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Downy Mildew: How to Control Downy Mildew | The Old Farmer's Almanac

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Downy Mildew

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How to Control Downy Mildew in the Garden

Andy Wilcox
That Garden Writer Guy
Pest Type
Plant Diseases
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Yellow spots, withering leaves, and cucumbers getting sunburned? Your garden could be victim to a fungal disease known as downy mildew. Learn how to combat and get rid of downy mildew in your garden.

Downy mildew is a member of the group called “water molds.” It can be a catastrophic problem in some areas of the United States. Various species of downy mildew target many of our favorite garden plants, from herbs to melons. 

Downy mildew gets its name from the gray, fuzzy appearance of the spores formed on the undersides of infected leaves. 

What Is Downy Mildew?

Downy mildew is a disease caused by a group of pathogens called Oomycetes. These microbes require a living host to parasitize. Many downy mildews are host-specific, meaning they will only attack one group of plants, such as cucurbits.

Downy mildew causes yellowing, molding, and death of leaves but does not affect stems and petioles. Even on plants that are not killed, severe crop reduction is likely. The whole harvest might be unusable if your crop was a leaf vegetable or herb. For cucurbits, crop loss is mainly due to reduced photosynthetic activity and sunscalding of the fruits due to missing leaves.

Commonly affected plants include basil and almost all of the common cucurbits—watermelon, zucchini, winter squashes, cucumbers, and pumpkins. Grapes, soybeans, spinaches, and sunflowers are also targets.

How Does Downy Mildew Spread?

Warm and humid conditions aid the reproduction and spread of downy mildews. Higher temperature and humidity enable elevated levels of spore production. These spores can be transported hundreds of miles on the wind when released. 

The pathogen overwinters in the Gulf States, travels north during the spring, and reaches the northern tier of states by late summer. Some years the mildew may not spread all the way to the upper Midwest states or only sporadically. 

For downy mildew of cucurbits, Cornell University has created a website that tracks the progress of the disease every year. You can also view archived forecasts and alerts showing when the pathogen was discovered in different areas of the country annually. Check it out at their Integrated Pest Management Pest Information Platform for Extension and Education (ipmPIPE) at cdm.ipmpipe.org.

Identification

Depending on your location, downy mildew can start to appear as early as late spring or as late as August if you garden in northern regions. Keep an eye out for symptoms.

Look for these signs of downy mildew:

  • A mosaic pattern of yellowing on the upper surface of leaves.
  • Yellow areas become more extensive and blocky-shaped but are frequently confined by the leaf veins–yellow on one side, green on the other.
  • Infected leaves will turn brown
  • Close inspection may show purple-brown mold in the interior of the lesions
  • Gray spores on the undersides of leaves may create a fuzzy appearance.
  • Leaves are affected, but stems and stalks are not.
Powdery mildew (left) compared to downy mildew (right). Photos by Yevhenii Orlov and AJCespedes/Shutterstock

Powdery Mildew vs. Downy Mildew

These two diseases are sometimes confused by gardeners but are easily told apart. Downy mildew has the symptoms listed above. Powdery mildew has a wider geographic range. It can easily overwinter even in harsh conditions and, most tellingly, will show itself as white dust or powder on the surfaces of the leaves. 

Severe infections can look like the entire leaf has turned white. While some yellowing may occur with powdery mildew, you likely won’t notice as the whole plant will have a frosted white appearance.

Unfortunately, it is not uncommon for a plant to exhibit both problems at the same time. Learn more about powdery mildew.

Control and Prevention

Prevention is nearly impossible with downy mildew because it is spread by the wind. It can travel across an entire state in a day or two. Even the best garden sanitation practices cannot guarantee a downy mildew-free squash patch. However, take advantage of opportunities to reduce the risk of bringing it home with you.

  • Closely inspect any transplants you purchase, especially basil, squashes, cucumbers, melons, and spinach. Downy mildew can overwinter in a greenhouse and may be present despite the best intentions of the growers. 
  • If you suspect the beginnings of downy mildew, make sure to disinfect tools and wash your hands before moving to a new area of your garden to work. Don’t make it easier for the pathogens to spread–make them work for it! 

Choose Tolerant/Resistant Plants

Basils, cucurbits, and other garden and market garden cultivars resistant to downy mildew are available. If you’ve had issues with severe infections in the past, planting resistant cultivars is well worth the effort. Talk to your local garden club or county extension office if you are unsure about the severity of downy mildew in your area.

Selecting early-to-harvest varieties can also minimize the impacts of downy mildew on your harvest. This pathogen arrives in mid to late summer in most areas. Choose an early variety to get your crop harvested before the mildew has a chance to infect your plants.

Treatments for Downy Mildew 

Besides choosing resistant varieties and keeping a clean set of tools, a few cultural practices can reduce the severity of infection if an ill wind blows your way:

1. Grow up!

Sprawling plants like cucumbers and melons can be grown on trellises to provide airflow and increased leaf drying. Remember, downy mildew likes humid conditions and needs wet leaves to invade. Keeping your leaves up off the ground can assist in rapid drying after the morning dew or rain shower. 

2. Use a drip line for watering

When watering your plants, use a drip line irrigation system that will provide water at the base of the plant and not on the leaves. These systems use less water as well, an added benefit. 

If you water with a hose, use a wand to make reaching under the leaf canopy easier without constantly bending over. 

If you use overhead watering systems, set a timer to water in the morning so that the leaves will dry quickly as the sun reaches them and the day warms up. 

3. Reduce watering by mulching

A nice layer of “armor” on the soil surface will reduce the evaporation of water from the soil, meaning you need to water less often. If you use an overhead sprinkler system, mulching is even more crucial to avoid water loss and prevent soil from splashing onto leaves. Read more about using mulch.

4. Prune lower leaves

Those lower leaves likely weren’t doing much anyway. Prune them up to increase air circulation around the plants and reduce favorable conditions for downy mildew to grow. Removing lower leaves can also reduce the likelihood of other fungal diseases infecting lower leaves when spores are splashed up from the soil.

5. Get rid of infected plants

If you notice a few infected plants, give them the boot. Remove and bury them or dispose of them to limit spreading into other areas of your garden. If only a couple of leaves have yellow mosaic spotting and you suspect downy mildew, trim and destroy them. Don’t forget to sanitize your shears with rubbing alcohol or bleach when you’re done.

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Comments

Geri Reski (not verified)

2 months 2 weeks ago

Oh yes! Am well familiar with this condition! I have a greenhouse and mildew just loves my zucchini and cuke plants but also loves green beans! My basil is just fine. I cut off the leaves affected and discard. Outside of that no clue how to prevent because I have drip lines,, mulch, lots of sun and temp controls. Oh well….will just keep plodding along!