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PM Demystified

Photo by Harry Resin

Powdery mildew (PM) has been the scourge of most farmers growing anything from tomatoes to wheat to cannabis; this plague leaves few crops unaffected.

I’ve written about PM before but feel that this problem can never garner too much attention—especially since in California, in the past few months, pesticides have been in the news.

NBC released an undercover story about contaminated cannabis products in California, and they discovered that more than 80 percent of samples were contaminated with some type of pesticide. The most common being Eagle 20 (chemical name Myclobutanil), which is used to combat PM. Its used heavily in the tomato and grape industry and has been found to be non-toxic at small doses when consumed and processed by the stomach and liver.

However, when heated above 250°F, it turns into cyanide gas, which is a deadly poison and carcinogenic. The pesticide is systemic as well, meaning that you could be buying clones that have been treated and will still contain the chemical unbeknownst to you.

Currently, there has been tremendous research done on powdery mildew as it is such a blight on so many crops.

Photo by Harry Resin

Using genetic testing, researchers have discovered several PM resistant gene sets in tomatoes and wheat. After bringing that information to Steep Hill, they ran the marker in the accessions they had and found a 65 percent match in several cannabis strains that were in the database.

As more information about this becomes available, I will write another article about it. This is promising because we could then use certain strains as PM resistant breeding stock.

Unlike most fungal pathogens, PM grows on the surface of the leaf making it hard to eliminate. PM is also comprised of two organisms a bacteria and a fungus, both are symbiotic and cannot live with out the other half.

It is also difficult to break down PM, as it is insoluble—this is why conventional sprayers do not work as effectively as foggers or paint sprayers. The key being to “wash the plant and not spray it,” according to @bruc3_bamm3r.

Recently, the taxonomy of powdery mildew has undergone an extensive revision based on DNA sequencing and was found to contain five tribes of fungi. This is important because PM would not culture on standard 3M plates. These are the plates used in total yeast and mold tests that labs offer; this is why it has been so difficult to detect PM alone in cannabis.

Since it will not appear on the plates, it cannot be specifically detected.

Steep Hill will soon be offering a DNA-based mold test, which will also help us better understand which species of mildew is affecting us. Currently, I gave the lab some PM samples from the Bay Area and the species they found it to be once they had it DNA tested was from the Tribe Erysipheae. By better understanding the strain specificity of the mildew, we can better understand what the most effective strategy to counter it will be.

One very powerful tool that can be used is paracetic acid which is formed by combining H2O2 and acetic acid. This chemical is super effective against mildew and is found in Zerotol 2.0 (OxiDate 2.0), which is manufactured by a company called Biosafe Systems.

Paracetic acid can be sprayed right up until harvest at around 20-30ml per gallon. The higher you go in strength, the greater your chances are of burning the hairs on your plant, so dial it down in later stages of bloom.

If you do have a problem at later stages in your flowering cycle, I’d recommend using it at least every day to knock it back, and then every other day to keep it bay. Once the spores are out floating in your room, it’s very hard to get rid of them unless your room is empty and you can clean it from top to bottom.

Photo by Harry Resin

Biosafe Systems also make a stronger dilution of the paracetic acid, which can be used to sanitize the room in between crops.

Another tool that is very effective against mildew is the invisible light spectrum.

This includes not just UVA and UVB but many other frequencies in between. One product called the Pathogen Death Wand is being adapted from the food industry, where it has been used to kill E. coli and other food-born bacterias and pathogens. The device is currently being tested at a facility in Colorado and one in the Bay. So far, the technology looks very promising at eradicating fungus and bacteria pathogens.

As more advances in PM research is released, I will be sure to get another article out to share the information.

Keep it green and keep on growing!

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