Using insecticides and fungicides is common practice in marijuana crops to prevent, clear plagues and avoid cannabis plant diseases. However, you need to be careful when using such products as plenty of them aren’t properly flushed by the plant and they end up being ingested by the marijuana consumer. These substances can be very toxic for people. It is always best to use organic products as they are less invasive towards the plant; and the obtained cannabis will be free of harmful substances.

How to prevent cannabis plant diseases

Marijuana will likely show signs of disease if it isn’t treated properly. Most common plagues are red spider mites, white flies and thrips; therefore, you can use preventive compounds all the way until the start of the flowering period in order to avoid them. General guidelines recommend 100% organic products. If you opt for chemical products, use them only during the growth phase.

Apart from using pesticides, there are other actions you can take to prevent plagues in your crop:

  • Use new substrate for all the plants
  • Sanitize the tools before using them on the crop
  • Keep the area clean and disinfected
  • Keep pets away
  • Use latex gloves to handle the plant

Fungi, plagues, bacteria and viruses

There are multiple parasites and viruses capable of causing diseases on marijuana plants. This guide is going to go through the most common ones, how to detect them, how to prevent them, what effect they have on the plants and how to fight them.

Common fungi

Each type of fungi affects plants differently. However, most of them show up under similar circumstances: excessive humidity and poor ventilation.

  • Powdery mildew: It appears on top of the leaves and it looks like some sort of white powder in circles.
powdery mildew marijuana
  • Downy mildew: Leaves turn rusty and become weak until they finally wither.
  • Botrytis: It rots the buds, especially when they are very big and excessively humid. The flowers turn brown or gray. The only way to get rid of it is by cutting the infected buds at least 3 cm below the infected area.
botrytis marijuana
  • Rust: This fungus comes up in the substrates of stressed out or weak plants. It can attack the leaves, the stem or both. Infected plants show orange, yellow and brown spots and a sticky substance.
rust cannabis plant disease
  • Pythium: This fungus appears in the substrate and attacks the root of the plants, mainly during the germination stage, causing the stem to narrow so much that it doesn’t get hard enough to grow.
pythium cannabis
  • Fusarium: Lives on all sorts of substrates and damages the stem by rotting the root and causing vascular withering. There is no way to fight this fungus, all you can do is prevent it.
fusarium marijuana

Marijuana plant diseases caused by fungi have similar prevention methods. Plant surroundings need to be at the right temperature, humidity and ventilation and all the tools have to be clean. Of course, plants should also be carefully monitored every so often and sprayed with neem oil or organic fungicides from the very start.

Horsetail is also very useful to prevent and fight most fungi plagues.

Most common plagues

  • Red spider mite: Small red mite that feeds on plant sap, covering the leaves with yellow spots. It lays its eggs on the back of the leaf and creates spider webs between the leaves when the plague is very extended. This is one of the most persistent indoor plagues. It usually requires the use of chemical products and even sulfur burners.
  • White spider mite: This micro-mite alters normal plant growth, forcing the leaves to twist and acquire a shiny appearance. In order to detect it, simply look for eggs on the back of the leaves. Mix potassium soap with neem oil to eliminate them.
  • Aphid: These bugs live on the back of the leaves or around soft stems. They produce a thick and amber substance that attracts ants and other insects. To remove this plague, spray the plant with organic pyrethrins and 3 days later, use potassium soap to clean the amber substance and any leftover residues.
  • Thrips: Small yellow insects visible to the human eye that leave yellow and white spots on the top part of the leaves. Eliminate them by spraying organic pyrethrins on the plant.
  • Sciarid fly: Fungus gnats that appears in excessively humid substrates. Its larvae feed off the roots and stems until they make holes in them. You need to get rid of the adult insect and its eggs in order to beat this plague. Neem flour combats any substrate plagues.
  • Whitefly: This yellow-white flying insect is very easy to detect as you’ll see it flying around the plants. They live and lay eggs on the back of the leaves and feed off the sap of the plant. They can be virus carriers and cause sooty mold.
  • Caterpillars and worms: They leave clear bite marks and feces on the leaves. If the plant has flowered already, such feces can cause rotting. They grow and reproduce at a considerable speed, managing to eat plenty of the yield if not caught on time. Eliminate them using a Bacillus Thuringiensis-based organic insecticide.

Neem oil and proper basic care can help prevent most plagues.

Most common bacteria

  • Erwinia amylovora
  • Xanthomonas campestris
  • Pseudomonas cichorii
  • Erwinia carotovora ssp. carotovora
  • Agrobacterium tumefaciens

Bacteria cause small lesions to the plant that end up withering and staining the leaves, sometimes even rotting the stems. The best treatment is prevention and the same goes for fungi. The available options range from biological products to homemade remedies.

Viral cannabis plant diseases

Although there are plenty of plant viruses, only 5 affect marijuana plants

  • Hemp mosaic virus (HMV)
  • Hemp streak virus (HSV)
  • Arabis mosaic virus (ArMV)
  • Alfalfa mosaic virus (AMV)
  • Cucumber mosaic virus (CMV)

Viral cannabis plant infections need to be diagnosed in a lab, which requires time and money. The only solution to sort out a cannabis viral infection is to burn the plant, therefore, it is easier to focus on prevention. Avoid planting cuttings or seeds of unknown origin unless they are quarantined and away from the rest of the crop until you are sure they are healthy. Also, check for plague signs often to act as soon as possible if needs be.

How to apply the treatments

90% of products to treat cannabis plant diseases are diluted in water and sprayed on to the foliage. This has to be done at night time or with the lights turned off to avoid causing burns to the plants.

Indoors, turn off the ventilation system to ensure treatment efficiency and turn it back on 2 hours before the lights are due, to make sure the plants are properly dried beforehand. Spray the plant with only water 2-3 days later to clean product leftovers.

Always follow product guidelines and apply with safety.

As explained throughout this post, there are plenty of diseases that can affect cannabis plants and cause partial or total yield loss. Although there are lots of chemical remedies on the market, our recommendation is to use good quality organic products to prevent cannabis from losing its qualities.

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Erik Collado

With more than 10 years of background in the cannabis sector, his experience and knowledge are the base of GB The Green Brand’s success

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