White spots on hemp leaves: why mold forms and how to solve it.

Why do white mold spots form on hemp leaves? Causes and solutions.

Modified on: 07/09/2021

Have you seen or heard of white spots or dots on cannabis leaves? It is a fungus called powdery mildew and here you can find out how to get rid of it.

From the moment the hemp growers plant the marijuana seeds to the moment of flowering, the plant goes through several phases in which it is possible to deal with changes in leaf color, pests, mold and much more.

Here we would like to clarify the phenomenon of white spots on hemp leaves.

Why do they appear?

Is it something serious?

This is powdery mildew, a fungus that takes the form of a white powder and which must be eliminated as soon as possible to prevent it from causing serious damage to entire hemp crops.

Here we explain why it appears and how to solve this problem.

Healthy hemp plant with no white spots on the leaves

White spots on hemp leaves? This is powdery mildew.

As with all other crops, hemp can also be affected by pathogens and molds.

One of these is powdery mildew, which is the formation of white spots on hemp leaves.

Powdery mildew is an attack by several fungi, belonging to the Ascomycetes family, which manifests itself in the form of a white powder grouped together in small, light-colored spots, which then spread from the leaves to the rest of the plant.

Powdery mildew occurs during the vegetative or flowering phase but is unlikely to appear immediately after the germination of cannabis seeds; however, to avoid nasty surprises it is important to monitor the plants from the beginning to the end, to avoid creating a particularly humid and poorly ventilated environment.

But are these white spots on cannabis leaves dangerous or is it an easily solvable problem?

Powdery mildew – also called white rot – although seemingly harmless, can actually compromise an entire marijuana plantation within a short time. 

This white mold saps the plants, slowing down or even stopping the photosynthesis process, causing yellowing of the leaves, but also causing dwarfing, curling and dropping of the leaves, poor flowering and if the plants are not treated, death.

How can we be sure that the white spots on the marijuana leaves really are powdery mildew and not another pathogen?

Powdery mildew is easily recognizable because it comes off the leaves simply by running a finger over them.

The fact that you can easily remove powdery mildew from the leaves in this way may seem like an advantage, but it means that the white powder can stick around easily and attack entire crops in a short time.

But here’s what to do if it does occur.

Read also: Basic principles of the vegetative phase in outdoor cannabis plants

Powdery mildew in hemp: natural and chemical remedies.

When hemp growers have to deal with white spots caused by powdery mildew, they can intervene either with natural remedies and practical measures, or with more “powerful” chemicals.

In outdoor hemp cultivation, it is, first of all, important to try to keep the humidity low and to suspend fertilization. 

In addition to this, if the powdery mildew attack has just occurred, it is possible to remove the stained parts of the plant, hoping that the ‘disease’ didn’t already have the time to spread.

In indoor marijuana cultivation, in addition to what has been said above, you can also take action by moving the plants away from each other so that they can enjoy better ventilation. You should also try to dehumidify the environment.

Fans are useful in the preventive phase to keep the environment dry, but, if powdery mildew is already present on the plants, they can facilitate the spread of fungal spores, and could therefore be a problem.

In both types of crops, it is recommended to use fungicides of natural or chemical origin, but what is important to know is that you must act as soon as possible because powdery mildew spreads very quickly!

Fungicidal spray for white spots on hemp leaves

Hemp and white spots during the vegetative phase: what to do?

When powdery mildew occurs during the vegetative phase (the phase from the stage after germination of hemp seeds to the beginning of flowering) it is easier to take direct action.

For impact action, some people use chemical fungicides, as if the plant is in the vegetative growth phase, it will have plenty of time to dispose of the chemicals before flowering and harvesting.

Acting immediately and with an impact product is important to prevent the entire marijuana crop from being compromised by the white spots that spread in an impressive short time.

If you only use organic and biological products, you can use a natural fungicide, although in this case, a more intensive pruning of the parts of the plant already affected by powdery mildew is also recommended.

Powdery mildew in the flowering phase of hemp: can something be done?

When powdery mildew occurs during the flowering phase, chemical treatments are not recommended and finding a proper remedy is more difficult.

Chemical products take a long time to be disposed of and could leave harmful residues in the crop.

The ideal solution is to use natural products that are not harmful to humans, although during the flowering phase they may not have time to act and solve the problem.

For this reason, sometimes the only solution is to eliminate the white-stained parts and harvest only the healthy inflorescences.

Read also: A basic guide to indoor growing: here are 5 key points.


When white spots appear on hemp leaves, farmers know that they have to intervene as soon as possible with an effective remedy, otherwise the disease could quickly contaminate the whole crop.

The best way to do this is to anticipate the problem by trying to limit the build-up of moisture as much as possible, dosing irrigation in the right way and, in indoor crops, paying attention to the ventilation parameters: all measures that make the development of fungi unfavorable.

Considering all of this, it is also important to highlight that to be certain that your cannabis crop will be able to withstand this type of attack in the best possible way, it is essential to start with the right genetics, sowing only auto-flowering, feminized and fast seeds of the highest quality such as the BSF seeds on sale in our store, Sensoryseeds.

After this brief explanation, we would like to remind you that it is not allowed to grow marijuana in France, but if you are passionate about this plant, you can buy all of the best collectable seeds in our store Sensory seeds.

We look forward to seeing you in our store: see you soon!