- Downy mildew is a fungal disease that is quite common in many agricultural crops, including cannabis cultivation, which is the focus of this discussion.
- This pathogenic fungus causes an infection within the infected plant, which is associated with various characteristic symptoms that appear as the infection spreads.
- In this article, we’ll explain exactly what downy mildew is, how to spot it on your plants, and how to get rid of it for good so you don’t lose your harvest.
- What exactly is downy mildew?
- What conditions promote the development of downy mildew?
- How can I get rid of powdery mildew in my cannabis crop?
It is certain that many growers with some experience in cannabis cultivation have already encountered problems with fungal diseases that cause mold, such as downy mildew, botrytis, powdery mildew, fusarium, or alternaria. These fungi attack cannabis plants gradually, as various symptoms appear progressively within the growing area.
Prevention is by far the best way to avoid any problems related to these issues. It’s best to be very vigilant, as these pests can lead to others—simply because once cannabis plants’ immune systems are weakened by this mold, harmful insects can easily infest your grow space, whether indoors or outdoors.
Even though down y mildew isn’t the most devastating disease for cannabis cultivation, it’s best to protect and preserve every plant in its entirety, since this mold attacks every part of the plant step by step.
Leaves can easily be affected, for example, which will gradually disrupt the photosynthesis process and interfere with a range of internal metabolic processes, weakening the plants over time and leading to either partial or total crop loss. That is why we are offering some tips on how to handle such a situation.
One of the first and most important pieces of advice we can give you is to carefully select high-quality cannabis seed strains, paying special attention to the specific conditions of your climate.
It makes sense that if you live in an area with high humidity for various reasons, you should choose cannabis strains that show a certain level of resistance to many of the fungal pathogens that cause mold. This is a basic starting point for preventing rot.
What exactly is downy mildew?
Late blight (Phytophthora infestans) is a pathogenic fungus whose initial visual symptoms appear on both the upper and lower surfaces of the leaves, where small spots ranging in color from yellowish to translucent can be observed, along with a white layer of powdery residue.
The stems and heads can also be affected; if the heads are infected, they will be unfit for consumption because this could cause serious respiratory problems in humans. It is possible to confuse downy mildew with powdery mildew, as the latter exhibits fairly similar symptoms, except that the circular spots appear only on the upper surface of the leaves in the case of powdery mildew, whereas with downy mildew, both sides of the leaves can be affected by this fungus.
Downy mildew then causes the leaves to curl upward, before they gradually but surely die. Downy mildew is a harmful fungus that most often spreads through its spores, which attach themselves to and colonize the leaves, facilitating both reproduction and the spread of the disease by forming the characteristic rot on various parts of the plant. The first visible sign will be a slowdown in the plant’s growth or flowering.
Widespread downy mildew infection can spread rapidly throughout a crop, because even if the mold first appears during the final weeks of flowering, it can still lead to a significant loss in the final yield of the high-quality buds we all seek—or even a total loss of the harvest—if the disease is not treated properly and in a timely manner.
It is also worth noting that fungal spores can remain dormant for long periods of time until all the right conditions are finally in place for them to fully develop on plants of all kinds.
What conditions promote the development of downy mildew?
What is actually happening internally within the cannabis plant is that it is already suffering from one or more nutrient deficiencies in its nutritional balance, which weakens its immune system. This allows us to rule out all factors related to its environment, and ultimately helps us better understand that each plant gradually loses its ability to resist a potential mildew attack.
That is why it is important to pay special attention to your plants’ nutrition, providing them with the right amount of water and nutrients while carefully monitoring the humidity level. Be careful not to add too much nitrogen to your nutrient solution, as this will accelerate the growth of this fungus.
As we mentioned in the previous chapter, great care must be taken to ensure that this does not lead to related problems, such as the arrival of certain insect pests. Downy mildew is caused by several species of pathogenic fungi.
These factors cause fungal diseases through the action of fungi or molds, which thrive in environments where air humidity is very high and where air circulation and ventilation are insufficient. High humidity alone already provides comfortable conditions—or an ideal environment—for the development of significant rot.
Good air circulation and proper air exchange are essential for climate control in the grow room, as it is crucial to avoid creating pockets of stagnant air in the various nooks and crannies of our indoor cannabis grow setup.
These air pockets provide an ideal environment for fungal spores to spread throughout each plant. For successful indoor cannabis cultivation, it is essential to closely monitor two key environmental factors: humidity levels and temperature fluctuations.
If you maintain consistent conditions for these two factors—both during the growth phase and throughout flowering—you should be able to avoid many problems. It is also important to prune the foliage to a greater or lesser extent depending on the varieties you are growing. The goal is to limit moisture buildup within the plant’s structure, which can lead to mold growth.
