- Alternaria belongs to the family of fungal diseases; it is a fungus that is occasionally found in cannabis cultivation.
- However, even though it rarely appears, Alternaria can wreak havoc on many crops. Once the disease has taken hold in a cannabis crop, it can persist for quite some time, as the symptoms can be misleading and are often mistaken for those caused by other molds.
- In this article, we’ll explain in detail what Alternaria is, the conditions under which it appears, the preventive measures you can take to avoid an infestation, and finally, how to effectively combat it to permanently eliminate it from your beloved cannabis plants.
- What is Alternaria?
- What conditions promote the growth of Alternaria?
- What preventive measures can be taken to avoid an Alternaria infection?
- How can you eliminate Alternaria in cannabis cultivation?
Any fungal disease can have devastating consequences in a cannabis crop, as fungal pathogens such as Alternaria, Botrytis, downy mildew, Fusarium, powdery mildew, and Pythium are not always easy to identify and eliminate.
For inexperienced growers, this can quickly turn into a real nightmare, since some of these conditions have fairly similar symptoms, making it difficult to tell them apart and determine the appropriate treatment for each one.
We recommend using organic fungicides whenever possible to protect the environment; use chemical treatments only when there is no other alternative. Furthermore, Alternaria, Fusarium, and Pythium directly attack the roots in the soil, which makes our task more difficult, as this underground part of the substrate is inaccessible to visual inspection.
What is Alternaria?
Alternaria is a pathogenic fungus belonging to the Pleosporaceae family, which is simply a category encompassing no fewer than 44 fungal diseases that are harmful to cannabis plants, as well as to many other plants.
As with many parasitic fungi, its spores can remain dormant in the soil for long periods of time before finally encountering the ideal conditions for their development on our beloved plants. Alternaria particularly thrives in the presence of dead leaves or decomposing organic matter, which it uses to colonize the environment and reproduce.
Alternaria alternata can also affect humans, as the World Health Organization (WHO) estimates that approximately 10 to 15% of the global population exhibits some form of allergy to these parasitic fungi. This fungal disease can also cause skin or respiratory problems, such as rhinitis, bronchial asthma, etc. Furthermore, it appears to be responsible for the loss of 15 to 20% of agricultural crops worldwide each year.
An Alternaria infection is characterized, upon its onset, by fairly dark, grayish circular spots that turn brown or even black on the leaves, as well as on the stems of the plants in your garden. These spots have a necrotic appearance with a yellowish border; they can reach up to 2.5 cm in diameter and are most often bounded by the main veins of the leaves.
We quickly see a widespread slowdown in growth or flowering, giving us the visual impression that the plants are suffering from various nutrient deficiencies.
What conditions promote the growth of Alternaria?
The mycelium of Alternaria reaches its optimal stage of development when temperatures approach 27 or 28 °C. We observe the formation of this fungus’s spores at temperatures between 20 and 23 °C; these spores are then dispersed into the indoor or outdoor environment, either by fans or by natural winds.
These spores are highly volatile in the air; they require a fairly stable surface to establish a new colony and reproduce. They can settle virtually anywhere due to their microscopic size; for this very reason, if they manage to attach themselves to the soil surface, this could also lead to infection of plant roots, as contamination deep within the substrate would occur via the water in the nutrient solution.
As you can see, Alternaria requires warm temperatures and high humidity to thrive under optimal conditions, and it also needs access to leaves or other plant debris to establish itself and reproduce. Caution is also warranted, as these fungal diseases can arise from a prior infestation by insect pests such as aphids, which can weaken the plants’ immune systems enough to facilitate the emergence of this fungus.
Aphids can also act as vectors for transmitting disease between different plants in your cannabis crop, as they can suck sap from the leaves or stems of different plants and gradually infect them as they feed on them daily.
These same aphids could eventually appear as a result of Alternaria infection, since both insects and parasitic fungi typically appear when a plant’s natural defenses are weak, regardless of whether the plant is in the growth or flowering stage.
