By: Amy Fang Shi, Sean Westerveld, Jim Todd and Melanie Filotas

Downy mildew of hemp (also known as cannabis downy mildew) has been identified in multiple hemp fields in southwestern Ontario. The disease first appeared in early September and was confirmed through microscopy and genetic tests to be caused by Pseudoperonospora cannabina. All growers of hemp (Cannabis sativa) should watch out for this disease, although the impact of the disease may depend on the end use.

It is important to note that this disease is very different from powdery mildew, which is common on hemp. Powdery mildew is caused by a fungus and grows mostly on the leaf surface. Downy mildew is caused by an oomycete (water mold) and grows within the leaf. Spores of downy mildew are produced on the lower leaf surface and appear mostly in the early morning before they are released into the wind. While there is limited information on this pathogen, downy mildews typically prefer long periods of high humidity and moderate temperatures.

Downy mildew of hemp has not been reported in Canada before, but there have been historical reports of the disease occurring in the US. Downy mildew spores can spread hundreds of kilometres on the wind, so there is no way of knowing the origin of the disease.

Look for irregularly shaped brown spots on the leaves with patches of light green and/or a yellow halo around the lesions (Figure 1). The lesions may begin as yellowish-green spots before turning brown. The disease will first appear on older leaves within the canopy where humidity is highest and then move up the canopy from there (Figure 2). This pathogen is specific to Cannabis species and is different than the pathogens that cause downy mildews of other crops such as cucumbers, lettuce, onions, hops, basil and tobacco (blue mold).

Figure 1. Symptoms of downy mildew of hemp leaves. Note the yellow halo in the bottom photo that may be more distinctive early on.

Figure 2. Symptoms of downy mildew are worst on older leaves within the canopy.

Damage caused by this disease has been minor so far, since the crop is close to harvest. The potential impact of the disease on total or marketable yield of hemp is currently unknown. Up to now, no damage has been identified to the flowers or seed heads. There are no fungicides registered for control of this disease in Canada. Products used for management of powdery mildew usually have no effect on downy mildews. Management of the disease will require good airflow through the canopy to reduce humidity around the leaves.