Downy mildew of hemp

The causative agent of hemp downy mildew is the oomycete Pseudoperonospora cannabina. This disease belongs to the downy mildew group and significantly affects the vegetative parts of the plant, compromising both the yield and the quality of the raw material harvested.

The primary host for this pathogen is industrial and medicinal hemp (Cannabis sativa). The organism enters the plant tissue through stomata and colonizes the intercellular spaces, drawing nutrients from the host and causing significant physiological stress to the plant.

Visible symptoms include chlorotic spots on the upper leaf surface, which eventually turn necrotic and brown. Under conditions of high humidity, a characteristic greyish or purplish fuzzy growth—consisting of the pathogen's sporangiophores and sporangia—develops on the underside of the leaves.

The disease thrives in environments with high humidity (above 80%) and moderate temperatures ranging from 15°C to 22°C. Spores are dispersed through wind, rain splashes, and physical contact with contaminated tools or clothing, allowing the disease to spread rapidly across entire cultivation areas.

The economic impact of the disease is severe, leading to premature leaf senescence, stunted growth, and a reduction in fiber or cannabinoid quality. If left unmanaged, the infection can devastate a crop, rendering it unsuitable for professional processing and causing major financial losses for growers.

Integrated management practices to control the disease include:

• Selecting resistant cannabis cultivars for planting.
• Practicing strict crop rotation to reduce inoculum in the soil.
• Removing and destroying infected plant debris after harvest.
• Applying preventive or curative fungicides containing copper or other specialized active ingredients.