Cause Powdery mildew is caused by a fungus, Podosphaera macularis (formerly Sphaerotheca humuli) or Golovinomyces (Erysiphe) cichoracearum. Although P. macularis also is reported on hop, strawberry, and caneberries, races that attack hemp are probably limited to Cannabis. How these pathogens overwinter in the Pacific Northwest is unknown.
Symptoms Infections on susceptible leaves appear as whitish, powdery spots on either the upper or lower leaf surface. Entire leaf surfaces can be covered with powdery mildew and leaf petioles can also be infected as well as flower bracts. As powdery mildew patches age, ascocarps (small, nearly spherical fruiting bodies) may become visible as they turn from yellow to dark brown-black. Disease may occur at all stages of growth, affecting leaves and buds.
- Removal of infected leaves early in disease development can aid in delaying epidemics.
- Maintain adequate nitrogen levels but do not over-apply because more succulent tissue is more susceptible.
Chemical control Thorough spray coverage is essential to protect leaves. Apply protective fungicides before disease is active and reproducing. Chemical and cultural controls can help to keep disease levels low, both in terms of disease incidence and severity before plants begin flowering.
- Bicarbonates help to control powdery mildew. Thorough coverage is essential.
- Bi-Carb Old Fashioned Fungicide.
- Green Cure/Organic Production.
- Kaligreen. May be applied up to the day before harvest. 4-hr reentry. O
- MilStop (85% potassium bicarbonate). 1-hr reentry. H O
- Oils help to control powdery mildew. Thorough coverage is essential. Do not spray if temperature is below 50°F or above 90°F or when plants are wet or under heat or moisture stress. See label for details. Do not use when foliage is wet as good coverage is essential.
- Bonide Bon-Neem II.
- Conc Worryfree Vegol Yearround Pesticidal Oil. H
- Mildew Cure.
- Monterey Horticultural Oil.
- Omni Supreme Spray.
- Safergro Mildew Cure.
- Trilogy at 0.5% to 1%. 4-hr reentry. O
- Sulfur formulations are registered for powdery mildew (Safer Brand Garden Fungicide /Flowers Fruit & Veg. Conc, etc.). Sulfur is fungitoxic in its vapor phase and, therefore, is effective only when air temperatures promote volatilization. Sulfur volatilizes above 65°F but becomes phytotoxic above 95°F. Using it above 85°F is not recommended. Although sulfur reduces sporulation of established infections, it is primarily a protectant and must be applied before infection. See label for details on rates and reentry intervals.
Biological control Efficacy unknown in the Pacific Northwest.
- Actinovate AG. 1-hr reentry. H O
- Double Nickel LC. 4-hr reentry. O
- Regalia (extract of Reynoutria sachalinensis). 4-hr reentry. O
- Serenade Opti. Can rotate with other fungicides. 4-hr reentry. H O
References Rodriguez, G., Kibler, A., Campbell, P., and Punja, Z.K. 2015. Fungal diseases of Cannabis sativa in British Columbia, Canada. American Phytopathological Society Annual Meeting, Poster 529-P.
Ward-Gauthier, N., Beale, J., Amsden, B., and Dixon, E. 2015. Greenhouse hemp in Kentucky exhibits many common diseases. American Phytopathological Society Annual Meeting, Poster 502-P.