Crippled leaves of a basil plant that lacks light

7 Tips to Avoid downy mildew in basil

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Basil is an exciting herb to grow both in soil and hydroponics. It is fast-growing, relatively easygoing, and aromatic. Industrial-scale growers also grow basil as a companion plant, and home growers have many reasons to grow it indoors.

Basils may easily catch downy mildew, which, untreated, can be fatal to the plant. This post covers tips on avoiding downy mildew and fixing it if it appears.

Related: Hydroponic Basil Pests and Diseases

What is downy mildew?

Downy mildew is a plant disease caused by various oomycete fungi in the family Peronosporaceae. The one that attacks basil is Peronospora belbahrii. These fungi spread through air, water, or with the help of other insects. For instance, ants travel from plant to plant, carrying the spores. These spores grow in the vascular system and block the path nutrients are taken up by the plants.

Like many other fungi, downy mildew thrives in cool and humid conditions and can withstand different seasons. It is often devastating to the plant. Try to prevent it rather than try to fix it.

Symptoms of Downy mildew in basil

Downy mildew symptoms are specific to the host. Since this post concerns only downy mildew in basil, let’s discuss it.

The most notable downy mildew symptom is yellow spots on the upper part of the leaves. On other hosts, downy mildew creates a fuzz on the bottom of the leaf surface. But basil rarely shows this symptom.

With time, the yellow spots turn brown. Eventually, the entire leaf withers and drops. This process continues to many or all the other leaves, and the plant dies.

Other symptoms include stunted growth and discoloration in the overall plant.

How to prevent downy mildew in basil.

As we now know, downy mildew is deadly, and treating it is more challenging than preventing it. Here are our top tips for protecting your basil plants.

1. Pick a resistant variety

Although basil does catch downy mildew, some varieties can fight well against it. For instance, in my garden, the lemon basil got downy mildew. But the sweet basil, only two feet away from the lemon one, didn’t catch it.

Most people grow sweet basil, known as regular or common basil. Fortunately, sweet basil can withstand downy mildew. Yet, there’s no proof that sweet basil is immune to downy mildew. Thus, read on for more tips.

2. Introduce airflow

Besides the variety, letting the airflow through the plant is the number one tip to prevent downy mildew. It works for basil as well as many other plants.

Downy mildew needs a wet environment to spread. By introducing airflow, you dry off the leaves and boost transpiration, which helps maintain a relatively dry atmosphere around the leaves.

You can move the plant to a windy area or install a fan to improve air circulation. Also, avoid planting basil beside other plants with large, dense canopies, as these plants may block airflow.

Related: How to Install Inline Fans in Your Indoor Grow Room

3. Do not wet the leaves unnecessarily

If you’re growing basil in the soil, do not water the plant from the top. This often happens when using sprinklers in home gardens. Instead of spraying water all over the plant, try dripping, which directly supplies nutrients to the soil.

Watering basil directly near its roots would keep it dry. But if you’re growing outside and the plant is exposed to prolonged rain, there’s a high chance it will get downy mildew. Protected environments like greenhouses would help keep the plant sufficiently dry.

4. Do not use the same scissors without disinfecting.

A common way downy mildew spreads from plant to plant is when pruning scissors are used without cleaning them. This is the prime reason basil doesn’t grow after a harvest. Clean your scissors with disinfectant wipes or diluted hydrogen peroxide.

5. Avoid contaminated seeds and cuttings

Basil grows fast and blooms fast. If we leave it as it is, they will produce seeds. Yet, if the basil plant has downy mildew, it spreads into the seeds, too. If the seed is not treated well, the plants that grow from that seed (if it does) may also be infected. Also, only buy seeds from trusted sources.

As we know, basils start with seeds and cuttings. If you prefer to grow from cuttings, make sure the cutting and the mother plant are free of downy mildew symptoms.

Related: How to propagate basil from cuttings

6. Harvest in regular intervals

Harvesting creates more room for the air to flow through. Since basil can grow fast in summer, harvesting every two to three weeks may help. In winter and indoor gardens, you can wait for one more week or follow the basil pruning rule. You leave the first leaves in every stem and cut the rest.

Related: How often can we harvest basil?

Pruning basil leaving two sets of leaves and stems

7. Monitor

We’ve done everything possible to prevent diseases, but the rest is pure luck. Downy mildew happens to every plant, including basil. Although we say by the time we notice symptoms, it’s too late, we can at least control its spread. Routinely check your plant’s foliage, flowers, fruits, and stems. If you grow in hydroponics, please check the roots too.

Are hydroponic basil more vulnerable to downy mildew?

There’s no evidence that hydroponic basil catches downy mildew more often than basil growing in the soil. As long as we don’t wet the leaves, the chances of an infection are low in both methods.

Related: How to Grow Hydroponic Basil: A Step-by-Step Guide

Yet, the rate of disease spreading is high in hydroponics. Downy mildew spores can disperse through the water and affect any part of the plant. Thus, spores in the roots can travel very fast through the nutrient solution to a different plant. All hydroponic systems have this drawback unless you grow each basil plant in separate containers.

Often, if you see a downy mildew attack in hydroponic basil, the best thing to do is discard the entire batch and start from scratch.

Root rots and downy mildew

Although root rot and downy mildew are separate diseases, root rot is often followed by a downy mildew attack.

Root rotting is caused by insufficient oxygen around the plant’s roots. In such conditions, fungi like Pythium grow in the roots, blocking them from uptaking nutrients. It’s common in hydroponically grown basil, yet soil basil can also experience this due to overwatering.

It’s unclear whether root rot and downy mildew have a causal relationship. Overwatering or a wet climate may induce both these diseases. Yet, the rotten roots could also prevent the plant from maintaining its immunity.

Root rot in basil is a mild problem that can be fixed quickly, but downy mildew can be devastating.

Related: Fixing Root Rot in Hydroponic Basil

Using fungicides to fix downy mildew.

Neem oil is our favorite fungicide to fight downy mildew. It’s organic and a solution for many other plant problems. It works on basil just as well as on other plants. Dilute neem oil per the manufacturer’s instructions and spray it directly to the foliage. You can apply again in a week. The drawback of neem oil is that it kills even the beneficial insects. It’s more concerning in fruiting plants as neem oil may chase bees that help pollinate. But for basil, it’s only a minor issue.

Copper sulfate is often used to treat downy mildew. However, copper is a heavy metal that can be retained in basil leaves. When consumed for a long time, it may be a health hazard.

The best time to apply fungicide is when you suspect a downy mildew attack. This news can also come from your neighboring farm or garden. Apply the fungicide as soon as you suspect a potential infection rather than waiting for the symptoms.

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