This board is about one habit. First, find out exactly what is attacking your plant. Only then choose a treatment. Then plan what you will do in the first week after you find the problem.
Why identification comes first
Most problems on cannabis fall into three groups. Sucking pests, such as mites and thrips, feed on the cells inside leaves. Fungi and similar organisms grow on leaves and buds. Root diseases attack the roots below the surface.
The usual treatments for these three groups have almost nothing in common. A product made for insects does nothing to a fungus. A fungus treatment does not stop mites. If you guess wrong, you lose the days when the problem was still small enough to stop.
What to do: before you buy or mix any product, write down which group you think the problem belongs to. Also write down what you saw that made you think so. If you cannot say, post in the Plant Infirmary first. The section near the end explains how to post there.
Where to start: five guides in reading order
- Integrated pest management (IPM): a way of controlling pests that combines prevention, regular checks, natural enemies and climate control. Chemicals come last. Read this first to set up a weekly checking routine.
- Spider mites and thrips: identification, life cycle and biological control. Spider mites are tiny eight-legged pests that live mostly under the leaves. Thrips are tiny, slender insects that scrape the leaf surface. Read this to learn what to look for under a lens.
- Botrytis (grey mould): prevention, detection and emergency response. Botrytis, also called bud rot, is a fungus that rots dense buds from the inside. Read it before you flower a crop in the rainy season.
- Powdery mildew: prevention, early detection and treatment. Powdery mildew is a fungus that leaves white, flour-like patches on the surface of leaves. Read it to learn to spot the first small patches.
- Root rot, Pythium and Fusarium: protecting the roots. Root rot is a symptom, not a single disease: the roots turn brown, soft and die. Pythium (a water mould) and Fusarium (a soil fungus) are two common causes. Read this if your plants wilt while the growing medium is still wet.
Three mix-ups to rule out first
Each pair below looks alike at first glance but needs a different response.
| What you see | It could be… | …or it could be | First check |
|---|---|---|---|
| Pale spots or damaged patches on leaves | Spider mites: fine pale speckles, tiny moving dots on the underside, webbing when the infestation is heavy | Thrips: silvery or bronze streaks on the upper side, with tiny black specks (their droppings) | Turn over several leaves from the top, middle and bottom of the plant. Check both sides with a strong hand lens. |
| White or grey growth | Powdery mildew: white powder on the leaf surface that wipes off with a finger | Botrytis: grey-brown fuzz inside a dense bud. Often the first sign is one leaf on a bud turning yellow and dry while the rest stays green. | Wipe the patch with a finger. Gently open any dense bud that has a dying leaf. |
| Yellowing, drooping or burnt-looking leaves | A failing root zone: the plant wilts even though the medium is wet, and the roots are brown, slimy or smell bad | A feeding error: too much or too little nutrient, or the wrong pH or EC | Measure the pH and EC of the water you give and of the runoff. Then look at the roots: healthy roots are white or cream. |
pH tells you how acidic the solution is. EC (electrical conductivity) tells you how much fertiliser salt is dissolved in it. If both readings are normal and the roots are brown, look at the roots first, not the feed.
What to do: once you have narrowed it down, open the matching guide above and write a one-week plan. List what you will remove, what you will change in the room and on which days you will check again. The plan lets you see after seven days whether your action worked.
Why mites, botrytis and powdery mildew have their own boards
Each of these three problems has its own life cycle. Each also has a different period during which action still works. Mites can build up over several generations if you wait. Botrytis cannot be reversed inside a bud, so you can only cut out the infected bud and stop the spread. Powdery mildew is easiest to control while the patches are still small and few.
Because the timing is so different, each one has its own board: Mites, Botrytis and Powdery Mildew. The Plant Infirmary is the open board for everything else, including problems you cannot yet name.
What to do: once you know which problem you have, post or search in its board. That way the answers come from growers who have dealt with the same life cycle.
How to post in the Plant Infirmary so the problem can be diagnosed
A thread can be diagnosed when it gives three kinds of information. Without them, other growers can only guess.
- Photographs. Show the whole plant, plus close-ups of the top and the underside of the affected leaves. If you can, include one photo of the roots. Photos let others see the pattern of damage, not only one leaf.
- Environment readings. Give the temperature and humidity in the day and at night, and the pH and EC of your input and runoff. These numbers show whether the climate or the feed could be the cause.
- What you changed recently. Mention any new nutrient, new clones, a spray, a transplant or a power cut. A recent change is often the cause, and it helps others rule out other possibilities.
What to do: before you post, take the readings at the same times on two different days. Two days of readings show a pattern. One reading can be a one-off.
Spraying in flower and the lab certificate
Anything you spray while the plants are flowering has a good chance of showing up later on the certificate of analysis (COA). This is the laboratory report that lists cannabinoid levels and contaminants such as pesticide residues. That is why these boards discuss biological control and environmental control before chemicals. Biological control uses living natural enemies, such as predatory mites. Environmental control changes temperature, humidity and airflow so that the pest or fungus cannot spread.
This matters most if you sell to any buyer who requires lab-tested product. A residue found on the certificate can make a whole batch unsellable to that buyer.
Translator’s note: in Thailand, use only pesticide products registered with the Department of Agriculture (กรมวิชาการเกษตร), and use them exactly as the label says. There is no separate list of products approved for cannabis. Read what you may legally spray under Thai GACP before you buy anything. Not every predatory mite species works equally well in Thai heat. Read this comparison of two predatory mites at 30–32 °C before you order natural enemies.
What to do: keep a spray log with the date, product name, registration number, dose and plant stage. If a test later shows a residue, the log tells you where it came from.
Thai and Southeast Asian conditions
Translator’s note: the rainy season keeps humidity high for months. That favours botrytis in dense buds and powdery mildew on leaves. Indoors, a power cut stops the air conditioner and the dehumidifier, so humidity can climb overnight without anyone seeing it.
Translator’s note: heat of 30–35 °C speeds up the life cycle of many pests, including mites. A small population can become a serious one quickly. Warm roots also favour root disease. If you grow in water, read running hydroponics at 31 °C reservoir temperature.
Translator’s note: clones bought from another farm are a common way pests and diseases enter a grow room. They can carry mites, thrips or hop latent viroid (HLVd). HLVd is a tiny infectious particle that stunts plants and reduces resin, and it spreads through cuttings and tools. Clones can also turn out to be a different strain from the one you paid for.
What to do this month:
- Record humidity in the morning and at night, every day, for two weeks. This shows you when your room enters the danger zone for mould.
- Keep new clones in a separate area, away from your main room. Check them under a lens several times before they join your other plants. This stops one bad batch from infecting the whole room.
- Use separate tools for new plants, or disinfect your tools between plants. This lowers the risk of spreading HLVd and fungal spores.
Have a plant you cannot diagnose, or a question about anything above? Ask below this topic, or post your photos and readings in the Plant Infirmary.