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Mildew and growing weed

Mildew is the general name for a certain set of mould infections that strike plants, in which various parts of the plant become covered with a white or grey fluffy mould. Real mildew (Erysiphe graminis) is common on various grains, with various specialised forms (forma specialis) adapted to each specific grain variety.

Real mildew.

  • Real mildew: real mildew forms a fluffy mould on the upper surface of the leaf

Fake mildew.

  •  Fake mildew: fake mildew appears on the underside of the leaf. Fake mildew is found on, cucumber, lettuce, and other plants

Some photographic examples to help identify mildew:



Real mildew:


Fake mildew:


Moulds, mildew to bud rot

Moulds tend to thrive best in damp conditions, especially if there is not too much circulating air. Under these conditions mould spores that are always present in the air find a spot where they can establish a foothold and start growing into adult fungi. When you have not succeeded in preventing this mould growth, then you need to get on top of the situation as fast as possible.

If the fungal growth is still moderate, immediately remove the infected parts and make sure that the growing environment of the cannabis is suited to the plant rather than the mould; that is, with good ventilation, control of the air humidity and temperature, and your plants not sitting in too wet a medium.

If the infestation is more serious, there is no other option than to spray with poison (fungicide). Repeat the application after a few days, even if it looks as if the first application cleared up the trouble. Even then, it still applies that you need to control your climate better. The application of fungicide should be seen as a last resort. It is not healthy for young plants or people, so this is another case of prevention being way better than the cure.

A very common fungal infection of cannabis is pythium. This mould causes root rot and rotting of the lower part of the main stem. It usually strikes young plants and clones. Bigger, healthier plants are less susceptible to pythium infection. In more serious cases, the plants can be stricken by ‘falling down disease’, the symptoms of which should be fairly apparent.

Pythium can be recognised by the way it turns the bark at the foot of the stem brown. At the early stage of an infestation this brown build up is easy to peel off, but later the rotting process penetrates deeper in to the foot of the plant.

Pythium is a mould, which does best in damp conditions. Pythium spores are only spread via water. There are 2 types of spore created, mobile ‘swarm’ spores and ‘resting’ spores. The swarm spores germinate best in temperatures around 15 degrees C., whist the resting spores germinate when it’s warm; around 28 degrees C.

If you want to try and avoid infestation by pythium, for a start you need to keep your soil temperature constant. Large swings in temperature should be avoided. Further, you will need to keep the air moisture at a good level (so not too high).

Leaf moulds (such as mildew) and filamentous fungi are less common than pythium. Mildew can cause among other things bud rot. Here too the same precautions can help: maintain an optimal control of your climate. Unlike many other moulds, mildew also does well when air moisture content is low.

Bud rot usually strikes towards the end of the bloom phase. The more compact the plant, the bigger the chance that it will rear its ugly head. You can recognize bud rot by the way bud leaves suddenly turn yellow. These yellow leaves also come loose from the bud with the slightest tug. To prevent the whole plant from becoming infected, I’m afraid you’ll have to remove the whole bud.

Bud rot can be prevented from striking to some extent by ensuring your grow space maintains a relatively low air moisture during its dark periods.


It was only about three years ago that South American researchers came up with an alternative prevention of real mildew. Brazilian scientist Wagner Bettiol found that weekly spraying with milk could keep infection by real mildew as well under control as by spraying with synthetic pesticides.

Not only was milk a good plant protector, it also turned out to be a useful nutritional supplement for the leaves, one that boosted the plant’s immune system. From the results of his experiments it appeared that a weekly spray with a concentration of at least 10% (1 part milk to 9 parts water) reduced the extent of the mildew infestation by 90%. Just make sure you do not increase the concentration too much, since above 30% it starts to serve as a nutrient base for infection of the leaves by another mould.