Preventing white mildew on Cannabis

White powdery mildew, or WPM, is a very common but serious threat to cannabis plants.
WPM can be easily spotted on young leaves of a cannabis plant and appears as a white or grey powdery-fuzzy substance. Once powdery mildew has infected a plant leaves, it can easily spread to other parts of the plant, including the buds, making them unusable.

Small bumps or blisters on the top of fan leaves are a clear sign that WPM is developing; if your powdery mildew infestation is severe, you will smell rotting vegetation.

When WPM has infected your plants, there are limited solutions. Immediately separate
visibly infected plants and closely analyses all surrounding plants to ensure WPM did not spread. Smoking and/or ingesting powdery mildew is unsafe and can result in dizziness, brain fog, fatigue, respiratory infections, and even lung disease.

The most effective way to avoid WPM is by prevention.

There are three times that you're most likely to see mould on your cannabis plants – during the vegetative stage, the last few weeks of flowering, and drying and curing. Powdery mildew thrives in over-watered and overcrowded plants and high humidity environments – which is why it. important to keep clean air circulating.

Spores spread in the air by workers of by infected plants, and they can survive for up to a week. In humid conditions (70%) the spore then germinates producing a germ tube and haustoria. The mycelium then develops, and eventually more spores are produced.

This happens within 5-7 days!

How can we reduce powdery mildew?

  • Prevent spore germination
  • Reduce mycelial growth
  • Reduce secondary spore production

How we can manage powdery mildew?

  • Plant resistant varieties
  • Chemical fungicides
  • Biological control agents containing microbes
  • Reduce risk produces
  • Enhance plant defense response
  • Manage environmental conditions

There are currently no synthetic fungicides registered for cannabis. To minimize damage cause by powdery mildew growers, apply vaporized sulphur (except organic), hydrogen peroxide (Zerotol) and potassium bicarbonate (MilStop). In addition, Regalia may induce resistance to powdery mildew. Other practices include using UV-C light and a physical means of control.

In this trial Rhapsody (Bacillus subtilis), Regalia Maxx, and MilStop were applied prior to viable mildew infection and were tested for 4 weeks. Both MilStop and Regalia provided good control of powdery mildew, with Zerotol and Rhapsody providing less control. MilStop aand Regalia application resulted in the collapse of mycelium in the infected leaves.

In addition to evaluating the efficacy of registered biological controls on the market, Zamir wanted to experiment with the use of UV-C light for controlling powdery mildew. In this Trial CleanLight Pro Unit UV-C Light was used for 3-5 seconds daily at approximately 3-6 mJ/cm 2 . UV-C radiation reduced mildew development but can be difficult to implement in a commercial setting.

The above is an extract from Zamir’s K. Punja from Simon Fraser University presentation

5 Top Tips from Avoid extreme temperature fluctuations Day and night temperatures should not fluctuate more than about 15°F. The optimal temperature range for cannabis production lies between 68°F and 85°F, but the tighter the variation, the less susceptible your crop will be to powdery mildew attacks.

These fluctuations are one of the most significant contributing factors to disease infection for indoor grow operations. When lights shut off, temperatures can plummet for a short period of time. As temperatures drop, the air holds less water, so it is released into the environment.

This can result in micro condensation and pockets of high humidity, setting the stage for spore germination. HVAC and dehumidification systems eventually catch up and stabilize the environment, but by then, spore germination has already occurred. Wall-mounted fans help to disperse the spores, and employees that brush up against the plants will inadvertently carry the spores to new spots in the grow room.

Even though the day and night temperatures may be steady, a 45 to 60-minute period of extreme fluctuations can aggravate the onset of powdery mildew infection.

Use Leaf Surface PH as a Weapon
It may be impossible to eliminate powdery mildew spores from the environment, but at least growers can make it more difficult for infections to take hold. The success of powdery mildew closely depends on the pH of the leaf surface, so raising or lowering this value can disrupt its ability to become established in the crop.

The application of products containing potassium bicarbonate will temporarily raise leaf surface pH, while products that contain sulphur will drop it. Although these products are approved for organic production, it is preferable not to use them on fully flowering plants. If you anticipate powdery mildew problems, consider using these products preventatively up to the second week of flower. Beyond this point, the application of these products could negatively affect the finished quality of your cannabis.

Fight Nature with Nature

Bio fungicides are another organic method of preventing powdery mildew outbreaks.
Products that contain Reynoutria sachalinensis use an extract of giant knotweed to trigger the plant’s natural defence mechanisms and produce disease-fighting biochemicals. This induced disease resistance is not systemic, meaning that only the treated area of the plant is protected. Still, it is translaminar, meaning that if sprayed on the top of the leaf surface, the bottom of the leaf will be protected as well.

However, new plant growth is not protected, so the product must be reapplied on a seven- to
14-day schedule. Although bio fungicides can be a powerful addition to a grower’s disease-fighting toolkit, make sure to verify that the use of fungicides — even organic ones — are permitted for use on your cannabis crop.

Filter out the Problem
Most indoor cultivation sites do not circulate air between individual rooms, but even closed-loop ventilation systems can perpetuate powdery mildew outbreaks by dispersing spores throughout the grow room. Installing air filters with a MERV rating of 9 or higher can help capture mould spores and prevent them from being re-introduced into the cultivation area.

Have your HVAC technician install these filters in the air handling system and be strict about cleaning or replacing them on a set schedule.

With severe infestations, cut your losses not the leaves…
When powdery mildew is first spotted in a crop, it makes sense to remove the affected leaves. But if none of the previous four recommendations are followed, it’s unlikely that an outbreak will improve through leaf removal alone. With heavy infestations, removing leaves can actually make the problem worse.

As workers pluck leaves, they inevitably cover their hands with spores and spread them through the crop every time they touch a plant. With severe infestations, this activity spreads as many spores as it removes. Furthermore, if the defoliation is extreme, the mildew will spread to the only part of the plant that remains to be infected — the flowers. Maintaining a crop that is heavily infested with powdery mildew will place the rest of the cultivation facility at risk. Sometimes it’s better to scrap the crop instead of providing life support until harvest day. Cutting down a damaged crop hurts, but infecting the entire facility will prove much more painful.

As with any disease, prevention is key. Since cannabis cultivators can’t rely on conventional fungicides to protect their crops, growers must be creative as they fight the good fight.

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