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How to Prevent Downy Mildew on Basil

Preventing Downy Mildew on basil.

Downy mildew is mildew that is spread by the wind and can reach the leaves of your basil plant. It infects basil plants in America every single summer and many other surrounding plants as well. For some farmers, the wrong amount of downy mildew could result in an entire lost crop and a significant financial loss for the season.

For home gardeners looking forward to their basil crop all winter long, downy mildew is going to feel as great a disaster. Use this guide to learn how to prevent downy mildew on basil every year, and you’ll be ahead of the mildew game all season long.

Related To: How to Bring a Dead Basil Plant

Signs of Downy Mildew on Basil

Signs that basil plants has Downy Mildew.

Signs of downy mildew on basil include a yellowing on the leaves and a fuzzy mold that occurs near the bottom. This is a pathogen or toxic chemical, that is called Peronospora belbahrii. The mold can be gray in color while the leaves will turn yellow. This yellowing moves through the veins of the leaves and will eventually turn the color of the entire leaf, starting with the underside of the leaf.

This is called chlorosis. Chlorosis is a sign that the leaves are lacking in nutrients. As it grows, you will notice that the spreading of the mildew remains on the bottom leaves entirely and on their surfaces.

You will see spores on the other side of the leaves facing the ground. The best time to notice this is early in the morning. If you think you might have some downy mildew present, remove an affected leaf and put it in a plastic Ziplock bag or container, and leave it in the dark for 24 hours. This will induce the development of more spores if downy mildew is on the leaves.

If you are really concerned, take the ziplock of the leaf to a university or public health unit to have the leaf examined. Most universities take great interest in plant pathogens

If the leaves begin to turn black, they have become necrotic or have died.  You may see some of this with the spores of mildew on the underside of your basil leaves. The spores of this pathogen are gray in color and look like little dots. Necrotic leaves with black spots will begin to fray at the ends, and the problem may not be exclusive to the underside of your basil leaves.

The mildew infection begins at the base of the leaf of the plant and grows up as the plant does. You will notice that the leaves will begin to die off on their own and fall off of the plant. As soon as you notice them, prune them to keep the plant as healthy as possible for as long as possible.

Seeds Treated to Withstand Downy Mildew

Treated basil plants to survive from downy mildew.

You can purchase basil seeds that have been treated to withstand downy mildew. Look for seed farmers and marketers that offer basil seeds treated to prevent downy mildew from occurring. These are not foolproof seeds that have been marketed, but they may help you to prevent downy mildew.

On average, seeds marketed as having been treated to withstand or prevent it generally works. The concept behind these seeds is that they have been developed with genes that are resistant to downy mildew. You may find that some seeds have been heat-treated to withstand downy mildew, and this is not the same as a seed that has been developed genetically to withstand downy mildew.

Not all seed growers are created equal, so you will have to do some research to locate the best seeds for you. Even though there are some varieties on the market of downy mildew-resistant basil seeds, you may just be better off using natural gardening methods such as pesticides to prevent downy mildew from occurring on your basil plants.

Pesticides to Treat Downy Mildew in Basil Plants

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Pesticides and fungicides are among the most common and most popular methods used to treat downy mildew in basil plants. When you use fungicides, the earlier you start, the better. You want to start treating the basil before you see any symptoms of the downy mildew.

Many municipalities will offer local alerts for the presence of downy mildew that is carried by the wind. Coastal areas are going to see this the most, as the moisture in the air can be an incredible tool for encouraging downy mildew growth. Do your research to find out when downy mildew is active in the area, and you can start your fungicides then.

Organic fungicides are another way to treat downy mildew and aren’t any less effective than traditional fungicides and pesticides. Some organic fungicides are labeled specifically for use with basil, but you don’t have to get them just for downy mildew. You won’t always have to treat downy mildew, especially if you are afraid of fungicides.

However, if downy mildew is prevalent in the area one season, you may want to begin your fungicide treatment as early as possible. In many cases, a single dose of the fungicide is all that you will need to treat your basil plants to prevent them from getting downy mildew. You may want to spray more than once a season, particularly if it seems like it’s a bad year for downy mildew.

Check the labels with your fungicides and pesticides to be sure what the recommended spraying cycles look like. These will be listed on the bottle or with its instructions, and you should follow those instructions to the letter.

