Powdery mildew is a fungus that grows as white, powdery spots on the upper side of living cannabis leaves, and later on stems and buds. It lives on the outside of the leaf, so a spray that touches it can kill it. This part explains which sprays to try first and what never to do. It also covers what to do when mildew reaches the buds in the last weeks, how mildew affects lab testing, when to watch most closely, and what changes in Thailand’s heat and humidity.
The fungus behind the disease is Golovinomyces ambrosiae. Botrytis (grey mould, also called bud rot) feeds on dead tissue and needs wet surfaces. Powdery mildew is different: it feeds only on living cells and does not need water on the leaf. Several points below come from the Pacific Northwest Plant Disease Management Handbook [2]. This is a crop-disease guide from Oregon State University Extension, the farm advisory service of a US state university.
Part 1 explains how the fungus infects, how to tell it from trichomes under a loupe, and how to prevent it.
This is Part 2. Part 1: Powdery Mildew on Cannabis: Prevention, Early Detection, and Effective Treatment
1. Treatment Once You Find Mildew
Botrytis is hard to stop once it is inside a bud. Powdery mildew, by contrast, can be treated after you find it. The reason is that it grows on the leaf surface, where contact sprays reach it. A contact spray works where it lands; the plant does not absorb it. Treatment gets harder as the infection spreads and as flowering moves on.
Translator’s note: In Thailand, you may use only pesticides registered with the Department of Agriculture (กรมวิชาการเกษตร), and only according to the label. The brand names in this section are registrations listed in the US handbook [2]. A US registration is not a Thai registration. Home remedies are not registered pesticide products either. That includes milk, kitchen-grade bicarbonate, home-made neem mixes and pharmacy hydrogen peroxide. Do not assume that a home remedy is accepted under GACP.
If your farm works under GACP (Good Agricultural and Collection Practice, the farm standard for medicinal plants), ask your auditor before you spray anything. Record every spray: product, rate, date and the plants treated. The Thai rules are explained in What You May Legally Spray Under Thai GACP.
Treatments in order, least toxic first
1. Potassium bicarbonate (KHCO₃)
- Use 3–5 g/L, plus 2 mL/L liquid soap so the spray spreads over the leaf.
- It raises the pH of the leaf surface and kills the mildew on contact.
- Registered bicarbonate products for hemp powdery mildew in the US handbook are Bi-Carb, Carb-O-Nator, Kaligreen, MilStop SP and Vacres. Apply them every 5–14 days, covering every leaf surface thoroughly [2].
- You can use it into mid-flower. Rinse off the residue at harvest.
- Before you buy, ask the distributor to show you the Thai registration.
2. Milk spray
- Mix 1 part milk with 9 parts water. Spray it while the lights are on.
- This is an old garden remedy. Use it to support airflow and cleaning, not as your main treatment.
- There are no published figures on how well it controls powdery mildew on cannabis specifically. For that reason, no figure is given here.
3. Neem oil
- Use 5 mL/L, plus 2 mL/L soap so the oil mixes with the water.
- Spray it at lights-off. Neem combined with light causes phytotoxicity, meaning the product itself burns the plant tissue.
- It works on the fungal mat on the surface. It does not get inside the leaf.
- Do not use it after about week 3 of flowering. It can change the taste of the buds and leave residue on them.
- Ask for the Thai registration before you buy a neem product.
4. Sulfur, and the temperature limit that decides whether you can use it at all
- Use a micronised (very finely ground) sulfur spray or a sulfur vaporiser, a heated burner that spreads sulfur through the room air.
- Use it as a protectant before infection, every 7 days [2].
- Apply it when the air is 18–29 °C (65–85 °F). Above 35 °C (95 °F), sulfur burns the plant [2].
- Never mix sulfur with any oil-based product, such as neem or horticultural oil. Never apply sulfur within 2 weeks of one either. The combination causes severe plant burn. If you used neem, wait 2 weeks before sulfur, and the other way round.
- Do not use sulfur during flowering, because it leaves residue on the buds.
5. Hydrogen peroxide (H₂O₂)
- Dilute 3% hydrogen peroxide with water and spray it directly onto the mildew colonies. It oxidises (breaks down) the fungus on contact.
- It leaves no lasting protection, so you need to repeat it often.
Translator’s note: Before you treat a whole room with any product, spray a few leaves first. Check them the next day for burn spots. Many Thai grow rooms and greenhouses run hot, and heat raises the risk of burn. A one-day test costs a few leaves instead of a whole crop.
What not to do
- Do not ignore it. Powdery mildew does not go away on its own.
- Do not raise humidity in the hope that moisture will help. High humidity is what you control for Botrytis, and powdery mildew does well in moderate humidity.
- Do not use systemic fungicides on cannabis meant for consumption. These are fungicides the plant absorbs and carries inside its tissue, such as myclobutanil (Eagle 20) or propiconazole. They work, but they are banned in regulated cannabis markets. Product treated with them fails testing and cannot be sold. They are also a health concern.
Translator’s note: Do not assume that a fungicide registered in Thailand for another crop may be sprayed on cannabis. Check its label and the Thai rules in topic 1455 first. How residues are tested, and why they cannot be removed afterwards, is covered in Pesticide Residues on Cannabis.
2. Mildew in Late Flower: Choosing What to Save
If mildew reaches the buds in the final 2–3 weeks, you have fewer options:
- Harvest now. Do this if most trichomes are already milky, meaning cloudy white rather than clear. Reading trichomes is explained in Harvest Timing. A slightly early, clean crop is worth more than a mature, contaminated one.
