Mold thrives on warm, humid summer days and can wreak havoc on the garden in several forms. Spotting mildew growth on one of your plants can be devastating and can be a sign of any number of species of mold or fungus. Downy mildew is one of the most common diseases to affect outdoor basil, decimating entire crops.

Thankfully, there are many ways to prevent downy mildew, and if it sneaks past your defenses, it is easy to control if you catch it fast enough. Learn how to keep basil downy mildew out of your garden.

What Is Downy Mildew?

Sweet Basil Downy Mildew Disease, Peronospora sp. appear on the surface of sweet basil leaves.

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Downy mildew on basil, caused by the Peronospora belbahrii parasite, is a water mold. It is often confused with the more prevalent, white, powdery mildew, which is a type of fungus. Powdery mildew doesn't affect the plant's health, making it easier to treat. It is common on a range of plants, including squash and cucumbers.

Although downy mildew requires a plant host to survive, the wind can spread it rapidly over large distances. Like many molds, downy mildew thrives in warm, wet, and humid climates. Environments with relative humidities greater than 85 percent are the perfect breeding ground. Unfortunately, basil downy mildew also thrives in temperatures as low as 59 degrees Fahrenheit, which can be a problem during the rainy season.

One of the first signs of this disease is the yellowing of the lower leaves, which is sometimes mistaken for nitrogen deficiency. The affected leaves may also curl or wilt, which can look similar to heat-damaged leaves but lower on the plant. As it progresses, the upper leaves will also turn yellow and brown. A defining characteristic is the gray, fluffy spores that form on the underside of the leaves that, if left untreated, can turn into black spots.

Now that you know how to spot downy mildew, it's time to learn how to prevent and treat it. 

Use Resistant Varieties

lemon basil

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The best way to prevent basil downy mildew is to buy resistant varieties. The following varieties are popular options; 'Rutgers Devotion,' 'Rutgers Obsession,' 'Amazel,' and 'Prospera Compact.' These varieties are often labeled 'DMR' for downy mildew resistant.

Red, Thai, spice, and lemon basil are also partially resistant. Sweet basil is the most susceptible, but if you are a fan of this type, 'Eleonora' is a variety that is less susceptible than others.

Peronospora belbahrii can survive on seeds; if possible, buy seeds that have been steam-treated to kill the pathogen.

Keep Leaves Dry

wet basil leaves in a pot

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Downy mildew thrives in wet environments, so wet basil leaves spell disaster. When planting your basil, you want to create an environment that isn’t conducive to mildew. This means planting your basil where you can keep the leaves dry.

Greenhouses and hoop houses are popular growing locations for basil due to the warmth and protection from pests but monitor the humidity levels. You may need to adjust ventilation and run fans throughout the warm summer nights to ensure the relative humidity stays below 85 percent.

When planting in outdoor garden beds, select a sunny location to promote fast drying of the leaves.

Growing basil in pots may offer an optimal solution, allowing you to move them as needed. During rainstorms and at night during high periods of humidity, you can readily move potted plants indoors to reduce the chance of infection.

When watering basil, water early in the day and avoid overhead watering methods that wet the leaves.

Plant and Harvest Early

Person holding pots of basil

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Basil downy mildew is usually worse in late summer when humidity soars and the dew settles in the early mornings. Planting your basil in early spring and harvesting before the high humidity and the impending rainy season hits can help to prevent this disease.

Space Plants

basil in raised bed

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Good air circulation is vital in preventing basil downy mildew. When growing basil, give each plant about a foot of space. If their leaves are touching, they are too crowded. Giving them space allows the leaves to dry properly after rain or watering.

Monitor Your Plants

inspecting basil plant in pot

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Downy mildew can be difficult to detect in the early stages, so it’s essential to closely monitor your plants. Check on the leaves regularly and closely examine any that are yellowing. Since mold often forms overnight, the morning is a great time to inspect your plants. Look at the underside of the leaves for gray mold spores. Doing so will help to confirm the presence of downy mildew.

Remove and Destroy

Basil downy mildew plant

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If you find mold spores on your basil, the only treatment is to remove and destroy all affected plants. Once you have pulled the plants, till the soil to reduce the spread of mildew. As an added precaution, don't replant in that area until the following season. For potted plants, discard the soil. Do not discard affected plants and soil in garden composts, as this may spread the disease.

The disease doesn't harm humans, and asymptomatic plants in the surrounding area are safe to eat. As a precaution, it is a good idea to harvest nearby plants and use them immediately.

Unfortunately, neither pesticides nor fungicides are effective in treating downy mildew. Certain fungicides can help to prevent mildew. However, since downy mildew isn't a fungus, and rain decreases the fungicides' effectiveness, their use is generally not recommended.

Prevention Is Key

Basil downy mildew is about the worst thing you can find on your beloved spice, and destroying affected plants is, unfortunately, the only treatment. Prevention is key; thankfully, there are many ways to avoid this disease taking over your crops. Plant disease-resistant strains early in the season. Keep your basil leaves dry until they are ready to harvest; this will go a long way to ward off downy mildew.

Do you struggle with downy mildew on your basil? Share your tips and tricks to prevent this disease in the comments!