Downy mildew of hemp

Pseudoperonospora cannabina is a specialized plant pathogen classified within the kingdom Chromista (phylum Oomycota). It is a well-known oomycete that acts as an obligate parasite on the genus Cannabis. Despite being often referred to as a fungus in traditional nomenclature, it is biologically distinct due to its specific reproductive cycle and cellular structure.

The disease it causes, known as downy mildew of hemp, is characterized by the appearance of yellowish chlorotic spots on the leaves. These spots are often delineated by leaf veins, giving them an angular appearance. Under high humidity conditions, a characteristic greyish-purple fuzz, consisting of sporangiophores and sporangia, develops on the undersides of the leaves.

The biology of Pseudoperonospora cannabina is heavily dependent on moisture. The pathogen spreads primarily through motile zoospores that require liquid water to swim and infect host tissues. The survival mechanism of the pathogen includes the production of thick-walled oospores, which allow it to persist in soil and crop debris for several seasons, posing a persistent threat to subsequent plantings.

Environmental conditions play a critical role in the development of the disease. High humidity (above 85%) and moderate temperatures between 15°C and 22°C are optimal for the pathogen's rapid colonization. Poor air circulation in greenhouses or dense field plantings promotes the spread of spores, leading to widespread infection within the crop canopy if conditions remain stable for even a few days.

The economic impact of the pathogen is significant, as it leads to premature leaf senescence and defoliation. This drastically reduces the plant's ability to photosynthesize, resulting in stunted growth, lower biomass yield, and reduced quality of fibers or secondary metabolites. Effective management is essential to prevent severe crop losses and to maintain the overall health of the hemp plantation.

• Crop rotation to reduce pathogen inoculum in the soil.
• Proper field and greenhouse sanitation to remove infected plant debris.
• Improving airflow and drainage to reduce leaf wetness duration.
• Targeted application of approved fungicides during high-risk periods.
• Selection of hemp cultivars with higher resistance to oomycete pathogens.