Fungal infections in cannabis plants: what is 'damping-off'?

Typically, it's recently germinated seeds, seedlings or unrooted cuttings that are killed by the damping-off fungus. As stated, there're many different types of fungi that can infest cannabis plants, and this disease, in particular, is not caused by just one of them. There are some common symptoms, though:

All in all, already germinated seeds develop white spots, become weak, like dehydrated, and finally collapse. Other signs include the appearance of discolored or brown and yellow spots around the same area or even the development of ill-looking nodes. The stem of the diseased plant will get thinner and darker as it rots and ends up dying due to lack of circulation. If the plant already has leaves, the lowest ones will wither as a symptom of damping-off.

If you happen to spot any of these signs, be prepared to act quickly for this disease hampers the absorption of nutrients, which can be fatal to your plants. If you fail to do so, they may die within days, or even hours, because it's when they're the most vulnerable. These early stages are crucial.

Causes and solutions

Damping-off occurs when a fungus colonizes the soil or substrate we're growing our plants in and it extends to the other parts equally affecting stems, leaves, and flowers. This disease, therefore, attacks the lowest section of the plant, namely the base of the stem. What triggers its emergence?

• Cause: The fungus is present in a non-sterile substrate or growing medium.
• Solution: Use sterilized tools and soil to germinate your cannabis seeds or root your plant cuttings. Thoroughly clean the area, including the material you may have used, in order to minimize the risk of infection.
• Cause: Excess of water/moisture in the soil. Poor drainage.
• Solution: Water moderately. During their first days of life, plants don't need too much water as their small roots are unable to take up most of it. Check the substrate on a regular basis to make sure humidity levels are just right.
• Cause: Dampness in the air. Lack of air circulation.
• Solution: Let plenty of fresh air into the room before sowing, make sure you don't place your seeds too deep into the soil and don't use a small plastic greenhouse to cover your jiffy pellets. If you're working with cuttings, you'll most certainly have to use the greenhouse for it favors their rooting. In this case, renew the air by lifting the cover for some minutes every day.
• Cause: Inadequate temperature.
• Solution: The pathogen that usually attacks cannabis plants can thrive in different temperature ranges. Pythium species, for example, love warm, damp soils. Botrytis cinerea prefers cooler temperatures, instead. Fusarium species can thrive both in cold and in warm conditions.

This huge variability caused by the many differences in moisture, air circulation, and temperature makes it necessary to have a deep knowledge of the different fungi that could lead to this fungal infection so as to be able to detect them before it's too late. Some of the most common ones are these:

Powdery Mildew: startling but not as lethal

Powdery Mildew is incredibly easy to identify. When a plant is infected, a white or ashy layer of dust will start to cover its leaves and buds. One of the biggest issues with this fungal disease is that many could think it's nothing but dirt. In fact, if they try to wipe it off, it disappears, just like dirt. Fungal spores travel in the air and land on the substrate or on the leaves maximizing the chances of having an outbreak of damping off.

The good news is that it's very easy to detect and get rid of. One of the best ways to combat it is by using a horsetail- or propolis-based organic fungicide specifically conceived for fighting this disease. The infected parts will remain infected but you can still protect the rest of the plant.

Botrytis: careful with moisture and caterpillars

Also known as gray mold, it is a parasitic fungus that could attack your plants at any time. However, its occurrence is particularly dangerous during the early stages, when the flowering is about to end, and all throughout the drying phase. Brown water-soaked spots appear everywhere. Little by little the plant starts to become less green and more like gray, the disease reaching its peak with the development of a velvety film covering the infected parts. The environmental conditions that trigger its proliferation are high moisture levels and constant rainfall with extreme temperature switches. Caterpillar droppings can also cause botrytis to appear and attack your plant.

There's no other moment as critical as the flowering. Once it's settled, it's very difficult to fight, although there are many products that could help. That's why the best you can do is take preventive measures.

Fusarium: the most aggressive one

We can most certainly confirm that there's no fungus as aggressive as this one. If it ever shows up, it is rather likely to end up killing your plants. It can strike in many different ways. While some Fusarium parasites colonize the root area, others go straight to the vascular system. The symptoms are clear, though. In just a few hours, your plants will dry out completely or partially, as if they'd run out of water.

Unfortunately, if your plants are infected by this fungus, it's highly unlikely you could get rid of them for there's no 100% effective treatment against Fusarium yet. Prevention is, therefore, your best card. Alternatively, you can use beneficial fungi such as mycorrhizae to act as natural protectors of the roots, giving them a boost of energy and vigor.

Pythium: a fake fungus

This is, without any doubt, one of the most dangerous and lethal fungal infections that could affect your plants at an early stage: it's not unusual to find seedlings bent down towards the ground. Pythium is not exactly a fungus, though, despite it being originally classified as one. It belongs to the Oomycetes group made up of small terrestrial and aquatic eukaryotic microorganisms that resemble fungi and live either in the irrigation water or in the soil. Commonly found in greenhouses, they expand rapidly in favorable conditions.

Using a sterilized growing medium is the best way to prevent its emergence. Mycorrhizal fungi can also prove very useful, as much as preventing the pooling of stagnant water.

These are just some of the most common fungi that could cause the so-dreaded damping-off. Remember that detecting and reacting in time could save your cannabis plants. However, taking preventive measures and escaping from any risky situations is the best way to avoid bud rot.

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Cover photo: Michelle Grabowski, University of Minnesota Extension - Horticulture, Bugwood.org