Problems with harvest: Illnesses and moulds


Plants are living things and they remain healthy if you create a good climatic environment. The most important factors for the growth of your cannabis plant I've already mentioned before, namely: air, light and soil. In addition, it is also important to cultivate in as sanitary a way as possible. Nevertheless, it is possible that your plant might get sick. Even my plants get sick from time to time. A common illness is deficiency disease where the plants are missing something in their nutrition.

Potassium deficiency

Potassium is present in the entire plant and controls numerous processes (such as carbohydrate metabolism). It is essential for the transportation of all substances within the plant. In addition, it ensures firmness and the quality. In cases of potassium deficiency, vaporization is slowed down, as a result of which the temperature in the leaf rises and the cells are burned.

Symptoms of potassium deficiency

Potassium deficiency can especially be recognized by the edges of the leaf (edge disease). They can change colour and show signs of necrosis. Remember that it is impossible to determine potassium deficiency merely by looking at the exterior symptoms because dead leaf edges can also occur in case of low atmospheric humidity for instance.

Causes of potassium deficiency:

  • Not enough or the wrong fertilization.
  • Cultivating on potassium fixating soil.
  • An excess of salt (sodium) in the root environment.

Solutions for potassium deficiency:

  • Use nutrition for cannabis plants.
  • Add potassium yourself.
  • Add extracts of grape varieties.

Suzy’s Tip:

Don't add too much potassium as this could cause salt damage, calcium and magnesium shortage and acidification of the root environment.

Phosphorus deficiency

Phosphorus is important for cell changes and energy transfer in the plant. It's essential for the cell walls, the DNA proteins and enzymes. Phosphorus is especially important for young plants.

Symptoms phosphorus deficiencyt 

Phosphorus deficiency can be recognized by purple/black dying leaf parts which deform the leaf and make it curl up. 

Causes of phosphorus deficiency:

  • Too low an environmental temperature.
  • Too high a pH value.
  • Bad plant nutrition.

Solutions for phosphorus deficiency:

  • In instances of a high pH value you can acidify the medium with a diluted solution of phosphoric acid.

Suzy’s Tip:

After a shortage has been determined, fertilizing with phosphatic fertilizers will have little to no effect because the presence of phosphorus is most important during the growth stage. So always use proper soil and good fertilizers from the beginning. Phosphatic fertilizers: guano, bone meal, blood meal, Thomas phosphate and animal manure.

Calcium deficiency

Calcium is an essential element for both plants and animals. It is present in the entire plant and is essential for many processes, but especially for the growth process. Symptoms calcium deficiency Calcium deficiency is first visible in the older, upper leaves. These will have yellow/brown spots and will get sharp brown edges. Growth is often slowed down and in case of a serious shortage the tops will remain small and won't grow thick. Causes calcium deficiency:

  • Not enough or the wrong fertilization.
  • Cultivating on calcium fixating soil.
  • Too much ammonium, potassium, magnesium and/or sodium in the root environment.
  • Problems with vaporization because of a high electrical conductivity or too high/low an atmospheric humidity.

Solutions for calcium deficiency:

  • Start with good, not too acidic soil and the right nutrition.
  • In case of too high an electrical conductivity, rinse or give clean water in time.
  • Improve your soil by means of dolomite, gypsum, calcium-activated potassium, calcium-activatedmagnesium, Thomas phosphate, triple superphosphate or calcium nitrate.
  • Add extra calcium with liquid liming materials (such as calcium nitrate solution) in the nutrition.

Suzy’s Tip:

Be careful, because too much calcium is not good for the plant. This could result in a dark plant, limited in growth and shortages of manganese, zinc and copper.

Sulphur deficiency

Sulphur is present in the plant, the seeds and in the cell fluid. In the form of sulphate it's essential in the soil and in the water system of the plant. When you cultivate outside, you will never have a shortage of sulphate because of acid rain.

Symptoms sulphur deficiency

Sulphur deficiency can be recognised by light green/yellow discolouration which is often the most visible in the older leaves.

Causes of sulphur deficiency

  • Too high a pH value and too much lime..
  • Wrong composition of nutrition.

