Downy mildew: a pathogen commonly found in cannabis

Downy mildew is one of the pathogens most commonly found in cannabis crops. It usually emerges when a series of conditions that favour its development occur at the same time. In this post we explain how to identify this fungus, under what circumstances it may appear, and, most importantly, how to eradicate it.

Downy mildew first appeared in Europe in 1878, when the French started to import American grapevine strains that were resistant to phylloxera, a plague that caused havoc in the French wine industry of the time.

Furthermore, the relevance of these new American species facilitated the arrival of downy mildew to the rest of the world. This pathogen can be detrimental to many plant species, especially fruit and vegetables, and therefore became a real nightmare for American and worldwide farmers alike.

How to detect downy mildew

Downy Mildew is a common pest in cannabis crops across the globe. It discreetly fixes to vegetable tissue, safely under cover, and has the ability to thrive without drawing much attention.

It settles in leaves, stems, and even in the plant’s flowers or fruits. The hyphae (a network of filaments that make up the structure of the fungus) can firmly fix to these organs, where the fungi can happily grow and propagate.

Why do we say that downy mildew is a discreet strategist? Because growers often spot it when it’s far too late, which is what usually occurs with most endoparasites: these only show symptoms on the aerial parts of the plants at very advanced stages of growth.

How does downy mildew manifest itself?

Most common symptoms include whitish or yellowish spots on the leaves, similar to translucent oil stains. If there’s too much moisture, a white powder can also appear on the underside of the leaves. However, it’s important not to mistake this for powdery mildew, which can show similar symptoms.

As the infection spreads, these stains are confined by the veins of the leaves and become darker until they turn brown. When moisture levels are high, they get coated in a fine layer of white fluffy dust.

Infection can also be found on the stem, with the appearance of dark stains that can eventually damage the whole area as infection advances. If no action is taken right away to curb the spread of the disease, the affected area will slowly wither away.

During the following phase of infection, the affected areas die and the leaves fall off prematurely, which translates into a decrease in cannabis yield and quality.

Favorable conditions for the appearance and propagation of downy mildew

Downy Mildew is a deceptive fungus for several reasons. In addition to developing discreetly in areas not visible at first sight, it can remain dormant for some time before becoming active again.

When conditions are favorable for its growth, microorganisms can remain dormant during autumn and winter, when they’re found in spore or mycelium form (fungus structure at a late stage).

In spring, when temperature and humidity levels rise, these microorganisms usually come to life again. Downy mildew requires a temperature between 50°F to 77°F to develop optimally.

Regardless whether you’re growing indoors or outdoors, the higher the humidity level and temperature, the quicker and more severe the propagation and development of the fungus will be.

Preventing its appearance

There are some guidelines that you can follow to prevent the appearance of this pathogen on your cannabis crop:

• Keep your grow space clean.
• Keep your crop free of plant litter.
• Minimize air humidity (good management of indoor ventilation and temperature) as well as during watering.
• Minimize heat (in indoor spaces) and light exposure (lighting).

If you don’t control these risk factors, there are more chances that this fungus could appear in your grow. It’s much easier to follow these guidelines in indoor spaces than outdoors, where the actions of a grower are more limited. Outdoors, if your plants become affected by downy mildew, limit irrigation frequency and quantity.

Preventive treatment against downy mildew

The number one source for these nasty fungi is plant waste, where they can hang out in their tough form, just waiting to spread through wind, water, animals, or even clueless growers. That’s why it’s super important to keep your growing space and surroundings clean and free of plant junk to keep your plants healthier, especially if you’re dealing with greenhouses or grow rooms that don’t get much air flow.

Another smart move is to try and air out the area when you can; maybe throw in an oscillating fan. It’s also a good idea to keep the temperature just right (either with some sunshine or a thermostat), trim your plants regularly, or space them out so they’re not all crowded together.

On the flip side, growers should avoid drowning their plants (you can hit pause on watering when it’s not needed or cut back on how much water your crop’s getting) and going overboard with fertilizer or nutrients. Also, skip watering at night and ease up or stop watering before you turn off the lights, so they can help dry things out and keep the humidity down.

Now, there are some natural preventive treatments you can try. One trick is using plants with germ-fighting properties, like chamomile or garlic, mixed in water and sprayed on your plants. Fungicides like copper oxychloride and stuff called dithiocarbamates are pretty common for prevention too. And organic preventives work great, like copper salts or Bordeaux mixture (that’s slaked lime plus copper sulfate) – remedies tons of farmers still swear by because they really do the job.

Healing treatment for downy mildew

If downy mildew manages to reach an advanced stage of development, the only way to eradicate it and save your cannabis crop is by applying effective chemical treatments. You can choose between two types of chemical treatments and two very different courses of action.

The first one is based on fungicides that act by contact (they’re sprayed on the plant tissue): these include chlorothanolil, cymoxanil, and imazalil.

The second option comprises systemic fungicides (which are added to the nutrient solution): triadimenol, metalaxyl, and myclobutanil. Before use, it’s important to learn the specific directions for each product. This includes dosage levels, effect duration, and also required safety measures in order to minimize health risks.

Downy mildew can be a real nightmare for cannabis growers. While some infections are just unavoidable, your best bet is to do everything you can to keep those fungi away from your precious plants. Lucky for you, there are some easy tricks you can use that won’t break the bank.