Powdery Mildew How To Guide: Containment, Control & Prevention

Powdery Mildew How To Guide: Containment, Control & Prevention

"How do I get rid of Powdery Mildew?" is one of the most common plant questions we get here at GrowersHouse. Controlling and eliminating Powdery Mildew takes more than one tool and some time but it is possible to contain an outbreak and prevent future contamination. We recommend a 3-step approach to keep your grow room or green house Powdery Mildew free:

1. Identify


2. Prevent


3. Treat

1. Identify: Do I have Powdery Mildew? What Does it Look Like?

Powdery Mildew and its easily identified powdery white spots is one of the most common diseases in Greenhouse and indoor agriculture. This crop destroying fungus can infect a wide array of plant species and spread rapidly through greenhouses both indoors and outdoors alike. Powdery Mildew can be transmitted by other plant pests such as aphids or simple contamination by touch or with infected tools, clothing, or boots, and sharing ventilation with previously contaminated spaces and clean spaces

First symptoms of infection are the dusty gray or white spots on either the upper or lower surfaces of leaves. Left untreated Powdery Mildew will begin to produce spores and begin to spread into stems, fruits and flowers. Although plant death only occurs in extreme cases, PM can greatly reduce fruit or flower yield and aesthetics, and can also decrease the overall value of contaminated crops.

*IMPORTANT*

If a Powdery Mildew infection has already been detected, any affected plants should be bagged and removed from the environment. Manual monitoring frequency should be increased to prevent further spread of spores and infection. Clean and sanitize any tools or equipment which regularly come into contact with multiple plants to keep mold spores from cross contaminating healthy plants.

By researching which specific species of fungi afflicts your specific crops, specific conditions and preventative techniques can be focused on to keep your garden powdery mildew free. It is important to mention that no matter which method you choose to treat or control powdery mildew, you will get best results by combining a systemic method and a surface method together. The combination of effects will provide killing pathogens on contact, suppressing spore growth, limiting systemic spread and provides plant system protection. Rotating use of multiple methods or products may be important, as commercial agriculture does. Strains of fungi and other plant diseases can become resistant to repeated use of the same treatment.

2. Prevention: Cleaning & Sanitation - Before, During and After

Molds and fungus particularly like dry foliage surfaces and humid environments, greenhouses and indoor gardens are especially susceptible to powdery mildew and mold infections. In general, relative humidity (RH) levels should be kept low (40% - 55%) and constant (fluctuating environmental factors are conducive to powdery mildew infection). Providing adequate spacing between plants will help manage the humidity level in and around the leaf canopy, while also allowing treatments to reach all surfaces of the plants. Limiting extra growth in lower leaves by trimming has shown to be helpful by limiting the most vulnerable leaves lower on the plant stem. Liquid water applied directly to high-risk surfaces can also discourage the germination of the fungal spores, but it is important to be sure this does not encourage the development of other plant specific diseases which benefit from liquid water.

Practice active sanitation methods with disinfection practices to minimize contamination.

It is important to sanitize tools, equipment, structures, and even people:

• Boot baths or sprays for walk-in greenhouse spaces are the first line of defense for preventing tracking in pests


• Hand washing, hand sanitizers, and antimicrobial soap  suggested between handling plants, use with alcohol based hand sanitizers for best results


• Tool Dip Disinfectant for scissors, trimmers, or other hand held tools. Immersing tools for 2 minutes in a disinfectant can prevent cross contaminating to healthy plants. (Such as Physan 20 Fungicide Virucide, 4 tsp/1 gal water


• Hard Surfaces sanitizing like pots & trays, walkways, re-useable equipment, evaporative coolers/water, hydroponic tanks/tubing, and replace or wash HEPA filters


• Area Disinfection between plant cycles to throughly sanitize rooms and all surfaces with a rated hard surface disinfectant such as (ProKure V Disinfectant, BioSafe SaniDate), and to spray or steam clean transportation vehicles or truck beds

Best practices like the ones listed above are easy to implement in a new environment or space as well as in current and established grows for controlling Powdery Mildew. Once your growing operation is set in motion, monitor and manually inspect plants for infection on a regular schedule, focusing on mid to lower leaves, as this is where infection most often begins. Between production cycles, all surfaces should be thoroughly sanitized and cleaned as well as all debris and leftover foliage removed.

