In recent years, the marijuana plant is becoming increasingly sensitive to the attack of the fungus called Powdery Mildew, also known as Oidium or White Mold.
It’s a parasitic fungus of the family of the erysiphales, which attacks the aerial parts of the plants.
Although this infection is relatively easy to cure, it can bring serious consequences, because it can cause damage to crops leaving them absolutely useless. In a first phase it develops on the leafs, to extend then to the petioles, stems and finally the buds, completely destroying the resin and leaving the marijuana plants totally unusable for smoking, since the spores of the fungus are solid matters that we would absorb during its inhalation.
This pest affects both indoor cannabis as well as outdoor or greenhouse crops; for its propagation it needs strong humidity variations, a situation found very commonly in crops under artificial lights and in greenhouses. Outdoors the plague is more seasonal and usually appears more often in spring and autumn.
Powdery Mildew is a fungus that is easily diagnosed. It manifests as a typical white powdery that can be confused with dust on leafs and buds. In a second phase the attacked leafs become yellow and end up drying.
Powdery Mildew is an external fungus and its behaviour is very similar to that of sucking insects, since its small spores are carried by the wind and when they lay down on a leaf they put down rootlets that suck and absorb nutrients from the plant.
Powdery Mildew in indoor marijuana crops
Indoors, as we mentioned before, Powdery Mildew is an increasingly common disease and in some cases it’s necessary to take preventive measures from the beginning of the crop, but what will help us more to avoid this pest is to control the climate of our growing space. The aim is to prevent that the relative humidity of our growroom neither exceeds 65% nor goes down under 40%. To do this perhaps we will need a humidifier, to avoid that – with the lights turned on – the humidity doesn’t descend too much due to the effect of the heat from the bulbs. During the night hours of our plants, we should adjust the ventilation so that plants don’t produce condensation inside the growing room. In extreme cases we can use a de-humidifier. Another detail that can help to prevent the propagation of the fungus is to leave enough space between the marijuana plants, because shady areas are formed in the lower parts of the plants when they are too close, what favours the appearance of Powdery Mildew.
If you can’t avoid the ups and downs of your hygrometer, you can use preventive phytosanitary treatments.
Powdery Mildew in outdoor or greenhouse crops
In outdoor cannabis plants, the plague of Powdery Mildew appears with a certain seasonality, being Spring and especially Autumn the seasons with more propensity to get this disease. To prevent it, we should keep plants free of dried leafs and avoid planting them in areas with few hours of sunshine. If we grow in plant pots it’s convenient to turn the pot to make sure that all parts of the plants have a good exposure to sunlight. We also should avoid placing the plants too close to one another. It’s important that the air can circulate between them.
In greenhouses we should try to have a proper air circulation. For this, it’s recommendable to install some ventilators inside the greenhouse to force the air movement.
In autumn it’s recommended to realize preventive biological phytosanitary treatments, specially if we live in areas whit fog and mist.
Control of the Powdery Mildew
Within the biological treatments for the control of Powdery Mildew, we can find products like Propolix, Oidioprot from Ecoprotect or Ospo. They are two products of proven efficacy, but due to the high resistance of the fungus, we should make the treatment each 10/12 days until 10-15 days before harvesting our plants.
There are also chemical synthesis products on the market like the Anti-Powdery Mildew from Compo, which can be used if we have exhausted the possibilities of biological origin, always taking into account that we have to respect the safety term that in this case is about 30 days.
Although in many books and gardening manuals it’s recommended the use of sulfur as a biological remedy to control powdery mildew, we do not recommend to use it with cannabis plants, given that even if we make it with great precocity, the smell of sulfur stays impregnated in the marijuana plant.