Alternaria, or Alternaria alternate, one of the most common fungi attacking cannabis plants, belongs to the family of Pleosporaceae, which includes 44 different species all over the world, and is closely related to the breakdown of organic matter. So much so that almost 20% of the global crop loss occurs due to the appearance of Alternaria.
Setting aside some variations, most Alternaria grow very much like Alternaria alternate, which propagates itself via asexual spores called conidia leaving little spots on the leaves in its path. These conidia are spores responsible for the reproduction of the fungus. Sudden humidity swings can cause conidia to fall and be moved by air currents to nearby plants. After landing on the leaves, they rest 'till a few hours after sunset to come out and reproduce with the help of dew. High humidity levels favor infection. In fact, in optimal environmental conditions, this fungus only needs 12 hours to penetrate the leave, settle and start destroying everything in the vicinity.
There are also some Alternaria causing other diseases such as the root rot that propagate at the speed of light through infected plant tissue. This genus of fungal pathogen species can destroy your crop within hours, be it a huge cannabis grow or a small marijuana plant.
Symptoms of Alternaria
The first sign of Alternaria presence in your crop is the appearance of tiny spots on the leaves. They could range from yellow to dark brown or even black, depending on the species. These small leaf spots can grow to form target-shaped concentric rings with a black spot right in the middle, which are the spores responsible for the reproduction of the fungus known as conidia.
Some may even show yellowish haloes around the spots as they become darker and, as conidia form, can develop a fuzzy look too. The spores reproduce when the room temperature is around 68-74°F, and the optimal environmental conditions for propagation are reached when the temperature gets closed to 80°F.
The spots may eventually coalesce into one large necrotic area that leads to the withering and death of the leaf. It's not uncommon for largely infected areas to defoliate, which can result in decreased vitality and even in the death of the plant should the infection be at an advanced stage.
How to prevent and fight Alternaria?
Alternaria can be prevented by following these tips: keep your crop clean, ensure proper ventilation and a balanced climate with no extreme humidity and temperature variations, water regularly and moderately early enough so plants don't feel damp during nighttime, let the substrate dry between each watering and make sure your plants aren't too close to one another.
Another important preventive measure is to avoid reusing the same soil several times in a row and to always get rid of decomposing vegetable matter. We cannot forget that these airborne spores may be everywhere. Besides, decomposing matter is a magnet for pathogens, so it's crucial to avoid piling up large amounts of vegetable waste.
A pretty interesting way to prevent its appearance is the so-called mulching. Mulch can hamper the spread of Alternaria spores already in the soil. However, if the infection is spreading rapidly, the plant is better off out of your grow. Otherwise, it could pass it on to the other plants.
Full-spectrum fungicides have proved to be very effective preventive measures too. Still, we recommend using natural fungicides such as horsetail or Neem oil when trying to fight it. If the problem persists, though, and chemicals are the only possible solution, make sure you use them before the flowering kicks off since cannabis flowers absorb all active molecules around them.
As we've seen, this fungus can be a real threat to plants if you fail to spot it and don't act promptly. That's why we recommend the abovementioned methods so highly. An infected marijuana plant is unlikely to come out well, even after the use of chemical fungicides. If that happens, get rid of her. You may see it as a defeat but it's the only way to keep the infection from spreading to the other plants as well. You're never getting back your time and effort by focusing solely on the affected plants.