>If you want to take it a step further because you believe your area is highly susceptible to powdery mildew, alternate use of powdery mildew preventatives such as Serenade Garden Disease Control or PM Wash at a diluted rate (we recommend ¼ strength).

3. Treatment: What is Safe to Use on Powdery Mildew in a Garden?

UV Ultra Violet Lights:

Because powdery mildew and other infections such as fungal spores spread easily via airflow and ventilation, UV air-filters can be an extremely helpful tool to prevent the spread of these diseases. UV light (in limited exposure) via a handheld light can also be directed at plant surfaces and leaves in order to deactivate/kill Powdery Mildew, fungus and bacteria already present or forming in the grow room. Use care and limit exposure time with UV lights because overexposure can be harmful for your plants too! Use UV lights specially made to treat powdery mildew and molds because they are made at an intensity that is tested to kill powdery mildew with minimal harm to your plants when used as directed.

Chemical Treatments:

Inorganic Treatments:  Despite the potent abilities that nature has in this area, often chemicals manufactured through industrial chemical processes can be more effective, efficient, and economic. Many chemical based disinfectant treatments are safe to use with live plants. For example, potassium salts and phosphorous acid can protect and treat many fungal and bacterial infections by providing nutrients to plants that cannot be absorbed by fungi, strengthening crops and weakening disease.

Stronger chemicals can be utilized in various ways to prevent bringing disease into the grow room initially. Peroxide solutions can be used as an eco-friendly way to clean hard surfaces. For hospital grade sanitation, Chlorine dioxide solutions can be used to immediately kill microorganisms and pathogens on any hard surface.

Organic Treatments: Organic fungicides are derived from various plants and natural mechanisms. A few of the most commonly used organic agents to control Powdery Mildew: herbal oils like Neem, insecticidal soaps, sulfurs and other elemental pesticides, citric acid, and bicarbonates, Milk and many more.

• Neem Oil along with various other oils are derived from trees and plants, which contain chemicals with broad pest control properties


• Garlic also naturally produces antimicrobial compounds, which can be extracted and used in spray solutions


• Milk or dry milk powder has been an effective tool for organic minded gardeners and some organic farms use a whey based foliar spray to inhibit growth of several species of powdery mildew


• Insecticidal Soaps are based on potassium fatty acids which have nearly no toxicity to mammals, making them safe around children and pets, and have been approved for use on powdery mildew in the United States since 2011


• Sulfur forms a natural elemental compound which prevents fungi from germinating on leaf surfaces due to decreases pH levels. Sulfur can be vaporized in the grow room or sprayed conventionally to form a layer of protection. It is important to mention that sulfur is an eye and lung irritant, and protective measures should be taken, especially ventilating away from living spaces around the area


• Potassium Bicarbonate works similarly to sulfur by making the leaf surface uninhabitable to fungi, and is most often mixed in solution and sprayed over the plants


• Sodium Bicarbonate, or baking soda, can also be used in solution to a similar effect


• Beneficial Microorganisms have evolved ways to prevent disease, and by introducing these organisms to the garden they can also provide that protection to the plants on which they live. Bacillus Subtilis is one of these organisms, often found in the digestive tract of mammals and humans

Climate Control:

Because powdery mildew relies on fairly specific environmental conditions, maintaining a regular climate control schedule is paramount to preventing an initial infection. By using dehumidifiers to keep relative humidity levels low, spores will not receive the environmental cue to germinate. This cue also comes with fluctuating humidity, thus using a system of humidifiers and dehumidifiers to maintain the optimal humidity, despite differences in temperature in and out of the grow room, can prevent mildew from ever becoming a problem. There are a wide variety of climate controllers on the market, many coupled with thermostats and hygrometers, even some with wifi connectivity to send notices when changes occur.

Sterilization by UV Light (Ultraviolet radiation) has been practiced in medicine since the 1800's, with successful application to air and water purification for agriculture iof both UV-C and UV-B. Because Powdery Mildew fungus is transparent and has no pigment for protection against UV light, Ultra Violet radiation can easily penetrate cell walls and damage their DNA, killing them directly or rendering them unable to perform essential life processes including replication.