Powdery mildew in cannabis; Photo: Ryan Douglas
Powdery mildew is unique in the mushroom world and can be difficult for commercial cannabis growers to control. In contrast to the bud rot caused by Botrytis cinerea , powdery mildew cannot be stopped simply by drying out the atmosphere.
Powdery mildew is considered a dry mold by many growers because it can spread quickly in environments with relatively low humidity. Because the devil is in the details.
Real powdery mildew pores do not need standing water to germinate, only short periods of time with high humidity. Temporary environmental fluctuations are all that spores come to life, stick to the surface of the leaves and start sucking energy out of the plant.
Preventing powdery mildew from spreading requires a multi-dimensional approach that approaches the problem from different angles. Keep these five tips in mind to help keep your crops free from this pesky disease.
Avoid extreme temperature fluctuations
Day and night temperatures should not fluctuate more than about 15 ° F. The optimal temperature range for cannabis production is between 68 ° F and 85 ° F, but the narrower the variation, the less susceptible your grow will be to cannabis.
These fluctuations are one of the most important factors contributing to disease infection in indoor growers. When the light is switched off, the temperature may drop momentarily. As temperatures drop, the air contains less water, so it is released into the environment. This can lead to micro-condensation and high moisture levels, which open the way for the spores to germinate. HVAC and dehumidification systems eventually catch up and stabilize the environment, but by then spore germination has already taken place. Wall fans help disperse spores, and workers brushing against plants inadvertently carry spores to new locations in the grow room.
Although day and night temperatures can be stable, a 45-60 minute period of extreme fluctuations can make powdery mildew worse.
Use the pH of the leaf surface as a weapon
Removing mildew pores from the environment may be impossible, but at least it can make it harder for growers to develop infections. Powdery mildew's success is highly dependent on the pH of the leaf surface, so increasing or decreasing this value may affect its ability to establish itself in the culture.
The use of kaliumbicarbonathaltigen products lowers the pH of the leaf surface temporarily, while sulfur containing products it increase. While these products are approved for organic growing, it is best not to use them on plants in full bloom. If you anticipate problems with powdery mildew, use these products preventively by the second week of flowering. Beyond this point, using these products could negatively affect the final quality of your cannabis.
Fight nature with nature
Biofungicides are another biological way to prevent powdery mildew outbreaks.
Products containing Reynoutria sachalinensis use giant knotweed extract to activate the plant's natural defense mechanisms and produce disease-fighting biochemicals. This induced disease resistance is not systemic, which means that only the treated area of the plant is protected. However, it is translaminar, which means that when it is sprayed onto the leaf surface, the underside of the leaf is also protected.
However, the growth of new plants is not protected, so the product must be reapplied within seven to 14 days. While biofungicides can be a powerful addition to a grower's disease control toolbox, make sure that fungicides, including organic ones, are allowed in your cannabis grow.
Filter the problem
Most indoor grow sites don't circulate air between rooms, but even closed ventilation systems can keep powdery mildew outbreaks by spreading spores throughout the grow room. Installing air filters with a MERV of 9 or higher can help trap mold spores and prevent them from being reintroduced into the growing area. Ask your HVAC technician to install these filters in the ventilation system and to clean or replace them on a set schedule.
If the infestation is severe, cut off your losses, not the leaves.
The first time powdery mildew is detected on a crop, it makes sense to remove the infected leaves. However, if none of the above four recommendations are followed, removing the leaves is unlikely to improve an epidemic. If the infestation is severe, removing the leaves can make the problem worse.
When workers pick the leaves, they inevitably cover their hands with spores and spread them throughout the crop every time they touch a plant. In the event of severe infestation, this activity spreads as many spores as it eliminates. When defoliation is extreme, late blight also spreads to the only part of the plant that has yet to be infected: the flowers.
Maintaining a heavily powdery mildew plant puts the rest of the grow facility at risk. Sometimes it is better to throw away the harvest than to provide vital assistance until harvest day. Cutting a damaged plant will hurt, but infection of the entire plant will prove to be much more painful.
As with any disease, prevention is key. Since cannabis growers cannot rely on conventional fungicides to protect their plants, growers need to be creative to fight the good fight.
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