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white spots on cannabis leaves not mites

White spots on fan leaves?!

Have you noticed white spots on your fan leaves or stems lately? Have you noticed leaves with white spots that look like small round patches of powdered sugar?

If so, then I may have bad news for you, this isn’t fairy dust left behind from your guardian weed angel, this is the sign of either a fungal disease or an aphid/spider mite infestation – but don’t panic.

If you haven’t seen bugs and your an indoor grower it’s most likely just White Powdery Mildew which is actually pretty common! and easy to fix if done quick enough.

As mentioned above, if you’re growing indoors, in a clean environment, what’s covering your leaves is most likely White Powdery Mildew, unless you are growing outdoors, the white dusting could be a sign of other problems such as Spider Mites, which love to lay eggs on the leaves, looking very similar to WPM at first glance, left untreated both can and will devastate crops.

I’ll explain how you can tell the difference between WPM and spider mites, how WPM is caused and how to get rid of it successfully.

What Is White Powdery Mildew?(WPM)

White powdery mildew is a fungal disease that only exists to eat, reproduce and live another day. WPM is quite literally a mold, that grows and leeches from the nutrients in your plants and will eventually cripple the plant if not stopped.

Often occurring due to poor air circulation and high humidity levels, the fungus develops a strong ecosystem within the humidity and shade of the canopy but it can be easily fixed if you catch it early enough – left untreated, WPM can turn into a catastrophe and ruin an entire crop.

How to identify WPM?

  • Plants infected with mildew look as if they have been dusted with flour/sugar.
  • Powdery mildew typically starts off as circular, powdery white spots.
  • WPM typically covers the upper part of the leaves, however, might grow on the underside of the leaves.
  • The lower leaves are usually affected first, giving the impression that a sudden outburst has bloomed overnight when it fact the fungus would have been “setting up shop” the whole time. The most affected leaves tend to be towards the bottom of the plant.
  • Leaves can also twist, dry and break.

How to control & eliminate WPM?

The main cause of WPM is high humidity – usually as a result of poor air circulation, cannabis plants require a RH level of 40 – 60% with air circulation, remove the air and you can be guaranteed that WPM will colonise.

Start by closely inspecting each individual plant and carefully remove any affected leaves, be careful to not let infected leaves come into contact with unaffected leaves, dispose of all infected leaves separately to be burnt, seriously – burn them.

Once you are confident that you have removed all infected leaves, I would also recommend that you remove the first 1 – 2 inches of soil as it’s likely that spores have fallen into the soil and replace it with new, clean soil.

Now you will need to treat your plants with a natural fungicide to kill the remaining spores and stop it from coming back again.

Natural Fungicides for WPM:
Fungicides can be easily made at home by filling with the help of a hand pump sprayer, a few popular solutions involve:

  • Milk (1:9 ratio of milk to water)
  • Baking soda (2 tablespoons per gallon of water)
  • Neem Oil* (4 teaspoons per gallon of water)
  • Hydrogen Peroxide (1 teaspoon per gallon of 35% H202)
  • Trifecta Crop Control Powdery Mildew Remover

*Add a drop of washing up liquid to prevent the oil from separating with the water inside the handpump, otherwise, the oil will spray out at a full concentration as a result of sitting on top of the water.

Apply a heavy foliar spray at night time before the lights go off to both the top and underside of the leaves, repeating this process daily. It’s important to wait until the lights are due to go off because the water droplets can be magnified under the intense lights and burn your leaves.

Try to not spray too much of the fungicide solution onto the mature buds because they are oils and can stick around for a while,

Disease Resistant Strains

If your growing environment is susceptible to mold or other diseases, did you know there are mould-resistant cannabis strains that exist?

White spots on fan leaves?! Have you noticed white spots on your fan leaves or stems lately? Have you noticed leaves with white spots that look like small round patches of powdered sugar? If

Tiny White Spots But No Spider Mites – Help Needed!

  • Jan 31, 2016
  • #1
  • flexy123
    Well-Known Member

    Info to my grow: Growing some feminized (Northern Lights, Somango) and some Autos, the problem doesn’t show on all strains/plants, some look perfectly fine without the dots.

    I am growing in soil, and as nutes I am using “PlantMagic Organic Oldtimer”, currently watering maybe once a week.

    This is not my first grow so I am “somewhat” experienced. I have my share of experience with spider mites, I had them last year. I checked the undersides of the plants, there are no mites.

    The problem seemed to have gradually developed once I switched from my old MarsHydro 300W lights to my new fixture which is 4x CREE CXB3590 lights. The light is currently about 35-40cm (13-14inch) from plants. Light bleach?? I am a total loss!

    Please see the pictures. You can’t “rub off” the dots. You see some leaves where there are more dots in the middle. Some leaves are starting to get yellow and some also have brown spots. I am baffled since my grow otherwise since I have the new light is great. I am just puzzled because of those spots.

    (I DO have a fungus gnats problem but I always had them, remedy for them is already on the way in the mail).

    Info to my grow: Growing some feminized (Northern Lights, Somango) and some Autos, the problem doesn't show on all strains/plants, some look perfectly fine…