Powdery mildew is a fungus that shows up as white, powdery spots on cannabis leaves. It can appear overnight and cover a plant within days. This part explains which fungus causes it and how it infects a leaf. It also shows how to tell it apart from trichomes under a loupe, and how to set up a grow where it struggles to take hold.
Part 2 explains what to spray once you find mildew and what to do when it reaches the buds in late flower. It also covers how mildew affects lab testing and what changes in Thailand’s heat and humidity.
1. Which Fungus Causes Powdery Mildew
Powdery mildew on cannabis comes from a group of fungi called the order Erysiphales. Older articles and labels still use an outdated name for the cannabis species: Golovinomyces cichoracearum. Early cannabis reports used that name in a broad sense (“sensu lato”), and that identification has since been replaced.
The current name is Golovinomyces ambrosiae. Researchers have shown that it is the same species as G. spadiceus [1]. Older cannabis studies that say G. cichoracearum are describing the same disease under the old name.
What to do: When you search for research or read a product label, look for all three names: G. ambrosiae, G. spadiceus and G. cichoracearum.
A second species that comes from hops
Podosphaera macularis causes powdery mildew on hops, the climbing plant used to flavour beer. It can also infect hemp. Researchers first showed this by putting its spores on hemp plants on purpose. Natural infections in hemp fields were then confirmed in 2020 in Oregon, a state in the north-west of the United States [2].
That report comes from the Pacific Northwest Plant Disease Management Handbook. This is a crop-disease guide from Oregon State University Extension, the farm advisory service of a US state university.
Hops are therefore a real source of infection, not just a theoretical one. The risk applies if your grow is near hops. It also applies if you share tools or clothing with someone who works with hops.
Translator’s note: Powdery mildew spores travel on clothes and tools (see section 2). If you or your staff visit other farms that grow hops, hemp or cannabis, change clothes and clean your tools before you enter your own grow. Keep one set of work clothes and tools that never leaves your site. This keeps spores from other farms out of your rooms.
It only feeds on living plants
Powdery mildew is an “obligate biotroph”. In plain words, it can only feed on living cells, and it dies without a living host plant. Botrytis (grey mould, also called bud rot) works the opposite way: it feeds on dead tissue.
This difference changes how you control each disease. Botrytis grows on dead leaves lying on the floor, but powdery mildew cannot. Instead, powdery mildew spreads fast across the surface of living leaves. It can cover a plant in a few days.
What to do: Clearing dead leaves off the floor starves Botrytis, but it does not starve powdery mildew. For powdery mildew, inspect the green leaves still on the plant, starting with the lower and inner ones.
2. How Powdery Mildew Infects a Leaf
Six steps from one spore to thousands
- A spore lands on the leaf. These spores are called conidia. Unlike Botrytis spores, they do not need water droplets or a wet film on the leaf to start growing. Moderate air humidity on a dry leaf is enough.
- The spore sprouts and breaks in. It grows a short tube along the leaf surface (a germ tube). The tube then forms a small pressing pad (an appressorium) that pierces the leaf’s outer layer of cells.
- It builds feeding organs inside the cell. These are called haustoria. They draw nutrients from the living cell without killing it straight away. The leaf therefore keeps feeding the fungus day after day.
- It spreads across the surface. Fine threads (hyphae) grow over the leaf as a white network. As they spread, they push new haustoria into new cells. This surface growth is the “powder” you see: whitish, powdery spots on the upper side of the leaf [2].
- It makes new spores. The fungal mat (the mycelium) raises short stalks. Each stalk carries a chain of new spores.
- The spores spread. The slightest air movement releases them. They also travel on clothing and tools, and through the ventilation system.
Steps 1 to 5 all happen on the leaf surface. That is why you can see an infection early with a loupe. It is also why a spray that touches the fungus can reach it (see Part 2).
Translator’s note: Step 6 means people carry spores from room to room. Visit healthy rooms first and rooms with mildew last each day. Wash your hands and change gloves between rooms. This stops you from spreading the disease yourself.
Conditions that favour powdery mildew
| Condition | Effect on mildew risk |
|---|---|
| Moderate humidity, leaf surface dry (no water droplets) | Best conditions for spores to sprout |
| Poor air movement | Spores settle and build up |
| Crowded canopy | Pockets of still air form between plants |
| Low light intensity | The plant’s defences are weaker |
| Too much nitrogen | Soft, lush growth is easier to infect |
| Stressed plants (drought, heat, root problems) | The plant responds poorly to attack |
One humidity setting cannot stop both diseases
Powdery mildew does well in the moderate humidity that many growers consider safe. Very wet leaf surfaces actually disrupt its infection. Botrytis is the reverse: it needs high humidity and water on the plant tissue.
So you cannot set the room humidity to protect against both diseases at the same time. Set it against the disease that is more likely in your grow. Your genetics, your climate and the stage of flowering decide which one that is. Then protect against the other disease with airflow, cleaning and canopy work.
Translator’s note: To make this choice with real data, keep a short log for every run. Write down which disease appeared, in which week, and the room temperature and humidity at the time. After two or three runs, you will see which disease costs you more. Set humidity against that one. Room climate basics are covered in Grow Room Climate Control.
3. What Powdery Mildew Looks Like
Early stage
- Small white or greyish-white round spots appear on the upper side of leaves. They usually start on lower and inner leaves, where air moves least.
