, the slightly purplish-grey spores on the underside of leaves are an indication of this problem.

Worse yet, it was discovered that it can be transmitted through infected seed, which is likely how it managed to make its way around the world.

Hot water sterilization of the spores isn’t viable. Some seed companies are sterilizing the exterior of basil seeds via steam treatments now with good effect.

While this version impacts basil, there are other new variations of downy mildew.

Peronospora lamii was discovered on coleus in 2005 in New York, and by late 2006 had spread throughout the rest of the country.

Downy mildew of impatiens, Plasmopara obduscens, has developed rapidly since 2011 on the eastern coast as well. While primarily centered in the area around Massachusetts, it has started to spread to other states via infected plant sales.

Difficult as it is to deal with, it’s good to be aware that this plant disease does in fact regularly evolve to impact other forms. This helps us in research to work towards an eventual complete cure.

Symptoms of Downy Mildew

I’ve roughly discussed the symptoms that appear from downy mildew, but let’s get into more detail.

Early Downy Mildew Detection

Downy mildew on sorghum
Downy mildew on sorghum. Source: CIMMYT

A colony of downy mildew initially forms on the underside of the leaf. This can give the leaf’s underside a whitish, purplish, bluish or bluish-gray speckling.

Once the spore has locked into the leaf structure, the top of the leaf begins to yellow in spots to mirror the location of the sporangia underneath the leaf. The sporangia itself is white to bluish-white in color as it begins to create new spores.

The downy mildew may further colonize the leaf it’s on and spread to form a fuzzy-looking coating of sporangia.

Later Downy Mildew Progression

Downy mildew on yellow squash
This squash has downy mildew and powdery mildew at the same time. Notice the yellowed leaf, plus the whitish dusting on the upper surface. Both diseases spread in wet, cool conditions. Source: lis chere

As leaf spots die out, the mildew sporangia turns grey and powdery, and spores release to fall on other leaves. Branches and leaves may become distorted or die off entirely.

From the time of infection to the time that new spores form usually takes 7-10 days, but can happen in as little as 4 days if the conditions are right. 85% or higher humidity at the soil’s surface tends to speed the progression.

Left untreated or constantly reinfected, the plant may die. However, it’s more common that a plant will be colonized by the mycelia.

This makes the plant a possible danger to others in the garden, as an outbreak could possibly recur. However, effective deterrent measures will reduce that likelihood. Control measures may wipe out the infection.

Controlling Downy Mildew

Downy mildew on coleus
Downy mildew on coleus. Notice the deformed center growth. It’s also got a pest problem! Source: bill barber

It’s tricky to control this particular plant disease, and in part that’s because we’re still learning a lot about it. While commercial growers make use of heavy chemical sprays to eliminate it,

Downy Mildew Treatment

Once downy mildew has begun to colonize a plant, it’s essential to strike quickly to try to lessen its ability to do severe damage to the rest of your garden.

Our old friend, neem oil, should be our first line of defense. Applied early on, it can stop downy mildew from colonizing a plant. However, it won’t kill any of the disease that’s already gotten into the plant’s stems.

I prefer to use a copper fungicide like Bonide Copper Fungicide to battle downy mildew. It’s an organic treatment which can be applied every 7-10 days or until the plant has visibly recovered from its current outbreak.

For more severe issues,  Organocide Plant Doctor should be used. This broad-spectrum systemic fungicide can be used as a soil drench around the plant as well as being sprayed onto plant surfaces. It works its way through the plant’s tissues, dealing with a variety of possible diseases.

It’s important to note that in neither case are these fungicides guaranteed to eliminate downy mildew. Both are considered to have moderate to good effect on this disease, but there are still strains which persist.

However, copper based fungicides are widely used throughout the Pacific northwest as both a preventative and a fungicide, and phosphorous acid treatments like Organocide are beginning to become popular.

While there are chemical treatments which are proving effective against downy mildew, they tend to be hard for the average home gardener to get. These are generally used in large-scale agriculture and can be dangerous to apply without proper safety measures.

Chlorothalonil fungicides such as Bonide Fung-Onil are typically the only chemical variations that home gardeners have ready access to. This can work, so if neither of the organic options work, it may be an option for you.

Overall, the best treatment for downy mildew is prevention.

Preventing Downy Mildew

Downy mildew closeup on basil
A closeup of the underside of a basil leaf with downy mildew. Source: Starr Environmental

When preventing downy mildew, the first place to look is at your watering habits.

Only water in the early morning hours. This ensures that your plants have time to dry out during the day.

Keep them pruned to allow for good air circulation, as this also helps any wet foliage to dry. Remove visibly-damaged leaves once you see them to try to prevent further spread into the plant’s system.

Stake plants or secure them in a fashion where air can still get around to all surfaces of the plant. If using a cage, thin out some branches to ensure you have full air circulation. This is especially important with plants like tomatoes that can form dense growth.

Using a soaker hose instead of a standard spray nozzle will reduce the amount of splashing of soil onto leaves. This can protect your plant’s foliage from damaging spores.

If growing in a greenhouse, reduce the ambient humidity by opening vents and adding fans to provide airflow. Doing so will also help prevent problems with powdery mildew and some pests, like whiteflies.

Many varieties of plant are available now which are resistant to downy mildew. When at all possible, choose those varieties to avoid the problem entirely.

Ensure the area beneath your plants is kept clean and free of debris. You can mulch if necessary, but avoid buildup of fallen leaves, especially those which may be harboring spores.

Control weeds around your plants to reduce airflow issues at the base of your plants. Removing weeds as they appear is always a good idea, as it reduces the appearance of weeds in subsequent years.

Finally, severely diseased plants should be removed and destroyed for the safety of the rest of your garden.

While nobody wants to lose a prized plant, the long-term effects of downy mildew in the garden can be catastrophic to the rest of your plants. It’s better to remove very diseased plants just to ensure others don’t get it.

The upshot of all of this is that if you practice good garden maintenance regularly, you likely won’t have severe problems from downy mildew.

Once the weather warms up, it’s far less likely to appear, which may give you the break you need to recover from the cooler months of the year!

Frequently Asked Questions

Downy mildew on maize
Downy mildew on maize tends to cause yellow streaking down the fibrous leaves. Source: CIMMYT

 

Q: What plants get downy mildew?

A: It depends on the type of downy mildew, and there are a lot.

Whether you grow ornamental plants or fruits and vegetables, there are forms of downy mildew which might develop in your garden. I’m currently battling downy mildew on some roses, for instance. And last year I had problems with downy mildew on cucumbers, which is a completely different species.

A number of varieties are regular colonizers of greenhouse-grown plants, both edible and ornamental. Downy mildew can appear indoors as well, so keep a watchful eye on your houseplants!

Researchers are still learning a lot about these oomycetes and how they strike. And as they can and have evolved in the past, there’s likely to be new strains that appear. It’s best to act to prevent these problems before they can take hold.

On the bright side, there’s likely to be advances in science which lead us towards better treatments in time. Until then, keep a watchful eye out for the symptoms of downy mildew, and practice preventative measures!


Hopefully you now understand downy mildew a lot better than you did before. Have you ever had this common infection in your garden? What did you do to treat it? Let us know in the comment section!

 

Downy mildew is a common sight in the garden. Our disease guide will explain what it is, how it spreads, and how to prevent it from coming back!
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