Very often, a plant’s immune system is weakened because the soil is out of balance—it isn’t receiving the right amounts of water and nutrients it needs. This is why we mentioned earlier that these symptoms can be misleading. It is also quite possible to confuse them with those caused by parasitic fungi such as Pythium (damping-off) or Fusarium, as there are significant similarities in how their symptoms manifest.
All three require warm temperatures and high humidity to grow and thrive in the root zone. Specifically, they are parasitic fungi that attack roots in the soil and can cause damping-off in many agricultural crops. It is essential to take the time to do the necessary research before selecting the high-quality cannabis seed varieties you plan to purchase.
It is crucial to ensure that these cannabis strains can withstand high humidity levels in both the air and the soil—conditions the plants might face during heavy rainfall—as well as potential attacks by fungi or insect pests.
What preventive measures can be taken to avoid an Alternaria infection?
To avoid dealing with the presence and damage caused by this pesky fungus, we recommend that you make sure to perform a thorough, periodic cleaning of your indoor growing space, removing all dead leaves and plant debris found there. As a preventive measure, you may want to consider applying various fungicides from floor to ceiling in your space, so that you can start your next indoor grow in a completely sterile environment.
This approach will help you avoid the damage caused by many harmful fungal diseases of this type, and at the same time, it has another beneficial effect: it will help prevent the potential infestation of various species of insect pests, such as aphids, scale insects, spider mites, thrips, nematodes, and so on.
When it comes to outdoor cultivation, this aspect is harder to control because it’s not a closed environment. A good tip we can offer is to apply a broad-spectrum preventive treatment at the end of the growth period, so your cannabis plants can begin flowering under the best possible conditions.
Good ventilation is also key to success in indoor growing, as moisture buildup combined with poor ventilation in various areas of the space creates ideal conditions for spores to develop on plant parts such as leaves, stems, or buds, which can lead to the imminent onset of fungal diseases like powdery mildew, downy mildew, botrytis, pythium, or fusarium.
It is also necessary to maintain a temperature that is as stable as possible (between 20 and 25°C maximum), as well as a humidity level appropriate for each stage of growth (70 to 80% for seedlings, 63 to 70% for larger plants) and during flowering (45 to 55% maximum).
It is important to remember that seedling blight is usually irreversible for young seedlings grown from even the highest-quality seeds or for seedlings during the growth phase, because the appropriate treatment may not be effective enough to combat the fungus at that stage, since the plant’s immune defenses are still too weak to counter the attack of these molds, which manifest as scattered spots at that time.
Under no circumstances should you consume cannabis flowers affected by these molds, as they are clearly harmful to your health and can cause serious respiratory problems.
Learn how to water your plants in a way that meets their specific needs for water and nutrients—both in terms of quantity and by ensuring the nutrient solution and growing medium have the correct pH—as this will be much more beneficial for their nutritional balance and will help you avoid many problems with fungal diseases and various species of insect pests. Another important point is to always use fresh growing medium when starting each new growing cycle.
As a final biological control measure, you can prepare a horsetail decoction and apply it as a spray during the growing season; it is a completely natural and highly effective product. This natural treatment will effectively neutralize the action of spores within your crops.
How can you eliminate Alternaria in cannabis cultivation?
If, despite all the advice we’ve given you above—such as regular cleaning and implementing all the preventive measures mentioned earlier—you continue to experience persistent problems with one or more of these harmful fungal issues, then you will have no choice but to use chemical products.
We always prefer to avoid using chemicals whenever possible, but sometimes there is no other alternative for protecting our crops, including our cannabis plants. In such cases—as a last resort—you may consider using broad-spectrum fungicides such as Mancozeb or Maneb.
There are also highly effective copper-based fungicides for eliminating these molds, such as Bordeaux mixture, Clorotanolil, or Zineb. If you need to use fungicides made from chemicals such as those mentioned here, we recommend that you first carefully read all the information regarding their use, including the various safety precautions to follow.
It is essential for your health to always make sure you have the appropriate safety gear (wetsuit, mask, gloves, specialized footwear, etc.).