Additionally, there are natural ways to prevent downy mildew in basil plants.

Natural Ways to Prevent Downy Mildew in Basil Plants

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When you are trying to reduce or prevent downy mildew on basil plants, there are some natural methods that you can start with. You want to watch the moisture level around your basil plants to start. In many cases, downy mildew is caused by wind-borne problems along with excessive moisture and humidity.

Finding ways to control these factors in a greenhouse or outdoor gardens is the way to begin to prevent downy mildew in basil plants. You truly need to watch how much you are watering as well. Not only will excessive watering stifle the basil plants, but it will also promote conditions that could result in the growth of downy mildew.

The key here is to ensure that the leaves don’t hold excess water. Having a little on them in the morning as they dry in the shade is a good idea, but you want them to have their drink and be done with it. If you are in an area where the humidity is high, fans and lights can help to decrease humidity.

While lights, even outdoors, can contribute to warmth, they will reduce humidity in the garden for your plants. Outdoor fans or fans in the greenhouse after watering can help to reduce humidity around your basil plants.

The science behind the fans is that they will prevent the water from staying on the leaves after you water your basil plants. You don’t want the leaves to be watered when the basil is in the full sun for the day, as the sun will dry out the water and burn the leaves. This will not only help to ruin the basil but will also help to contribute to downy mildew.

Water in the morning or when the basil is in partial sun or shade. Fans will help to blow the water off of the leaves. For outdoor gardens and greenhouses, water the soil of the basil plants so that you can avoid damp leaves altogether.

You want humidity levels to be below 85 percent when you are trying to suppress downy mildew. You can purchase humidity sensors that will help you to keep a check on this if downy mildew on your basil is a serious concern. It might be, but it also might be a serious concern for other plants as well. Remember that if basil is infected, then the other plants around it are at risk as well.

Using Night Lights to Control Downy Mildew in Basil Plants

An indoor basil with night lights.

Using lights as a method to control downy mildew is a method that has been used for a little while, and that is because downy mildew likes to grow at night. In this instance, the lights are used at night, and many studies have been conducted to test lights on many everyday plants to determine if downy mildew is prevented with this. The answer is usually yes.

In this method, you only have to illuminate the surface of the leaf plants at night in order to suppress any growth or spreading of downy mildew. This will prevent spore production and inhibit any existing growth of downy mildew. Fluorescent bulbs that are 20 W are considered ideal for this method of preventing and controlling downy mildew growth.

In some cases, red lights and red light bulbs have been used to accomplish the same thing. Red lights help to not only work with preventing downy mildew but also with promoting leaf count and leaf growth and even helps to grow bigger plants.

One of the reasons that the light works are due to high temperatures. Keep a thermometer in the room where you are using this method. You want to see the room reach up to 113 degrees Fahrenheit. The spores will grow and be happy in temperatures between 80 and 88 degrees Fahrenheit. They will die at temperatures much higher than that. Keep the lights on them for at least six hours and not longer than nine hours.

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In some studies, researchers have learned that downy mildew is killed by light under certain conditions. The effectiveness of light in killing downy mildew depends on the duration of light and also the wavelength of light. These researchers found that every strain of downy mildew or pathogen could be sensitive to different wavelengths and different duration of light.

You will have to play with your lights to figure out which is the most effective solution for you. At the same time, you don’t want to make a mess of it. If you have lighting going and your basil doesn’t seem infected, leave your routine as is.

Interestingly, solar lighting is another method that is very effective in accomplishing the same goal. That is because it will trap in the heat that is necessary to kill the pathogen. Unfortunately, unless your basil garden is already indoors or in a greenhouse, that is going to be a very difficult task.

Keeping this heat and light component in mind, when you are destroying basil leaves with spores on them, hot and sunny days are the best days to do so. At the same time, when you are trying to kill or prevent downy mildew from occurring, keep your seasonal timelines in mind.

In seasons where there is less sun, such as winter, keep your plant night lights on for longer to prevent and inhibit the growth of downy mildew. You will know by the way your basil looks when it is growing. Healthy basil looks smells and tastes like healthy basil. Infected basil does not. Play with your lights and temperatures and you will know what is the right combination for your garden.