black spots on weed

How To Recognise And Prevent Alternaria In Cannabis Plants

Alternaria fungus is a real problem for gardeners, seeing as it can be quite devastating and difficult to treat. In this article, we discuss the different symptoms of Alternaria in cannabis plants, how to treat it, and ultimately, how to prevent it from occurring in the first place.



Alternaria is a genus of fungal pathogen species that, if you are not careful, can grow on your crop. It is a major plant pathogen that is responsible for at least 20% of all agricultural crop spoilage. This pathogen travels through the air, soil, and cuttings, and can easily attack your cannabis plants, especially when environmental conditions favour infection. These conditions include high humidity, warm temperatures, and wet leaf surfaces. Once on your plant, this fungal pathogen weakens it and gradually settles down, particularly when the crop displays strong potassium and oxygen deficiencies.

Other factors that make your plants more susceptible to Alternaria include:

  • Previous nematode infection
  • Poor or inert soil
  • Bad nutrient management
  • High humidity
Propolix Fungicide


Other than cannabis, this fungal infection potentially affects all varieties of carrots, tomatoes, potatoes, beans, crucifers, and less often, other herbaceous plants. While it attacks plants at all growth stages, it is most common during the flowering and fruiting stages. A plant that is infected may show signs of damping off and stunted growth.

Most often, farmers will notice the fungus when it appears on leaves as spots that can grow up to 2cm in diameter. Sometimes, the purplish-brown spots of Alternaria may be topped by other, smaller black spots called conidia. Conidia are the spores responsible for the reproduction of the fungus.


Alternaria is a seed-borne fungus. This means that it’s transferred to the plant through the infected seed, then spreads to nearby plants. While most commercial seed houses screen for infection before distribution, they do not always catch the problem. Since the infection develops slowly, an infected plant may not show any signs until it is well into growth.

This infection is rarely noticed until transplantation or fruiting begins. As a result, it ends up affecting all the nearby plants. Even so, the bolting of seed and flower is almost always the first symptom of Alternaria in all plants. This disease tends to get more noticeable in late summer or early fall and is quite common on a highly susceptible host.

Some of the notable characteristics of Alternaria include dark purplish-brown spots with a yellow edge that’s due to chlorophyll deficiency. In case the infection lands on a vein, it can cause the leaf to become distorted with some yellowing and leaf drop. On the stems, Alternaria spots appear elongated and sunken, and grow up to a few centimetres long.

Other symptoms of Alternaria include:

  • Necrotic areas
  • Slowed plant growth
  • Poor nutrient absorption that may result in various deficiencies

The brown, multi-celled spores of Alternaria can be moved by air currents and overhead irrigation to nearby plants. The symptoms of this infection may appear on the leaves, stems, cyathula, petioles, and bracts. It normally has a quick impact on the saleability of your crop. Since this disease is generally unknown, it is usually mistaken for Botrytis blight.


When a plant is infected with Alternaria fungus, its production stunts and produces infected fruits, which are not edible even to animals. Feeding these infected fruits to animals is suspected to cause a gastrointestinal problem, especially diarrhea in some livestock.

Infected tomatoes display slowed production and smaller-than-average fruits. Carrots, on the other hand, wilt in the ground without growing to potential. Coles and beans become stunted, with cabbage heads never developing correctly.


The risks associated with Alternaria are not limited to plants only. This fungus can infect humans as well. Its consequences vary in gravity and may sometimes cause respiratory and skin allergies like rhinitis, hay fever, and asthma. According to research, at least 10% of the global population is allergic to Alternaria in varying degrees.


  • Avoid growing cultivars that are susceptible to the disease
  • Avoid exposing your leaves to long periods of wetness
  • Adequately space your plants
  • Always water your plants early enough in the day so they don’t sit wet all night long
  • Avoid watering overhead, instead watering from the base
  • Keep your crop clean
  • Do not reuse the same soil for several grows in a row
  • Provide proper ventilation, especially in a greenhouse to avoid humidity buildup


In most cases, once a plant is infected with Alternaria, nothing much can be done except removing and destroying the whole plant, including fallen leaves. This way, you eliminate the risk of contaminating other plants in the area. However, if your plant is isolated or you are unwilling to destroy the entire plant, there are a few different ways you can attempt treat the fungus.

