As winter approaches, cannabis growers are all too familiar with the havoc that white powdery mildew (WPM) can wreak on their crops. This fungal disease can appear seemingly out of nowhere and quickly spread, leaving a trail of destruction in its wake. What can you do to prevent losses and finish your flower harvest with maximum yields.

Understanding WPM

It is important to understand the nature of the disease. White powdery mildew is a fungal disease easily recognizable by its white, powdery appearance on the leaves and buds of cannabis plants. WPM Spores can survive dormant on plant surfaces for extended periods until humid conditions are suitable for it to grow.

Prevention is key!

The first step in controlling WPM is prevention. There are several steps you can take to reduce the likelihood of your plants becoming infected with WPM.

• Select strains like Blue Dream and others that are WPM resistant.
• Ensure good air circulation and ventilation in your grow room or greenhouse.
• Maintain appropriate humidity levels (below 60%) and a temperature between 20 to 28 deg. With outdoor growing towards autumn, this is not possible and this is when we see the worst infections.
• Avoid over-crowding plants.
• Regularly inspect your plants for signs of WPM.
• Remove leaves with visible signs of WPM.

Start with good root and plant health.

Establish healthy roots by colonizing the rhizosphere with a healthy population of beneficial microbes. Microbes protect against root disease, helps increase root mass and facilitates increased nutrient uptake. A healthy plant is more WPM resistant plant.

• Use Eco-T (Trichoderma asperellum) as a soil drench at 2,5g/10l water. Apply to seedlings and then monthly.

• Use Rhizovital (Bacillus amyloliquefaciens) as a seed soak and/or soil drench at 4ml/10l of water after planting. Repeat drench at 4 weeks.

In addition to microbes, add silica to your nutrition programme. Silica is an essential element that hardens cells and makes plant tissues tougher and more resilient to stress and disease. WPM hyphae will find it more difficult to penetrate leaf tissues hardened by silica.

• Use Agrisil K50 (potassium silicate) as a soil drench weekly at 2-4 ml per 10l of water.

*Note: Agrisil reacts with other nutrients when in high concentration. Dilute your Agrisil first in water and then add to to your nutrient blend. Agrisil is very alkaline, so add and then correct your pH to 5.8. before application.

Optimize your feeding programme with Mega Nutrient Solution and V-12 bio-stimulants to ensure that your plants are strong, healthy and productive.

A plant that is healthy, will photosynthesize optimally and turn light energy into sugars. Some of these sugars are deployed to the roots as root exudate: food for microbes. The microbes protect the plant and make nutrients in the soil more plant available. Once in the plant the elements are transported upwards to build stems, leaves and flowers. The nutrient cycle is completed.

A plant that has been supported by microbes, fed a correct balance of nutrients and bio-stimulants will be healthier and more resilient to WPM.

Fill the “space” and start IPM early!

There are several biological sprays that can be used to control WPM.  These microbial products colonize the plant surfaces with good fungi and bacteria, which flight and outcompete the WPM fungus for nutrients and space.

We suggest the following spray program.

Treatment during Veg

Every week during Veg, treat your plants with BioSulpher @ 1-3ml per L of water and Double Nickel @ 4g / 1L water.

Notes: Biosulpher can burn new leaves of some sensitive cultivars, so always start with lower dosages. If you have used an oil-based pesticide make sure you wait for 2 weeks before applying Biosulpher.

Treatment during flower

Week 0 - Before flipping, spray Biosulpher at 1-3ml per litre of water.

Week 1 - Spray Biosulpher at 1-3ml per litre of water.

Week 2 - 1 week after the flowering starts, spray your final application of BioSulphur.

Week 3 - Spray AQSF at 5-15ml per L water, combined with Bio-Tricho at 3-5ml per L water.

Week 4 - Spray AQSF at 5-15ml per L water, combined with Eco-T at 2,5g/10L water.

Week 5 - Spray AQSF at 5-15ml per L water, combined with Bio-Tricho at 3-5ml per L water.

Week 6 - Spray AQSF at 5-15ml per L water, combined with Eco-T at 2,5g/10L water.

Week 7 – Spray final application of AQSF at 5-15ml per L water, combined with Bio-Tricho at 3-5ml per L.

*In high humidty areas spray Spray AQSF at 15ml per L water, combined with Bio-Tricho at 5ml per L water, twice a week until Week 7