How to detect, treat, and prevent powdery mildew

Powdery mildew, which is characterized by white powdery spots on leaves, stems, and flowers, is among the greatest disease issues faced by hemp growers, regardless of being indoors or outdoors. Controlled environment production facilities generally incorporate
intensive cropping systems and can be ideal environments for
the development of many diseases caused by fungi. This article will discuss powdery mildew signs/symptoms, infection, cultural treatment, preventative fungicides, and sanitation.


White powdery growth on hemp leaves infected with powdery mildew. Image: Erica Hernandez, Griffin GGSPro

Identification
Powdery mildew in hemp is a pathogenic fungus comprising several closely related species in the genus Golovinomyces. It is most recognizable by white patches of powdery-looking spots. These
fungal colonies can cover large areas on leaves, stems, and flowers.
Powdery mildew-infected plants may also appear stunted, and leaves appear brown. Though it rarely kills plants, heavily infected leaves can limit photosynthesis, reduce nutrient availability, and lead to
leaf necrosis and defoliation. Infected flowers will also display white mycelial growth, which would be considered a lower-quality product and may trigger a rejection of the project based on a state’s microbial pathogen testing.

Optimal conditions for development
Research on powdery mildew indicates that conidia can survive a wide variation of temperatures and humidity. Optimal temperature ranges for powdery mildew are believed to be between 69-84 ˚F. High humidity levels promote fungal growth, but low humidity promotes spore dispersal. Low leaf moisture is also more favorable for powdery
mildew.

Genetics as a control
Utilizing cultivars that are more resistant to powdery mildew is one of the best first steps for powdery mildew control. Unfortunately, the scale to which hemp breeders screen for disease resistance is
not the same as it is for traditional crops. In the future, we may see more hemp cultivars specifically selected for powdery mildew resistance.

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