Causes of Powdery Mildew in Cannabis Plants

Environmental & Climate Conditions

Powdery mildewis a fungal disease that infects a variety of trees, flowers, and fruits and can infect a plant at any stage in its life cycle. When contaminated with the spores, a cannabis plant displays symptoms that include patches of white powder on the upper and lower surfaces of leaves, flowers, stems, trichomes, and other parts typically within 14 days of infection. A plant infected with powdery mildew will experience a loss in nutrition, water, and cell functionality. Plant nutrition starts to deteriorate as the infection prematurely impacts the process of photosynthesis – causing discolored leaves, abnormal growth, and destroying cannabis resin. This means when left unchecked, powdery mildew can not only reduce product potency but also quality and yields in the long run.

Once powdery mildew shows up in a facility, finding the source of the infection is key because when conditions are favorable enough, one diseased plant can produce spores that spread quickly over long distances and cause secondary outbreaks within 7 days of initial infection. While it’s unusual for the fungal disease to kill its host, powdery mildew’s ability to reproduce and survive makes it critical for commercial growers to identify the cause so they can do their best to save the plants and even prevent infection from recurring. 

Once introduced into an environment spores of golovinomyces, the pathogen most commonly attributed to powdery mildew in cannabis, can spread quickly through the air. When wind or greenhouse fans are present, infection is facilitated even further. Powdery mildew prefers humid conditions but, unlike other fungi, is suppressed by moisture. While it tends to thrive in environments in which relative humidity (RH) is 95% or greater, powdery mildew has proven to be productive even when RH levels are below 50%. 

Along with high humidity and low air exchange, temperature can also facilitate the spread of powdery mildew. The pathogen commonly shows up in mid-to-late summer months because temperatures between 68° – 86°F mark a sweet spot for infection, and higher temperatures have proven to be at least somewhat fatal to spores. But when conditions are humid enough, powdery mildew is known to thrive even in shady areas.

The perfect storm of high humidity, poor air circulation, and moderate temperatures makes commercial indoor and greenhouse facilities particularly susceptible to outbreaks. And growers who cultivate in facilities that have yet to optimize their indoor environment can expect to encounter powdery mildew at some point.

Plant Nutrition & Production Practices

The industry’s frequent use of moms, cuttings, and clones for propagation spotlights why so many cannabis cultivators have experience with powdery mildew. After all, poor plant nutrition in one cutting could put the entire garden at risk. 

Infected clones are a common source of the disease, and since powdery mildew needs a living host in order to survive, bringing one infected plant into a facility has the potential to infect an entire crop. Five to 10 days after the initial infection, a single diseased plant becomes a source of spores and is able to spread the disease to other plants. 

When temperature and humidity conditions are optimal for infection, fungal spores of powdery mildew can survive in the shoots, buds, leaves, and other plant parts. Leaving infected plant debris unattended or adding it to a compost pile can contribute to the pathogen’s spread, resulting in infected soil that if utilized will simply result in the infection of every healthy new plant brought into the facility. Overcrowding rooms can facilitate outbreaks even further, making healthy plants more easily susceptible to airborne spores simply due to their proximity to the contamination.

Part of being the commercial cultivator of an agricultural product means accepting that fungal diseases like powdery mildew basically come with the territory. The more growers understand the conditions that can cause powdery mildew the better prepared they’ll be to mitigate the disease anytime it appears. From sourcing and breeding cultivars that show resistance to the disease, to developing best practices and SOPs surrounding facility cleanliness, to adopting an integrated approach to pest and pathogen management, cultivators can also have an impact on how and whether powdery mildew evolves into more of a nuisance than a nightmare.