- Wash the buds. After harvest, wash the buds in a weak peroxide bath or a lemon and bicarbonate bath. This removes spores from the surface. It does not repair damaged tissue, and heavily infected buds may still fail the lab’s limits.
- Throw away the infected flowers. Process the clean rest of the crop as normal.
Translator’s note: Make this decision before late flower, not on the day you find mildew. Write down in advance at what trichome stage you will harvest early, and what share of infected buds you will throw away. If you wash buds, plan where they will dry. Thailand’s humid air slows drying, and slow drying raises the risk of mould. See Drying Cannabis at 85–95% Ambient Humidity and Mycotoxins on Cannabis Flower.
3. Mildew and Lab Testing: Why a Batch Gets Rejected
In regulated markets, where cannabis is sold under government rules, labs test flower for three things:
- total yeast and mould counts;
- specific disease-causing microbes;
- mycotoxins, which are poisons made by some moulds.
The mycotoxins with limits for cannabis in some markets are aflatoxins and ochratoxin A. The mould groups (genera) of main health concern are Aspergillus, Penicillium, Fusarium and Mucor [4].
Powdery mildew does not make these toxins. Its cost at testing is indirect: mould you can see on the buds can get a batch rejected (see below).
Pass and fail limits differ from country to country. Check the limits that apply to your own licence. Do not rely on a figure quoted in a grower article, including this one. How to read a lab report is covered in How to Read a Cannabis Certificate of Analysis.
For GACP and GMP operations (GMP is Good Manufacturing Practice, the standard for processing medicines), the lab number is not the only test. Visible mould is enough reason to reject a batch, whatever the numbers say. Preventing mildew therefore decides whether you can sell the batch at all, not only how much you harvest.
Translator’s note: Ask the lab you use which limit its report is measured against. If you export, also ask for the destination country’s limit. Before harvest, check every plant for visible mildew. Set aside any plant with spots before it can mix with the clean batch.
4. When to Watch Most Closely
Indoors, the risk follows the crop cycle, not the calendar. Powdery mildew risk is highest:
- during vegetative growth, when the canopy is dense and not yet trained;
- during the stretch in early flowering, when plants grow taller fast and the canopy fills up quickly (see Stretch);
- during the final 3 weeks, when lighter feeding and lower temperatures weaken the plant’s defences.
Outdoors and in greenhouses, the riskier periods have moderate temperatures and swinging humidity. Very hot or very cold periods are less risky.
What to do: Check lower and inner leaves with a loupe more often during these three indoor stages.
Translator’s note: Keep a simple calendar of the date you first see mildew each year. After one year, you will know which months need extra checks on your site.
5. Powdery Mildew in a Tropical Climate
Translator’s note: This section applies the points above to Thailand and Southeast Asia. Three things change here, and one of them removes a treatment that most guides assume you have.
- For much of the year, sulfur may be unusable. The guidance is to apply it at 18–29 °C and never above 35 °C, where it burns the plant [2]. In a Thai greenhouse without air-conditioning, or outdoors in the hot season, temperatures often rise above that range. Build your programme on airflow, canopy work, silicon and bicarbonates instead. Treat sulfur as an option for the cool season or air-conditioned rooms only. What to do: Before planning sulfur, put a min/max thermometer at canopy height for a week. If the maximum passes 29 °C, sulfur does not fit your conditions. Greenhouse set-ups are covered in Greenhouse Growing.
- High outdoor humidity does not protect you. Powdery mildew does not need water on the leaf. Many indoor rooms use dehumidifiers to hold 50–60% humidity against Botrytis. That range is comfortable for powdery mildew. A room with little Botrytis is not automatically a room with little powdery mildew. Dehumidifier sizing for Thai rooms is covered in Dehumidifier and Air-Conditioner Sizing.
- Genetics matter more here than in cooler climates. Sulfur is limited, and your humidity setting is already used against Botrytis. That makes the PM1 question to your seed supplier [1] a real buying rule, not a detail. How to check a vendor is covered in How to Vet a Thai Seed Vendor.
- Power cuts. When fans stop, the air goes still and spores settle. When dehumidifiers stop, Botrytis risk rises too. Put at least the circulation fans on a battery backup (UPS) or generator. After a long power cut, check lower leaves with a loupe for the next few days.
- Rainy season outdoors. Wet leaves slow powdery mildew but help Botrytis. Keep checking for both diseases through the wet season instead of assuming one is gone.
Have you dealt with powdery mildew in Thailand or Southeast Asia? Reply below with the treatment that worked for you, such as bicarbonate, sulfur or something else. Please include your room temperature and humidity so other growers can compare. Also name any cultivars that stayed clean in your conditions.
References
- Mihalyov, P.D., & Garfinkel, A.R. (2021). Discovery and genetic mapping of PM1, a powdery mildew resistance gene in Cannabis sativa L. Frontiers in Agronomy, 3, 720215.
- Pacific Northwest Plant Disease Management Handbook. Hemp (Cannabis sativa) — Powdery Mildew. Oregon State University Extension.
- Dixon, E., Leonberger, K., Amsden, B., Szarka, D., Munir, M., Payee, W., Datnoff, L., Tubana, B., & Gauthier, N. (2022). Suppression of hemp powdery mildew using root-applied silicon. Plant Health Progress, 23(3), 260–264.
- Gwinn, K.D., Leung, M.C.K., Stephens, A.B., & Punja, Z.K. (2023). Fungal and mycotoxin contaminants in cannabis and hemp flowers: implications for consumer health and directions for further research. Frontiers in Microbiology, 14, 1278189.
This article is part of the Asiannabis Community series on pest and disease management. The content is for education in places where growing cannabis is legally permitted.