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


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.

Leaf Septoria (Yellow Leaf Spot) On Cannabis Plants

Cannabis plants, unfortunately, are susceptible to a wide variety of diseases and pests. Leaf septoria is one such disease, and can greatly damage the foliage, development, and yield of your cannabis plant. Learn how to properly treat and prevent leaf septoria from devastating your crop.

Leaf septoria is a harsh plant disease that regularly affects the foliage of a variety of plants, including cannabis.

If not handled properly, leaf septoria can be devastating to plants, destroying foliage, stunting their growth, and ultimately affecting the size and quality of their yields.

In this article we take a close look at cannabis leaf septoria, what it is, and how to treat/prevent this disease from affecting your cannabis crops.


Leaf septoria, also known as septoria leaf spot or yellow leaf spot, is a plant disease caused by a specific kind of fungus known as Septoria lycopersici.

This fungus usually overwinters on dead foliage or common garden weeds. Fungus spores can also spread onto equipment like garden stakes and netting, before germinating when conditions are right.

Leaf septoria can be an extremely damaging disease, greatly affecting the foliage and growth of a variety of plants, including tomatoes, parsley, and obviously cannabis.

As the name suggests, leaf septoria is characterized by yellow and brown spots forming on both the upper and lower sides of leaves. The spots tend to be circular with dark brown margins and tan or greyish centers. They usually measure between 1.5 to 6.5mm.

Leaf septoria usually affects plants just after they enter the flowering stage and usually forms on lower leaves first. As the disease develops it spreads its way upwards, quickly affecting multiple leaves all across the plant.

Affected leaves will usually turn slightly yellow, then brown, and eventually wither completely. Leaf septoria rarely spreads on to fruit, so it generally won’t affect cannabis flowers.

If left uncontrolled, the disease can destroy a lot of foliage. This ultimately creates a lot of stress for plants and stunts their growth as well as the size of their harvests. Leaf septoria is particularly prevalent in areas affected by extended periods of wet, humid conditions.


As with most garden pests and disease, early detection is extremely important when dealing with leaf septoria. Make sure to pay close attention to your plants during extended periods of hot and humid weather, as well as during the early stages of the flowering cycle.

Once you’ve identified the disease, make sure you follow the following steps to control it and stop it from spreading:


The first step to effectively dealing with leaf septoria is removing infected leaves. If caught early, you can usually prevent the spread of the disease by simply removing all infected lower leaves and burning/destroying them.

However, if the disease has spread to the height of your flowers, you’ll generally want to skip this step. Removing foliage from flowering areas will greatly weaken a plant and reduce the quality of its buds.


Proper air circulation is extremely important for cannabis plants and plays a big role in the management of pests/diseases.

If you’re growing indoors, improving air circulation can be as simple as adding an extra fan into your room and creating some space between your plants. If you’re working outdoors, however, this might be a bit more difficult.

Pruning is also a great way to help create airflow in and among plants. Try trimming down extremely bushy areas of your plants and avoid having leaves touching or laying on top of each other.

If you’re working outdoors, you may want to try elevating your plants slightly so that they catch a bit more wind. Alternatively, also consider running an electric fan on outdoor plants if possible.


Moisture is another major player in the spread and germination of fungal spores. Hence, you’ll want to avoid moisture as much as possible.

Avoid overhead watering as this will wet the leaves of your plants and consider watering slightly less regularly in order to give the soil a chance to really dry out. Also water early in the day to allow the soil to dry out during daylight hours.


Fungal spores often spread into soils where they hang out over the winter until conditions are right for germination.

While you won’t be able to change your growing medium mid-grow, there are some steps you can take to avoid any spores from the ground spreading onto your plants further.

Start by removing any dead foliage and raking the soil to remove any possibly infected vegetation. Next, dry out your soil properly. Finally, apply a thick layer of mulch to the top of your soil then water your plants.

This will help stop the spread of fungal spores from the soil up onto your plants.


Night shade and horsenettle are common hosts of Septoria lycopersici spores. Hence, make sure you run through your garden and remove any weeds that could possibly host the fungus.


Remember that leaf septoria naturally strikes in hot, wet conditions. Hence, if you’re dealing with an infection, try driving down the humidity and temperature levels in your grow space (where possible).


If you’re dealing with a minor case of leaf septoria, steps 1-6 might be enough to kill the disease and stop it from spreading any further. However, if you’re dealing with a more serious infection you may need to rely on some heavy-handed fungicides.

Broad spectrum fungicides and disease control sprays will usually do the trick. For extra protection, try opting for a copper-based fungicide. Either way, remember to carefully follow the package instructions when using any kind of disease control agent, and avoid getting any of it on your buds.

If you’re after a more natural alternative, we suggest turning to essential or horticultural oils like neem. Neem oil is commonly used to treat all kinds of garden pests and diseases and can easily be applied to your plants using a mister.

Alternatively, consider trying eucalyptus, rosemary, lemon, and cinnamon oils. Either way, remember that these oils have strong aromas and should never go near your buds to avoid contaminating their aroma/flavour.

Neem oil serves as a completely natural way to protect your cannabis plants against pests.

Leaf septoria, also known as yellow leaf spot, is a fungal disease that can greatly damage cannabis crops. Here's how to deal with leaf septoria on weed plants.