Powdery mildew is a white powdery growth that forms on the surface of the cannabis leaves. The fungus first appears as small bumps on the upper side of the affected leaves, and if left unchecked it can extend to the petioles, stems and finally the buds, completely destroying the resin and crippling the plant entirely. Also known as Oidium or White Mold, the pest affects both indoor and outdoor cannabis. It is necessary for farmers to take preventative measures from the beginning to keep powdery mildew at bay.

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Powdery Mildew (Wikipedia)

Erysiphaceae (commonly known as powdery mildews) is a family of ascomycete fungi in the order Helotiales. The members of this family are obligate parasites with a superficial mycelium that forms a characteristic white, powdery coating on their host plants.

Powdery mildews
Golovinomyces sordidus demonstrating typical white patches on a leaf of Plantago major
Scientific classification
Kingdom:	Fungi
Division:	Ascomycota
Class:	Leotiomycetes
Order:	Helotiales
Family:	Erysiphaceae Tul. & C.Tul., 1861

The family contains seventeen genera and around a thousand species. Combined they infect over 10,000 species of plants, many of which are commercially valuable.