How to get rid of powdery mildew with hydrogen peroxide

Powdery mildew is a fungus that can spread very quickly in a cannabis crop and severely damage the plants. Furthermore, if it affects the buds and they are smoked, it can cause lung damage. That’s why it is crucial to identify and eliminate it as soon as possible. Below, we explain how to recognize it and how to get rid of powdery mildew with hydrogen peroxide.

What is powdery mildew?

Powdery mildew is a formidable fungus that arrived in the Iberian Peninsula in 1851 from England and France. Since then, it has become a nightmare for growers as it can cause severe damage to any plantation, whether outdoors, indoors, or in a greenhouse. Humidity above 70% and temperatures between 15 and 35ºC are precisely what powdery mildew needs to develop rapidly. 

How to recognize it

Before disease symptoms appear on the plant, vesicular spots can be seen on the leaves. These vesicles will later develop into a white powder that is easy to identify. It is abundant, as if the leaf had been dusted with flour. There is another fungus called downy mildew that looks very similar to powdery mildew, although they are different fungi. Generally, powdery mildew appears on the upper side of the leaf, while downy mildew usually affects the underside.

Types of powdery mildew

There are many species of powdery mildew, often specific to particular crops. For example, some only affect grapevines, while others target certain varieties of cabbage. Sometimes they are so selective that they distinguish between strains. For instance, one type of powdery mildew affects cereals but can only infect wheat or rye, not both. 

The varieties that affect cannabis are Leveillula taurica and Sphaerotheca macularis. The latter also infects other plants like strawberries and hops. 

How powdery mildew infection occurs

Spores land on cannabis leaves, and if the right temperature and humidity conditions are present, they can germinate. Once germinated, the spore develops a filament that penetrates the leaf and serves to feed on the plant cells. As it grows, it forms more filaments, and over time, it produces reproductive organs that release new spores, further spreading the infection. 

Powdery mildew spores need moisture to germinate, but for growth, the leaf must be dry. This means there is a higher risk of infection spreading during dry, hot periods when morning dew forms on plants. 

Dangers of powdery mildew on cannabis plants

An infection weakens the plant by stealing nutrients from the leaves and causing stress, leading to stunted growth and flowering. Additionally, it affects the quality and taste of the harvest. Buds affected by this fungus should not be smoked as they pose a serious risk to respiratory health. 

How to get rid of powdery mildew with hydrogen peroxide

There are different ways to eliminate powdery mildew, but one of the simplest and most cost-effective methods involves using a household product: hydrogen peroxide. 

To do this, you need 3% hydrogen peroxide, which means that in one liter of solution, there are 970 milliliters of distilled water and 30 milliliters of H2O2. It’s important to use this concentration, as a higher one may damage the plant. To combat and remove powdery mildew with hydrogen peroxide, mix it with water in a 1:3 ratio (one part peroxide and three parts water). Then, simply spray this solution on the affected areas.

Additionally, hydrogen peroxide can be used for watering plants, helping to maintain root health. It is recommended to dilute the peroxide in one liter of water and water the plants. Afterward, allow the soil to dry completely before the next watering.

How hydrogen peroxide works

This common household product helps to get rid of powdery mildew from three different angles:

• Antifungal Properties: Hydrogen peroxide releases active oxygen, which oxidizes and destroys fungal cells. This process damages the cell walls of the fungus and stops its spread.
• Spore Cleansing: Powdery mildew spores on the leaf surface are eliminated upon contact with hydrogen peroxide, reducing the fungus’s ability to reproduce.
• Improved Oxygenation: In addition to fighting the fungus, hydrogen peroxide can benefit the plant by increasing oxygen levels in the root environment when used correctly.

Preventive measures

As we have seen, powdery mildew can be eliminated with hydrogen peroxide, but prevention is always the best approach since this fungus can cause significant damage. There are several guidelines you can follow to minimize the risk of this disease appearing in your crop.

• Maintain cleanliness at all times, removing plant debris and dry foliage.
• For indoor cultivation, it is recommended to avoid entering with outdoor clothing and wash hands thoroughly before handling plants.
• Keep humidity levels below 60%.
• Be cautious with nitrogen levels (powdery mildew thrives on nitrogen-rich plants).
• Ensure proper ventilation in the grow area and between plants.
• Keep pots clean and disinfected.
• Monitor and control insect pests such as whiteflies and aphids, as they can carry spores.
• Use a powdery mildew fungicide occasionally. There are biological fungicides such as trichodermas, which naturally protect plants against fungi.

It is crucial to identify and get rid of powdery mildew as soon as possible because, in addition to being a serious disease for cannabis plants, its spores can also pose a risk to people who inhale them.