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How to Treat Downy Mildew on Basil

With downy mildew and healthy basil plants.

Downy mildew on basil is a fuzzy grey substance that begins on the underside of your basil leaves and grows up to infect the entire plant. It is a pathogen that can impact not just your basil leaves and plants but surrounding plants as well.

You will start to notice yellow spots on the leaves that will eventually spread to kill the entire leaf. Although it can be very difficult to bring back a plant that has been infected by downy mildew, you can garden your plant to life if you catch it in time.

You will have to keep an eye on it. Farmers and gardeners that do not catch downy mildew in time are susceptible to losing their entire crop, and it can be devastating. Learn more about how to catch downy mildew here.

Related To: How to Bring a Dead Basil Plant

Facts About Downy Mildew

Basil plants with downy mildew.

Downy mildew for basil starts as a grey fuzz that looks almost like velvet. It is also accompanied by black spots that are spores of the pathogen. The disease begins on the underside of the leaves of the plant and will grow upwards. Although it is caused by a very specific pathogen, it is spread when the plant is undergoing excess moisture or wind.

The pathogen is an airborne pathogen that really likes its water. It can be found on the leaves of basil plants, but it is also found on the seeds of basil as well. It is a difficult pathogen to treat, but it can be done. The pathogen for basil is called Peronospora belbahrii and is a pathogen that will grow with watering.

Downy mildew is a pathogen that all plants can be at risk of catching but almost has its own variety. Peronospora belbahrii is the pathogen connected to basil, and every plant that is at risk will usually have its own downy mildew connected to it.

This is a pathogen that not only likes water but also does well in cooler temperatures. It is known to produce infection in temperatures as low as 59 degrees Fahrenheit. It does best in warmth and humidity, although it does have a peak temperature that will begin to kill off the spores that are producing problems in your basil.

How Downy Mildew for Basil Survives?

Basil plants with downy mildew.

Downy basil starts as little organisms called oospores that germinate under the right conditions. The humidity needs to be over 85 percent, and the temperature needs to be over 50 degrees Fahrenheit but not warmer than 75 degrees Fahrenheit.

These oospores are creatures that have thick walls and are almost as hardy as the basil plant, and during germination will begin to create specimens known as sporangiophores.

Those sporangiophores are the pathogens that will begin to be seen on the plant stomata or the pores of the plant. At first, they look like grapes. It is these small organisms that are the organisms that are transmitted.

They can be transmitted by air or water. So, if your neighbor is having a problem with downy mildew, then there is a chance that you might as well. Or, your infected basil may cause a problem for your neighbor’s plants.

Each sporangium or sporangiophores will hold many different cells that are called zoospores. As the sporangia are wind or water blown, they will land on the plant, and each zoospore within the sporangia will feed on the basil and germinate on its own.

This is when the infection starts, and it does not take long for the physical effects to come into effect and be seen on the plant. Zoospores are also swimmers and can move from one plant to another under the right watering conditions.

The entire process will take approximately 7 to 10 days. In a seasonal climate, the spores will not survive the fall and will die with the rest of the crops. Warmer climates, however, such as those in Florida and California, struggle with downy mildew every year. They have climates of both wind and water in the air that are conducive to downy mildew transmission.

When the weather gets warmer in the north, the spores will be transmitted north and begin causing problems there. You can watch your local news or public health units that monitor for these pathogens to see if there is an outbreak in your area.

Treating Downy Mildew on Basil

Man watering basil in pot to treat downy mildew.

When you are working with downy mildew on basil, you should know that there is no treatment or cure for the pathogen. You can, however, control the problem through a number of methods. You won’t necessarily lose an entire plant just because you have a few spores on the underside of leaves, but you are at risk for it if you don’t manage that quickly.

Start by monitoring your basil as soon as you hear word that there may be some pathogens in your area. You will want to harvest or prune as many leaves as possible as soon as you see them. Place them in a secure bag and put them in the trash, where the spores can’t be wind-borne any longer.

You should also increase the amount of space between your plants in order to promote better air movement between them and prevent the spores from traveling from one plant to the next.

For watering, you want to be sure that you water as early in the day as possible. You also want to be sure that you do so before there is direct sun on the plants. If your basil is outside, watering should be done at the base of the plant and never on the leaves directly, as tempting as it may be to add gloss to beautiful green and lush basil leaves.

A system of drip irrigation that includes watering at the level of the ground can help water go right into the roots of the plants. Although it does have an initial investment, you can eventually save money with this sprinkler system as it will consume less water on your plants. You will also save less on plant replacement over time as well.

Another important component of treating plants for downy mildew is making sure they do not have soil on the leaves. Soil is moist and may also carry pathogens. Adding mulch to the soil will help your leaves from getting soil on them, which could result in the spreading of downy mildew. Mulch also keeps water in the ground and keeps your plant roots as moist as they need to be, which prevents downy mildew from traveling.

If your basil is in pots outside, it is a good idea to bring them in overnight when the humidity is high or the rain is excessive on any given day. This will decrease the amount of infection, if any, and also prevent spreading to other plants.

When you do this, you want to be sure that you keep your basil away from other plants indoors, if you suspect there is any chance at all that the basil has been exposed to the downy mildew.

If you do suspect that you have a plant that can be infected, you want to keep it away from all other plants for up to 14 days to make sure it passes the 10-day window where plants can be infected with downy mildew without showing a symptom.

Related: How to Treat Powdery Mildew Without Nasty Chemicals

Preventing Downy Mildew

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Preventing downy mildew in basil is much easier than treating it or removing it together. The most common method of preventing downy mildew is by using fungicides. The best time of year to do this is when the humidity levels are high and the temperatures are low, such as in the later spring and in the early fall. These are an ideal way to set it up and forget it when it comes to downy mildew.

You will still need to check for spores and keep an eye on your plants to make sure the basil isn’t showing any indicators at all of the pathogens. Fungicides at the stems of the plants can also help to keep its vascular system healthy and free of downy mildew. You also want to spray the leaves of the plants on the top and the underside to ensure that the spores won’t be able to reach any of the plants.

A good copper fungicide is a popular choice in preventing downy mildew. The Bordeaux fungicide which is blue and composed of lime and copper is a popular one. You might even see it on tree trunks on occasion, and it is a good one to treat many fungal diseases.

Follow the directions carefully to ensure that the fungicide is not impacting the soil and any organisms, like earthworms that are needed in the soil. Another popular fungicide is neem oil, but you may want to avoid that one as it has a tendency to destroy more than just the downy mildew and also other life such as bees and helpful bugs.

Baking soda on the leaves of your basil is another common fix for downy mildew. If you add one spoon of baking soda with one liter of water, then you have an organic way of treating downy mildew and preventing it from getting onto plants.

Can You Completely Avoid Downy Mildew?

Healthy basil in pot on dark background.

You may not be able to completely avoid downy mildew. You may want to avoid planting sweet basil if you are convinced that downy mildew treatment will mean too much work.

Downy mildew doesn’t like the other strains of basil as much as it likes that one. But, if you want your sweet basil and are okay with a little effort, you will be able to avoid downy mildew. In the end, avoiding it is easier than treating it once it starts.