Powdery Mildew
No matter how hard you try to escape the confines of nature, growing plants is inherently an ecological endeavor. This means it is a practice in which many biological (plants, beneficial organisms, and pests) and non-biological factors (air quality, humidity, light, water, etc.) come together to form a system. Each of these constituents must be monitored and managed for the system to coalesce into something desirable and productive.
The prefix eco- in “ecology” comes from the Greek word for house, meaning that an ecosystem is a structure—like a house or building—made up of different parts. Some parts are foundational; others form the walls or serve other functions. Together, these parts create an emergent whole that is greater than the sum of its parts. If well built, it may even develop its own integrity and ability to stand on its own.
In this frame of thought, every emergent problem that an indoor cannabis grow faces can be understood as a systemic failure that demands an ecological, whole-systems solution. Let’s take a close look at one of the most common problems faced by indoor cannabis growers: powdery mildew. We’ll examine the pillars of the problem—the conditions that set the stage for an outbreak—and how they can be tweaked to build greater operational resilience.
What is Powdery Mildew?
Powdery mildew is an ascomycete fungus, also known as a sac fungus, meaning it forms and distributes its sexual spores within microscopic sacs called asci. Other familiar Ascomycetes include morels and Penicillium (the source of penicillin).
A morel, an ascomycete fungus mushroom.
Ejdzej. (2006, May 4). Morchella conica [Photograph]. Wikimedia Commons.
The phylum Ascomycota, to which these fungi belong, is a large and diverse lineage that diverged from the Basidiomycota roughly 500–650 million years ago, likely in the late Precambrian or early Cambrian period. Basidiomycetes include mushrooms like Amanita, Pleurotus, Ganoderma, as well as rusts and smuts.
Why does this distinction matter in an indoor cannabis facility? Because Ascomycetes and Basidiomycetes—though both highly successful—have evolved very different strategies for reproduction, colonization, and metabolism. These physiological and anatomical differences have direct implications for how they’re managed in controlled environments.
For example, Basidiomycetes primarily spread through basidiospores, which are sexual spores produced on specialized structures called basidia. Their fruiting structures (mushrooms, brackets, etc.) take longer to develop and require specific environmental cues for spore release and germination.
By contrast, Ascomycetes like powdery mildew also produce sexual spores (in asci), but their primary means of rapid spread is through asexual spores called conidia. These conidia are mass-produced and easily dispersed by air currents, water movement, or human activity.
![Ejdzej. (2006, May 4). Morchella conica [Photograph]. Wikimedia Commons.](https://commons.wikimedia.org/wiki/File:Smardz-Morchella-Ejdzej-2006.jpg){kind=link}
Microscopic image of a conidiaspore from the ascomycete Alternaria, another plant pathogen.
If you’ve ever seen or handled a plant infected with powdery mildew, that white, dusty coating is actually millions of these conidia. Touching a healthy plant after handling an infected one transfers them easily—one reason worker and room sanitation protocols are so critical.
In nature, powdery mildew’s strategy is to produce a cloud of spores that are picked up by air or water, reaching new host plants and restarting the life cycle. Basidiomycetes, by contrast, spread more slowly and often persist in soil or substrate as saprophytes (decomposers).
Most Basidiomycetes are saprophytic, helping to break down organic matter into simpler molecules that can be recycled into plant and microbial nutrition. Many are intentionally introduced into living soils—for example, Pleurotus (oyster mushrooms)—to enhance decomposition and soil health. Others, like Rhizoctonia, are pathogenic and cause root diseases.
Powdery mildew, unlike saprophytes, infects living plant tissue, triggering plant immune responses and extracting carbohydrates from photosynthetic cells in a decidedly parasitic fashion. The infection drains the plant’s metabolism by both diverting photosynthates into fungal growth and by damaging the leaves, which reduces photosynthetic capacity.
Understanding that different kinds of fungi—Basidiomycetes and Ascomycetes—proliferate under different environmental conditions is key to managing the ecological trade-offs of an indoor grow. Knowing that powdery mildew thrives under certain temperature and humidity ranges pushes us to become more data-driven growers, adjusting parameters to suppress its life cycle.
In future installments of this series, we’ll explore practical changes—how to use humidifiers and lights more strategically, how to select resistant genetics, and how to implement sanitation practices that disrupt powdery mildew’s reproductive strategy. Over time, by manipulating what we can control in a grow environment, we can reshape the ecological “house” we’ve built—strengthening its foundation and walls so the system itself becomes more resilient and self-supporting.