Powdery mildew is a fungal disease that can affect a wide range of plants. It is one of the most common and costly diseases for nursery plants such as flowers, vegetables, woody plants and cannabis. Temperature, relative humidity, light, and air circulation are all prominent environmental factors that influence powdery mildew and its prevalence. Once it becomes active on a contaminated plant, powdery mildew will leach away a plant’s nutrients, making the plant weaker and bloom less, ultimately reducing yields.
Prevention is the strongest tactic against powdery mildew, but if you get an infestation steps to remediate the disease need to be taken. Two steps to immediately take whether you grow organic or not should be to clean and quarantine. In large greenhouses and indoor growing operations, quarantining an infection can still mean a large quantity of plants have been or are exposed. Spores can hide on any surface in a grow room or greenhouse (including employee’s clothes). Rinsing and wiping down all surfaces (including ventilation, fans, all equipment, ect.) top-to-bottom, inside and out with a bleach and water solution will kill any spores present. Likewise, scrubbing your air if not already doing so is of utmost importance to catch airborne spores. These efforts, in conjunction with remediation efforts listed in this article, will better position you for success in mitigating and halting the spread of powdery mildew.
Preventive steps against powdery mildew that still allow you to maintain an organic standard of growing means any commercial products you use have OMRI approval. If used for cannabis and industrial hemp crops, special attention should be paid to make sure products are approved for use by relevant state and federal agencies.
- Milk: A popular solution for decades, milk has been touted as an antifungal spray, specifically against powdery mildew on cucurbits, such as cucumbers and squash. It is thought that the protein in milk offers an antiseptic-like effect once in the sun.
- Sodium Bicarbonate: Household baking soda is an organic approach long used to stop the spread of powdery mildew (and blight) without affecting vegetable and cannabis plants. It creates an alkaline environment on the leaf which powdery mildew cannot colonize (powdery mildew needs a neutral pH to imbed, survive, and thrive).
- Garlic: Garlic naturally contains sulfurs, and while there are sulfur treatments offered in stores, you should be wary of added chemicals and metals (and the literal stink it will add to cannabis buds). This remedy requires garlic bulbs, not cloves, to make. This should not be sprayed on leaves in direct sunlight or the heat of the day.
- Compost tea: Compost tea stands to provide many benefits to plants outside of powdery mildew remediation. Compost tea is thought to work against diseases like powdery mildew by inoculating plants with beneficial organisms that either create a physical barrier against pathogens or compete and attack them. There are mixed reviews regarding the actual effectiveness of compost tea in combating powdery mildew, but it’s overarching positive effects in other ways make it a worthwhile step to incorporate into your grow operation.
- Apple Cider Vinegar: Acetic acid in vinegar is what makes it helpful in combating powdery mildew. If used in too high a quantity though, it can burn plants.
- Plant oil blends: Blends of organic plant oils (such as garlic, corn and others) can help fight powdery mildew, especially when they are used in conjunction with baking soda.
- Bacteria: Bacillus subtilis and Streptomyces lydicus are naturally occurring bacteria. Bacillus subtilis works by feeding on powdery mildew as its nutrition source. Streptomyces lydicus establishes itself on plant surfaces to attack powdery mildew at its binding site. Be wary of your source for these bacterial components, as they may have other added ingredients that don’t meet organic standards.
- Copper Sulfate: Copper Sulfate can act as both a fertilizer and a fungicide. It can be detrimental to plants if applied excessively or the soil already contains high levels of copper. It can also affect the flavor of vegetables and cannabis.
- Hydrogen Peroxide: Milk has proven more effective and faster acting than hydrogen peroxide in testing, but hydrogen peroxide has still been shown to damage powdery mildew.
- Foggers or Bombs: Typically thought of when dealing with pests like mites, they also exist for fungal diseases like powdery mildew. These approaches are highly flammable and the grow space will require at least 24 hours of ventilation to ensure the product is completely cleared before it is safe for human entry. Foggers can cause damage to plants if triggered too close to them or if used in spaces smaller than recommended.
- Fungicides: The two most common fungicides used to combat powdery mildew are demethylation inhibitors (DMIs) and quinone outside inhibitors (QoIs).
- DMIs are a systemic approach. They must be inside the plant’s tissue in order to be absorbed by powdery mildew. They work by inhibiting biosynthesis in powdery mildew, but will not prevent germination of spores already present.
- QoLs operate external to both plants and powdery mildew. They work by inhibiting respiration and spore germination.
- DMIs and QoLs are best when used in conjunction with one another.
Build Your Defenses
Your best defense against powdery mildew is prevention. If you have an outbreak, the rush to prevent the spread is a challenging endeavor. Look around your grow operation? How realistic is it for you to access and clean every surface of spores? Contact us today to learn how the E1 Air Filtration unit is easy to use, easy to clean and the most hygienic option out there for capturing airborne contaminants.