Downy Mildew on Cannabis: How to Spot and Manage It
Downy mildew is a leaf disease caused by Pseudoperonospora cannabina, a water mold that attacks cannabis and hemp. Look for yellow-green spots on the upper leaf that turn brown, with gray to purplish fuzz underneath. It thrives in cool, humid, wet weather, and hemp specialists point to canopy airflow and removing infected leaves to reduce spore sources.
What Is Downy Mildew?
Downy mildew of hemp is caused by Pseudoperonospora cannabina. Ontario specialists note it is an oomycete, or water mold, that grows inside the leaf, while powdery mildew is a fungus that grows mostly on the leaf surface (OMAFRA). They add that this pathogen is specific to Cannabis and differs from the downy mildews of cucumbers, lettuce, onions, hops, basil and tobacco. If you see white powder on top of leaves instead, see our guide to powdery mildew.
What Does Downy Mildew Look Like on Cannabis?
UF/IFAS describes irregular brown to blackish spots on the lower leaf surface, with matching yellow spots on top, and a purplish to gray growth underneath when conditions favor it (UF/IFAS PP379). Purdue adds that lesions start greenish-yellow and water-soaked, then turn brown, with gray to black fuzz under the lesions (Purdue). In Ontario fields, symptoms showed first on older leaves inside the canopy, where humidity is highest, and then moved up (OMAFRA).
Source: OMAFRA ONspecialtycrops, downy mildew identified on hemp in southwestern Ontario (field hemp).
Downy mildew vs powdery mildew on hemp (sourced table):
| Trait | Downy mildew | Powdery mildew | Source |
|---|---|---|---|
| Organism | Oomycete (water mold) | Fungus | OMAFRA |
| Where it grows | Inside the leaf | Mostly on the leaf surface | OMAFRA |
| Spores | Produced on the lower leaf surface, mostly in the early morning | Not covered | OMAFRA |
| Powdery mildew products | Usually no effect | Not covered | OMAFRA |
What Conditions Favor Downy Mildew?
Purdue reports that downy mildew thrives in cooler temperatures with high humidity and free water (Purdue). UF/IFAS notes that cool weather below 65°F and wet conditions encourage spore production, and that leaf wetness encourages infection (UF/IFAS PP379). Spores form on the lower leaf surface, mostly in the early morning, and OMAFRA says they can spread hundreds of kilometres on the wind (OMAFRA). Indoor growers can read more in our post on factors that affect relative humidity.
How Do Growers Manage Downy Mildew?
Ontario specialists say management relies on good airflow through the canopy, and that products used for powdery mildew usually have no effect on downy mildews (OMAFRA). Purdue advises removing infected leaves into a paper bag and not leaving infected tissue on the ground, to reduce inoculum for the next season, and avoiding scouting in the early morning or after rain (Purdue). These steps come from hemp field guidance. Other leaf and bud diseases are covered in pathogenic mold. Check the product label and local rules for your crop.
Getting ready for an outdoor season? Take a look at our outdoor seeds.
Sources
Some sources below studied hemp or other crops rather than drug-type cannabis, and their findings are reported as results for those crops.
- OMAFRA ONspecialtycrops: Downy Mildew Identified on Hemp in Southwestern Ontario
- UF/IFAS PP379: Field Guide to Hemp Diseases
- Purdue Pest&Crop: Multiple Diseases Found on Hemp
Everything here is general growing information from Kind Seed Co, drawn from the sources listed above.
FAQ
What causes downy mildew on hemp?
Downy mildew on hemp is caused by Pseudoperonospora cannabina, an oomycete or water mold rather than a true fungus. Ontario specialists say it grows inside the leaf and is specific to Cannabis species, so it is a different pathogen from the downy mildews that affect cucumbers, lettuce, onions, hops, basil or tobacco crops.
Is downy mildew the same as powdery mildew?
No. Ontario specialists explain that powdery mildew is a fungus growing mostly on the leaf surface, while downy mildew is a water mold growing inside the leaf. They also note that products used to manage powdery mildew usually have no effect on downy mildews, so OMAFRA treats them as separate problems.
Where on the leaf does downy mildew show?
UF/IFAS describes brown to blackish spots on the lower leaf surface with matching yellow spots on the upper surface. Under favorable conditions, a purplish to gray growth can appear on the underside. Purdue also describes gray to black fuzz under the lesions, so the underside of the leaf is where the fuzz shows.
Does airflow help against downy mildew?
Ontario hemp specialists say that managing downy mildew will require good airflow through the canopy. That fits what Purdue reports about the disease, which thrives with cool temperatures, high humidity and free water. OMAFRA says that airflow is meant to reduce humidity around the leaves, and UF/IFAS adds that leaf wetness encourages infection.