To manage Alternaria, you need to spray fungicides directly on the infected plants, coupled with improved sanitation and crop rotation to prevent future outbreaks. If you are an organic farmer, you will be limited to sprays of copper or captan fungicides, which might make controlling the pathogen even more challenging. Conventional farmers, on the other hand, can use trifloxystrobin (FRAC 11), chlorothalonil (FRAC M5), triflumizole (FRAC 3), and other chemicals on plants listed on the label of each product.

Mulch can also help slow down the spread of Alternaria spores that are already in the soil when applied immediately after planting.

Alternaria can result in considerable losses to farmers if not taken care of immediately. Make sure you implement all the prevention tactics listed above to keep it from getting into your garden. When growing cannabis, an infected crop cannot really be saved by fungicides, especially during the flowering phase. As such, it’s best to remove and destroy the plants, and start your grow-op fresh. It may be upsetting and time-consuming, but you won’t be wasting your energy on bad buds.

Alternaria is a fungal pathogen that stunts plant production and ruins fruits and flowers. Here's how to recognise and prevent Alternaria in cannabis.

How to Tell if Your Stash of Weed Has Gone Bad

Avoid the conundrum by just storing it well in the first place.

Let’s say you chance upon a stash of weed you forgot about some time back. Happy day! But … is it even good? You don’t even remember stowing it away, let alone when you bought it. Can you still smoke it?

Can Weed Go Bad?

The good news is, weed doesn’t really “go bad;” its chemical makeup just changes. Marijuana is a plant, just like the veggies and herbs in your kitchen. Cells break down, molecules oxidize. Just as the dried oregano in your spice rack becomes less flavorful over time, weed becomes less potent. Old weed won’t kill you, but it also won’t get you all that high.

How to Store Weed

The best way to avoid this question in the first place is to store your weed properly. Glass or ceramic containers are your best bet. You want to make sure the container is airtight, and won’t transfer smells or flavors onto your precious weed. Store your pot in a dark, cool refuge that’s not your refrigerator or freezer — too cold environments can suck the sweet, sweet moisture out of those leaves.

Fresh weed is full of the cannabinoids CBD, CBC, and THC, but sit your weed down for a bit in ultraviolet light (i.e., sunlight), and your THC will break down into CBN, a cannabinoid that is way less potent and much more disappointing. Basically: Keep your weed in a still, dark, cool environment, and you’re guaranteed the best flavor.

But let’s say you didn’t get around to storing your weed like you should have, and now you’re stuck with a Ziploc bag of mystery marijuana that may or may not be good.


If your recently discovered stash doesn’t look much like weed anymore, just some dried out powdery leaves, it’s probably not going to be all that enjoyable to smoke. If you spot a fuzzy white powder growing on it, your weed is moldy and you definitely shouldn’t smoke it, unless you want to risk heart and lung problems. Mold spores, like moist places without a lot of airflow, which can happen if you store your weed where it’s too moist.


When you pull apart your re-discovered weed in your hands, you should hear snaps, not crackles, which can mean your weed is too dry. If your weed is silent, it has a lot of moisture, and you should be on the lookout for mold.


If your old stash smells musty, or like urine or a locker room, mold is probably the culprit. Chemical or plastic scents are a result of poor storage or pesticide contamination. Old weed might not smell as strong as the day you stuck it somewhere and forgot about it, but it shouldn’t have any off smells. Weed should smell like weed.


If it looks like your old stash is free of mold, and you decide to light some up, you’ll know pretty quickly if it’s really gone bad just by the taste. It should still taste like weed, without off flavors.

Smoking poor quality weed won’t hurt you (with the exception of mold) — it just won’t be quite as enjoyable as the fresh stuff. You probably won’t get as high thanks to a lower THC content, but that’s about it. Enjoy your re-discovered stash!

Avoid the conundrum by just storing it well in the first place.