Powdery mildew is a fungal disease caused by obligate parasites in the order Erysiphales that live on the plant surface. On cannabis the main species is Golovinomyces ambrosiae, which was previously reported as G. cichoracearum. It forms white, flour-like patches of mycelium and spores on leaves, stems and, later, buds. Infected tissue yellows, photosynthesis drops and buds become unsellable.
In practice
Unlike most fungi, powdery mildew does not need free water on the leaf, and moderate humidity, still air and low light within the canopy favour it. It is spread by airborne spores and on clones; the fungus has also been reported to persist inside some infected plant materialbeing reviewed. Prevention is airflow, sensible humidity, plant spacing and clean starting material. Resistance genes such as PM1 have been mapped, so cultivar choice matters. Once it reaches flowers, options are very limited; check what is registered for cannabis locally.
Don't confuse with
- Trichomes: resin glands on buds and sugar leaves; mildew spreads across fan leaves too and wipes off as dust.
- Downy mildew: a water mould with grey-purple growth under leaves and yellow patches above.
- Spray residue or hard-water spots: no spreading growth.
The process in chloroplasts that uses light energy to turn carbon dioxide and water into sugars and oxygen; cannabis uses the C3 pathway.Pest and disease prevention by changing how the crop is grown, such as humidity, airflow, spacing, watering and cultivar choice, so pests and pathogens struggle to establish.Plant density is the number of plants per unit of canopy area; more plants raise yield per area up to a point but shorten veg and reduce uniformity.A gene is a stretch of DNA that carries the instructions for one functional product, usually a protein such as an enzyme, and is the basic unit of inheritance.A hair-like outgrowth of the plant's skin; in cannabis the glandular types make and store the resin holding cannabinoids and terpenes.A small leaf growing from within the flower cluster, often coated in trichomes, which is trimmed off and commonly kept for processing.A large palmate cannabis leaf with several long leaflets on a long petiole; fan leaves do most of the plant's photosynthesis and carry few trichomes.An oomycete leaf disease of cannabis, historically attributed to Pseudoperonospora cannabina, causing yellow patches above and grey-purple fuzz under leaves in cool, wet weather.Botrytis cinerea, the fungus behind bud rot or grey mould, which rots dense colas from the inside in cool, humid conditions late in flower.A heated vessel that vaporises elemental sulfur to suppress powdery mildew and mites in veg rooms; it carries fire, gas and timing risks and is never used in flower.The pest or disease level, set in advance, at which a grower takes a specific control action because waiting longer would cost more than acting.Cleaning and disinfecting rooms, tools, containers, irrigation and people's clothing so that pests and pathogens are not carried from plant to plant or crop to crop.
