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Powdery Mildew on Cannabis: Detection Myths & Realities

ByTrichome Team

August 24, 2023

Reading Time: 3 minutes

Powdery mildew is caused by fungal pathogens that exist as parasites on living plant tissues. They cannot live apart from their host, and the visible mildew itself is a “symptom” of the infection that you see in the form of spores from the fungus across the plant. Powdery mildew is particularly common in cannabis plants because it enjoys the warm and dry environments that are best for cannabis flowering.

The extent of powdery mildew is staggering: With over 400 species, it colonizes nearly 10,000 plant species. Its ubiquity means that practically anyone who has cultivated plants has encountered it. Although it might appear concerning, powdery mildew is harmless to humans and can be easily washed off vegetables from the garden.

Despite its mold-like appearance, as a parasite, its inability to survive separately from its host means that it won’t register on standard laboratory yeast and mold tests. In the context of cannabis, this translates to the fact that its presence wouldn’t impact microbial compliance test results. Nevertheless, should it infiltrate your cannabis cultivation, it has the potential to significantly diminish both crop yield and potency.

Passing With Powdery Mildew?

A common question that arises is, “I can visually identify PM in my facility, so why doesn’t the sample fail compliance testing for mold?” This confusion stems from the fact that mildew, unlike mold, requires a living host to thrive, making it unable to culture on artificial media commonly used in laboratory testing.

Testing for powdery mildew is more complex than some might assume. Unlike other fungi such as yeasts and molds, powdery mildew fungi cannot be cultured on artificial media. Consequently, they won’t show up in the required yeast and mold or other microbial compliance panel test, as they require their host to survive. This means that even if cannabis plants are affected by powdery mildew, standard tests and panels won’t indicate a failure.

To reliably detect powdery mildew, DNA-based methods like PCR tests are the only dependable option. This applies to all types of powdery mildew, including downy mildew and rust fungi. Through a PCR test, it’s possible to identify fungi responsible for powdery mildew in cannabis, such as common strains of Golovinomyces spp. If the facility’s powdery mildew issue is caused by these specific species, it can be accurately detected using this method.

Detecting and Addressing Powdery Mildew

Visual observation remains the main means of identifying powdery mildew on cannabis plants. Any white patches you notice are likely indications of its presence. The symptoms of powdery mildew can vary, but they often manifest first on older leaves, typically lower down the plant. Early signs to watch for include a loss of color and the emergence of bright green or yellow spots. Typically, powdery mildew spores develop on leaves, though they can also be found beneath leaves, on stems, and petioles.

Unfortunately, if you spot it anywhere, it means that it is present throughout your entire growing area. Eliminating it from the grow room is essential before initiating a new crop cycle. As with any microbial contamination, cleaning the room thoroughly with a 0.5% sodium hypochlorite solution (AKA, diluted bleach) is the most effective.

In cases where a grow has suffered a severe infection and the grower seeks certainty about eradication, an early detection method like a PCR test can be employed. This test can confirm that the powdery mildew has been effectively eradicated, preventing any future spread.

Fortunately, powdery mildew, despite its impact on plant yield and health, is not considered harmful or toxic to humans. However, it’s important to note that even though it won’t be identified in a yeast and mold test, a sample heavily covered in powdery mildew might not pass a foreign matter inspection. As per New Jersey guidelines, “If visible foreign material such as sand, soil, cinders, dirt, mold or mildew exceeds 25% of the total sample area, the sample shall not meet specifications.”

Have questions about testing for powdery mildew in cannabis? Reach out to Trichome Analytical for guidance.