This will promote airflow while also allowing light to pass through, which will simultaneously enhance the yield potential of the delicious, fragrant buds packed with resinous trichomes produced by each cannabis plant in our garden. We say this often, but it’s essential to take great care when selecting the high-quality cannabis seed varieties you plan to purchase. Make sure their characteristics are well-suited to the climate conditions in your region.
If a plant has too many leaves on its structure, there is also a risk of contact transmission, meaning that plants can spread the infection to one another simply through the friction of their leaves, stems, or buds. As we mentioned earlier, fungal spores are highly volatile, invisible to the naked eye, and can therefore settle anywhere they please.
How can I get rid of powdery mildew in my cannabis crop?
If you are not familiar with the preventive measures to take, you will likely have to deal with the harmful effects of this fungus. That is why we have provided some recommendations below for eliminating these fungi, which could jeopardize the success of our harvest.
Remove fallen leaves and yard waste
First, it is necessary to sanitize the various surfaces of the contaminated parts of the crop and plants (excluding the buds) using water with a high pH between 8.0 and 8.5. The effects of this high-pH water will be very beneficial for our beloved cannabis plants, as the fungal spores will no longer be able to attach themselves to the various sensitive parts of the plants (leaves, stems, buds) to reproduce and colonize the environment.
To do this, please use paper towels or a clean cloth; however, be sure to dispose of them well away from the growing area once you’ve finished cleaning. You’ll also need room-temperature water, especially to remove spores from the plants, to avoid causing any thermal shock to the plants.
Also, be sure to collect all fallen leaves and other plant debris and quickly take them to a recycling center far from your growing area, just as you would with the rags used during disinfection. Keep in mind that fallen leaves and plant debris provide an ideal habitat for the reproduction and growth of many fungi, such as downy mildew, botrytis, powdery mildew, fusarium, and alternaria, but they are also a prime destination for many insect pests, which could join in on these unwelcome festivities.
Prune the plants if necessary
Don’t hesitate to prune all affected parts of the plants, including diseased leaves that are drying out, wilting, turning yellow, and gradually dying. This also applies to the heads, which will be unfit for consumption anyway if they are infected by the fungus, and finally to the stems whenever possible.
Of course, be sure to promptly dispose of all this plant waste far from where you are working. This practice also has the added benefit of optimally stimulating the hormones in each plant (auxins and cytokinins, which are important for the structural growth of cell walls), thereby promoting their structural development.
Avoid repotting if the plants are diseased
If you think about it, it’s definitely best not to transplant your cannabis plants into soil that’s still very moist at that point. This makes perfect sense, because when we shake the plants—often unintentionally—to place them in a larger pot containing a new, high-quality potting mix, we will spread fungal spores across the soil surface or into our high-quality potting mix.
Caution is advised, as this could lead to new pest problems in the soil—specifically affecting the roots, which could fall victim to a fungal infection caused by Pythium, for example. A Pythium infection is devastating to roots, as its effects on seedlings lead to their almost certain death within a short period of time.
Pythium, or "damping-off," is difficult to treat because the problem originates in the soil, a part of the plant that is hard to reach. For those who enjoy growing seeds for various personal experiments, it’s important to know that Pythium can lie dormant in the seeds you harvest.
If you’re growing infected plants that will produce seeds you intend to save for future use, expect to run into some problems—don’t say we didn’t warn you. A certain percentage of the new plants that germinate will develop root rot caused by Pythium at some stage during their growth or flowering cycle.
It is also important to take certain precautions against a problem that could arise due to a lack of knowledge, because since Fusarium and Pythium directly affect the roots in the soil—which are invisible to the naked eye—we might mistake certain symptoms for those caused by viruses, such as the hop viroid, whose symptoms are quite similar to those of the two pathogens mentioned above.
Treat effectively
You can apply a fungicide treatment that acts either by contact with plant tissue or systemically; this means mixing a biological or chemical product into the nutrient solution used to water the cannabis plants. This is how this type of treatment is absorbed; its active ingredients are distributed through the plant’s internal water and nutrient transport system to all parts of each plant for optimal effectiveness.
We recommend using various contact fungicides, such as cymoxanil, chlorobutanil, or imazalil, as soon as the first rains arrive to provide a protective barrier for the plants. As for systemic fungicides, we recommend treating with metalaxyl, triadimenol, or myclobutanil.
Each product must be applied within 72 hours at the latest to effectively eliminate these pests from your crops. Be sure to carefully read all information regarding dosage and safety precautions before applying any treatment. As you have likely gathered from this article, prevention is always better than cure.
In short, this is a good reason to conclude that if you can maintain proper temperature and humidity levels in your indoor grow space— matching the required values during both the growth and flowering phases—you will drastically reduce the likelihood of these fungal problems occurring in your cannabis crop. Also make sure there is sufficient space between the plants, as well as consistent airflow throughout the grow space.
