Any experienced cannabis grower has likely encountered one of the most common fungal diseases for plants – Powdery Mildew (PM). Powdery mildew is a white, powder-like fungal disease that reproduces on the leaves of many different kinds of plant species. Thriving in high-humidity environments and moderate temperatures,PM can severely stunt your plant’s growth and yields by blocking your leaves ability to absorb sunlight and convert it to energy, a.k.a. photosynthesis.
Some facts about powdery mildew…
- Powdery mildew reproduces sexually and asexually. It is a fast-growing fungi that is spread by spores. These spores most commonly infect susceptible plants via airborne transmission.
- PM will quickly move throughout your plant, smothering it in white mildew – many instances in just a few days time. Left untreated, powdery mildew will effectively ruin your weed harvest.
- This fungi can be spread through the wind, hitch-hike on your clothes or pets, and come through air-ventilation systems. Wooly aphids are also a known common transmitter of this infection.
- Common plant hosts are cabbages, tomatoes, cucumbers and beans – even trees such as apple, pear and Japanese maple trees can carry PM. Make sure to eliminate ALL of your local powdery mildew, not just selectively your cannabis plants.
- Powdery mildew will literally suck the life out of your marijuana – once identified you must take action ASAP. As PM takes over, you will see your whole plant yellowing and falling into slow, stunted growth.
- If you grow indoors, be sure to clean all your grow room with a bleach-water solution, making sure to cover all surfaces and areas the fungi may be able to reside.
- Getting rid of powdery mildew is not for the faint of heart. It is an arduous battle, often tending to go away for a few days to a few weeks, only to reappear later.
Those who have experienced massive outbreaks of powdery mildew outdoors or indoors will be the first to tell you – PM takes some real wizardry to cure.
There are many known remedies for powdery mildew available at-home and commercially. While bringing back a matured plant can be close to impossible if you want safe smoke, kicking the mildew out of younger plants can be done quite effectively.
Sunlight can be a lifesaver – if you have potted plants outside, ‘follow the sun’ with them. Make sure plants are in full sun for as many hours as possible, and also make sure the sun is not missing any sides of your marijuana, if possible. Hot sunny days combined with multiple foliar sprayings can be the KO punch to powdery mildew you need.
Stay away from sulfur and copper remedies, unless your plants are very young. Although these often work, they are toxic and not fit for smoking purposes. Once upon a time, a budding Blueberry Hash plant was sprayed down with a sulfur spray – it sure did get rid of the powdery mildew. A white sulfur residue was left behind that was impossible to remove, even after rinsing. Harvested, the resulting buds were completely unsmokeable – marijuana smelled and tasted precisely as if lighting a match – Fail.
A somewhat effective solution to PM is baking soda (sodium bicarbonate). Be wary though, case studies have shown baking soda to be only mildly successful. It requires quite a bit of baking soda to really get a good punch at powdery mildew, only seeming to knock out mildew growth for a short period of time. After a few days the powdery mildew usually sets back in – unless the mildew outbreak was minuscule, or the growing environment has been adjusted.
While foliar treating plants, make sure you cover up your topsoil. Although these remedies are ‘organic’, they still can be harmful to your soil’s microbiology. Simply cut pieces of slick cardboard to fit snug against the main stalk weed plants, effectively covering the topsoil and protecting from any dripping foliar spray.
Using a wetting agent, or spreader-sticker helps the spray stay on the foliage longer. There are many suitable kinds on the market, be sure to use one if possible. You can substitute 2 – 4 drops of Dr. Bronners soap per quart of spray soution as a spreader-sticker. Keep it out of your soil and don’t over do it, it’s easy to burn your plants up using this; 2 – 4 drops per quart is plenty.
Potassium bicarbonate is the preferred alternative to sodium bicarbonate. In a study performed on gooseberries, an initial outbreak of over 90% infection rate throughout plants was reduced to approximately 10% affected plants due frequent foliar spraying of potassium bicarbonate – read the study here.
Well-known cannabis grower/activist Ed Rothenthol prefers to mix an ounce of sodium bicarbonate in a gallon of water together with 1.5 cups of milk.
Milk, has also been proven to help remedy powdery mildew – but beware, do not spray budding plants with milk, as the proteins will create an easy environment for blue/black mold or botrytis to grow. Furthermore, potassium bicarbonate is beneficial in fighting other mildews, rusts, and molds.
Onyx Spider Mite and Powdery Mildew Killer
Specifically designed for cannabis growers, ‘Onyx Killer’ contains a malady of different oils including neem oil, hemp oil, coconut oil and a few others designed to cure plants rid with powdery mildew, mites, or both. It has been proven to work effectively in the outdoor marijuana garden, as experienced personally by growers.
An elephantine South African sativa presented herself gargantuanly gorgeous, except for the unfortunate infection that swept over her quick. Always skeptical, who would think ‘Onyx Killer’ was going be that great, kind-of looking to like a ‘snake oil’ product – is even stated as “Can Use Through Harvest!”.
To much surprise, Onyx Killer effectively wiped out nearly all of the powdery mildew on that jumbo sativa, with the harvested flowers clean while reeking like a skunk.
Apple Cider Vinegar
The uses and benefits of apple cider vinegar (ACV) are enough to write a book about. Many of us know people who drink, brush their teeth and bathe with the stuff. Relating to powdery mildew, the acetic acid contained in vinegar will help rid your plants of the infection. Spray two to three times a week for maximum effectiveness, making sure to cover the leaves fully.
It should be noted however, that although apple cider vinegar is strong enough to stop most moderate-level PM infections apple cider vinegar is not effective in eliminating a well-developed outbreak of powdery mildew.
Mix 2 teaspoons of organic apple cider vinegar into a quart of water – be careful not to overdo it, high concentrations of vinegar can burn plants.
Neem oil, produced from the seeds of the Indian neem tree, is a well known wonder product for plants. Naturally fungicidal, weekly application of neem oil will take care of most powdery mildew problems.
Professionals have had better results controlling powdery mildew with neem oil compared to baking soda (sodium bicarbonate) in controlled side-by-side studies. While normal doses won’t completely eradicate powdery mildew from mature infected plants, regular spraying will clean off the mildew, keeping it at bay. If neem oil is sprayed before plants have powdery mildew, you may rest assured they will not be easily infected with PM.
Being an organic insecticide and miticide as well, most gardeners should already have neem oil on hand. Some people complain about the smell, while others find it enjoyable, akin to a pine sol freshness.
Most remedies require more than one application to really be effective. Remember any moisture will likely wash the foliar spray off, even morning dew on plants can remove the coating off the leaves. Best time to spray is before noon on a dry, sunny day.
When trying to gain control over powdery mildew on cannabis, try to get your plants in a open, breathable spot. Fungi love closed humidity.