How to Manage an Inherited Powdery Mildew Outbreak

Question: I recently took a position at a cultivation facility in a tropical island environment that is infected with powdery mildew. How can I best eliminate the current outbreak? How can I prevent further outbreaks or proliferation? And what contributing factors can be addressed to minimize the risk of reinfection?

Answer: Powdery mildew (PM) outbreaks can be costly, especially when products fail contaminant testing. Your question is three-pronged, so let’s address the most pressing issue: eliminating the outbreak you inherited.

Handling a PM Outbreak

Destroying all infected plants or plant material is the easiest method of solving a PM infection, although it is rarely the preferred option. There are multiple products available to treat PM, including neem oil and fungicides made of natural ingredients like essential oils.

Each jurisdiction overseeing cannabis testing has slightly different specifications regarding acceptable products to use on crops, so it’s important to consult with your regulator before you start a treatment.

While the majority of these products help treat outbreaks, most do not completely eliminate the possibility of further proliferation. For that, growers need to find the root cause.

PM Sources

PM can enter a facility in many ways. Were new clones introduced recently? Growers must always quarantine all new clones or plant material before introducing them into the cultivation facility. All new clones should be heavily scrutinized and decontaminated before introduction.

Is the air entering the facility sterilized and filtered? I heard of a greenhouse that was horribly infected with PM from being a couple of miles downwind of a pumpkin patch. The pumpkins’ leaves were covered in PM, and the wind would carry the PM spores into the greenhouse’s intake ventilation.

The only realistic option in such a situation is to employ a combination of positive pressure in the greenhouse, air sterilizers, and high-efficiency particulate air (HEPA) filtration.

Creating a positive-pressure environment makes it difficult for pests and fungal spores to enter the greenhouse by forcing air out of the greenhouse when there is an opening. This is done by pushing more air into the room than what’s being exhausted. Air sterilizers and HEPA filters are added to the greenhouse intakes to purify that incoming air. All of this can be complex and very expensive to accomplish.

The best way to prevent cross-contamination is to always be aware of your surroundings, as neighboring plants and crops are susceptible to both infection and infestation of diseases and pests that can accidentally spread to your facility, especially if it is not yet contamination-proofed.

Establishing and practicing clean decontamination measures, such as changing attire before entering cultivation environments, decontaminating hands and feet before entering and touching plants, and utilizing blowdown chambers before entering cultivation environments, can also help growers avoid accidentally bringing in contaminants from their homes.

Outside of limiting external sources, multiple environmental factors can be optimized to minimize reinfection risks once you have the initial infection eliminated.

Optimize Your Environment

Ensuring proper ventilation can eliminate hot, stagnant air pockets where PM can proliferate. Moving air through a cultivation space is more nuanced than simply adding a few fans: airflow, air velocity, and air exchange must be carefully balanced. Using a combination of deliberately placed vertical air flow (VAF) and horizontal air flow (HAF) fans along with appropriate plant placement will help keep air moving and help wick away moisture.

It is also imperative to maintain a proper vapor pressure deficit (VPD) to maximize plant health, yield, and quality. VPD is the difference (deficit) between the amount of water in the air and how much moisture it can hold when saturated. When air becomes saturated, water will condense on leaf surfaces, making it more susceptible to fungal infections.

The ideal VPD is between 0.8 and 0.95 kilopascal (kPa). A high VPD (greater than 1.0 (kPa)) indicates that the air is still capable of holding a large amount of water and may overdry plants.

Both temperature and relative humidity influence an environment’s VPD, with a higher temperature generally raising VPD and higher relative humidity (RH) lowering it. When measuring temperature, it is important to take the leaf surface temperature measurements during the middle of the photoperiod—when transpiration would be highest—to help make sure the VPD is within preferential ranges.

Many new technologies and equipment are evolving rapidly around environmental controls today—a quick tour of a trade show today will yield a plethora of environmental controls, dehumidification systems, CO₂ recirculation units, and more.

The ability to tightly control environmental conditions and reduce outside contaminants from entering your facility will reduce the chances of plants becoming infected with PM.

Kenneth Morrow is an award-winning grower and breeder. He is the owner of Trichome Technologies, a research and development company. He also is a consultant and author of the book “Marijuana Horticulture Fundamentals” and a founding advisory board member of Cannabis Business Times, where he has been a columnist since 2015. Find him on Instagram @trichometechnologies or reach him via email at: [email protected].