Prevention and Treatment of Powdery Mildew on Flowers

Powdery mildew is one of the most widespread plant pests. It tends to affect flowering plants and fruit trees or bushes (roses, apple trees, etc.) as well as cannabis. The spores are in the air or on contaminated plants in the garden and can easily be drawn into your grow room by ventilators. In internet forums and cannabis cultivation books you may find lengthy lists of possible treatments that look temptingly promising for a complete recovery at all stages of mildew infestation. But the authors have rarely tested these products themselves, and the compounds are seldom differentiated between those applicable only for growth, and those also safe to use during flowering. This guide is therefore specifically orientated towards publishing some first-hand experiences with a variety of fungicides, and a review on the specific utilisation of products during growth and flowering in cannabis plants.

In warm, dry climate powdery mildew spreads widely. But we have also observed heavy infestations particularly after a long and wet winter. The symptoms even disappeared occasionally during a heat wave. It is therefore unlikely that powdery mildew only affects cannabis plants during a warm and dry weather period.

Powdery mildew can be detected by the white-coating on leaves and shoot tips, which succumb to the infestation by atrophy and eventually die. Only the surface of the leaves is contaminated by the fungus. Very heavily infested plants often cannot be saved. Plants affected by this disease and wilting plants should be removed from the garden if you don’t want the spores to spread. If the infestation of powdery mildew arises during the last few weeks of flowering it is possible to control an infestation until harvest. A good fungicide can fully combat powdery mildew, but the treatment is often lengthy and takes several weeks. Mother plants and cuttings should be treated preventively so that valuable genetics are not lost.

Mildew is a surprisingly aggressive fungus taking second place only to bud/grey mold (Botrytis). If you not only require a fungicide for preventive means, but also to combat an infestation, you may need to alternate between several products for the total period of treatment. In the instructions for use it is stated how often a specific compound can be applied per crop. Since fungicides allowed in organic agriculture can be less effective than synthetic products, all preventative management measures to reduce the primary infestation and to improve the microclimate should be exploited optimally. Fungicides are applied at the crucial stages in the development of the plant and the epidemic. Specific traits of the products must be taken into account for outdoor applications (such as rain fastness and hazards to plant health under intense heat or sunlight).