Growers of any crop, be it cannabis, ornamental shrubs and flowers, and even vegetables, will likely deal with powdery mildew at some point.

Powdery mildew is not actually a single pathogen, but rather any number of fungal diseases that present similarly on leaves and flowers. Knowing how to identify powdery mildew in a cannabis grow room, and monitoring for it, can keep damage by it to a minimum.

Symptoms of Powdery Mildew

Fortunately, powdery mildew is easily identified. It appears on leaves as a white or gray powder-like substance, hence its name. It usually starts on the underside of lower leaves and works its way up the plant. This is why it seems to many growers to appear from nowhere, when in fact it may have been “setting up” shop for some time on the bottoms of the lower cannabis leaves.

Powdery mildew will interfere with your plant’s ability to photosynthesize. Untreated cases of powdery mildew will lead to stunted growth and poor yields. It is not a death sentence for your plants, however. In small amounts it is largely a cosmetic issue, but if left untreated for a long enough period of time, powdery mildew can kill your plants.

Causes of Powdery Mildew

Powdery mildew is caused by the spread of spores. Spores travel easily and effortlessly on the wind, in water, on animals, on clothes and any other object that has the misfortune of brushing up against it. They are long lived. Once deposited in your soil or on your cannabis plants, they may sit idle for some time until the conditions are favorable for its appearance.

Powdery mildew likes high humidity levels and high temperatures. Make sure to keep the humidity in your growing area below 50 to 55 per cent in order to prevent the development of powdery mildew.

Treatment of Powdery Mildew

If caught early, the spread of powdery mildew may be stopped by the physical removal of any affected leaves. This should be done with sterile shears which should be disinfected afterwards to reduce the likelihood of spreading pathogens.

Unfortunately, most powdery mildew is not caught before it has spread to more than a few leaves. At this point, the grower needs to decide if they are going to use chemical fungicides or organic ones.

The market is rife with over the counter chemical fungicides. Chlorothalonil is an effective solution. Look for pesticides with this active ingredient (this is not the brand name, but rather should be the first chemical listed).

Other chemical options include the active ingredients Thiophanate-methyl, myclobutanil, propiconazole or tebuconazole. Whichever formulation is decided upon, make sure to alternate between at least two different types so that the disease does not build up a resistance to any single type of treatment. This is true among organic solutions as well.

For growers that with to attack this problem from a natural or organic stance, there are actually many options. Sulfur and copper have been used in the control of fungi on plants for centuries, but sulfur should not be used on stressed cannabis plants or when the temperature is at or above 90F (32C).

Sulfur also provides some control against spider mites on cannabis plants. Neem oil is another option that offers both fungicidal and insecticidal protection to cannabis as does horticultural oil.

Horticultural oil, however, has the same caveat as sulfur in that it should never be applied in high temperatures and it has the potential to burn leaves if used at high rates. Make sure it is a vegetable-based oil and not a petroleum-based horticultural oil.

The beneficial bacteria Bacillus subtilis can also be used and is found in some commercially available fungicides.

Prevention of Powdery Mildew

Like anything else, the best way to deal with a disease is not to get it in the first place. While there is no strain of cannabis proven to be impervious to powdery mildews, by making the conditions in your growing areas unfavorable to its development, you will yield benefits both literally and figuratively.

Fungi enjoy heat, darkness, and moisture. Make sure that all three of these environmental factors don’t exist in concert long enough to set the stage for powdery mildew to arrive. Proper lighting and airflow are critical, as is the proper spacing of your cannabis plants so that air can circulate around each plant.

Make sure not to overwater the soil, and, if at all possible, avoid overhead watering as well. Dry leaves are less likely to develop powdery mildew. When in doubt, favor being a bit too dry over being a bit too wet.

Not sure if you're dealing with powdery mildew? It could be a bacteria or virus. Read More: Diagnosing Common Cannabis Plant Problems.