Solutions for sulphur deficiency

  • Check the pH value and lower it if necessary.
  • Add inorganic fertilizers such as epsom salts and kieserite.
  • organic fertilizer such as finished mushroom compost and animal manure.

Iron deficiency

Iron is important for the metabolism of the plant and the production of chlorophyll. Iron is hard for the plant to absorb and it can only do so it under the right circumstances. Cannabis plants almost never have a shortage of iron, only in case of quick growth or high stress levels can a deficiency occur which in principle will resolve itself.

Symptoms iron deficiency

Iron deficiency can be recognised by a strong yellowing of the young leaves and the flushes between the veins. In case of a serious deficiency the older leaves and the smaller veins will also turn yellow and die.

Causes of iron deficiency:

  • The pH value in the root environment is too high.
  • High presence of manganese and zinc in the roots environment.
  • Iron concentration in the roots environment is too low.
  • The soil is too wet, as a result of which the oxygen supply in the root is slowed down.
  • The root system is functioning poorly due to damaged, infected or dead roots.
  • Too much light on the nutrition tank as a result of which algae growth occurs (algae use iron) and iron chelates are removed.

Solutions for iron deficiency

  • Lower the pH value in the roots environment.
  • Foliar fertilization with iron chelate, solution of EDDHA-chelates or EDTA chelates.
  • Raise temperature.
  • Add organic manure such as farmyard manure, chicken manure and mushroom compost in the first stage of cultivation.


Moulds are very dangerous for your cannabis plant. They can reduce production both quantitatively and qualitatively. Unlike animal diseases, moulds can really destroy your plant. As it happens, moulds don't need the plant to survive. Mould traces can be active for an extremely long period of time, not only in the soil, but also on walls, in clothing and in the air. Weakened plants especially are susceptible to contamination by mould. Some moulds are also dangerous for human beings, because they produce toxins that can be damaging for your health. There are many different plant and soil moulds. The best known moulds I will describe in detail.


A common mould among cannabis plants is pythium. This mould causes root rot and rot of the bottom part of the stem. Young plants and cuttings are very susceptible to pythium. In case of severe pythium the plants will get 'fall over disease'. Pythium can be recognized by the browning of the bark at the bottom of the stem. In the beginning the brown tarnish can be easily removed yet later on the rotting process continues in the entire plant.


Mildews is a mould that often occurs in plants that grow fast and can cause the tops to rot. Mildew is characterised by the white/grey mould fluffs which can be found on the different parts of the cannabis plant. A distinction is made between real mildew and false mildew. Real mildew is powder like and forms moulds on top of the leaf, whereas false mildew creates fluffs on the bottom of the leaf.


Botrytis is a mould that often occurs on plants that grow fast. If your plant is infected with botrytis you will see grey blue fluffs on your cannabis plant. This mould takes advantage of damage to the plant that the cultivator (consciously or unconsciously) has made.

Prevent moulds (preventive)

Preventing is better than curing so try to keep the chance of mould infection as small as possible. You can reduce the risk of mould infection by cultivating as sanitarily as possible, by carefully regulating the climate for the plants, ensuring a consistent soil temperature, providing a good atmospheric humidity, by doing a precautionary spraying or by altering the plants by means of crossing the genes of the plants. Precautionary spraying against moulds should occur once in 5 to 7 days.

Controlling moulds (curative)

If your cannabis plants are infected with moulds you can control these in different ways: naturally, chemically or dynamically. If you control moulds by means of spraying, it is of the utmost importance that you thoroughly spray the plant, including the top and the bottom of the leaves. During the first week you should spray on a daily basis, after that, 2 to 3 weeks, depending on how serious the infection is. Real mildew can be easily controlled by spraying the plant with a mixture of milk and water. The concentration needs to be at least 10%, but less than 30% to reduce the risk of other mould forming. You can also treat your damaged plant with sulphur. Controlling moulds with sulphur happens with the usage of sulphur evaporators. Sulphur is very efficient in case of mildew but be careful because it’s very unhealthy to inhale sulphur. Other disadvantages are that many insects which serve as fungicides also die because of the sulphur and a layer may develop on your plant that can't be easily removed. The best way to control soil moulds is to set up a root stimulator.

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