- The spots look powdery and wipe off or smear under a finger. Trichomes do not.
- At first, the leaf under the spot looks normal.
Moderate infection
- White patches grow and join together across the leaf.
- Lower leaves turn yellow under the coating.
- The mildew moves onto stems, leaf stalks (petioles) and calyxes. Calyxes are the small pods that make up a cannabis bud.
Severe infection
- Whole leaves are coated. They curl, turn brown and die.
- Mildew grows on the flowers. This is the most damaging stage.
- Infected buds develop a musty, stale smell that stays after drying.
What to do: Check the lower and inner leaves first, because the early spots appear there.
Trichomes or powdery mildew: how to check with a loupe
Trichomes are the tiny resin glands that cover cannabis flowers and sugar leaves. Sugar leaves are the small, resin-coated leaves that grow out of the buds. The name refers to the frosty resin, not to the leaf’s colour. Trichomes make the bud look frosty, so new growers sometimes mistake them for mildew. The opposite mistake also happens.
A loupe is a small hand magnifier. Under a loupe, a trichome looks like a tiny mushroom: a stalk with a round head on top. Powdery mildew looks different. It is a flat, web-like net of threads lying on the leaf surface, with no heads.
Check a suspicious white spot in this order:
- Where is it? Mildew starts on the upper side of leaves, with lower leaves first. Trichomes sit on flowers and sugar leaves.
- Does it wipe off? Rub gently with a fingertip. Mildew is dry and dusty and comes off. Trichomes are sticky and stay put.
- What does the loupe show? Mushroom shapes mean trichomes. A flat web means mildew.
- Does it sparkle? Trichomes glitter under light. Mildew is dull, matte white.
- How does it smell? Mildew smells musty and stale. Trichomes smell of terpenes, the plant’s aroma compounds.
| Feature | Powdery mildew | Trichomes |
|---|---|---|
| Texture | Dusty, dry, wipes off | Sticky, resinous |
| Under a loupe | Flat, web-like net on the surface | Mushroom-shaped stalks with round heads |
| Location | Upper side of leaves, lower leaves first | Flowers and sugar leaves |
| Shine | Dull, matte white | Glitters under light |
| Smell | Musty, stale | Strong, pleasant terpene aroma |
Translator’s note: Wash your hands after a wipe test before you touch another plant, because spores stick to fingers. If you are still unsure, post a sharp close-up photo in Plant Infirmary. Other growers can then help you confirm what it is before you spray anything.
4. Prevention: Make Your Grow Hard for Mildew
4.1 Airflow
Spores need still air to settle and take hold. Constant, gentle air movement across the leaf surfaces is the most effective single way to prevent powdery mildew.
- Move air across and through the canopy, not just over the top.
- Leaves should flutter all the time. They should not whip back and forth.
- Pay most attention to the lower canopy and the inside of each plant.
Translator’s note: During a power cut, fans stop and the air goes still. Those are exactly the conditions spores need to settle. If power cuts are common where you grow, put at least the circulation fans on a battery backup (an uninterruptible power supply, or UPS) or a generator. After any long power cut, check the lower and inner leaves with a loupe over the following days.
4.2 Canopy management
Each step below removes pockets of still air where spores settle.
- Remove inner fan leaves (the large leaves) that block airflow between branches. This is part of defoliation.
- Do not crowd plants. Air must move around the whole canopy.
- “Lollipop” the plants. Strip the weak lower branches and leaves that sit in still air. The lower stem becomes bare and growth stays at the top, like a lollipop.
- Train plants into a flat, open canopy, not a dense one with several layers. Training methods are covered in LST, HST & Supercropping.
4.3 Ultraviolet light
Powdery mildew is sensitive to ultraviolet (UV) light. LED lights that include UV-A and UV-B may help reduce mildew pressure, but this has not been well tested on cannabis.
Some growers also use UV-C germicidal lamps. These lamps give off short-wave UV that kills microbes. Growers switch them on for short periods during the dark period to kill spores on exposed surfaces.
Caution: UV-C harms eyes and skin. Never run it while people or animals are in the room.
Outdoor plants get UV from sunlight. This may be one reason why outdoor crops often have lower mildew pressure than indoor crops grown under lights without UV.
Translator’s note: Read the specification sheet of your LED lights to see whether they include UV-A and UV-B. If you add UV-C, put it on a timer and keep the room locked while it runs, so nobody walks in by mistake. UV-C only kills spores on surfaces it shines on. Shaded leaves and inner branches stay unprotected, so UV-C does not replace airflow.
4.4 Silicon
Silicon has direct evidence in hemp. In a trial published in 2022 by Dixon and colleagues [3], silicon was applied to the roots of hemp plants. The plants grew in a peat-based potting mix with no field soil.
The researchers found a straight-line link between silicon and mildew. The more silicon that built up in the leaf tissue, the smaller the share of each leaf that mildew covered. The effect also depended on the leaf’s position on the plant. In the upper canopy, a rate of 300 kg per hectare reduced the disease significantly. The middle canopy needed 600 kg per hectare for similar control [3].
Two points to keep in mind before you use these results:
- It was a root application. Silicon works by building up inside the plant tissue, not by coating the leaf. Spraying silicon on leaves is a different treatment, and this study did not test it.
- The indoor rate does not come from this trial. Many indoor growers use 50–100 ppm (parts per million) of silicon in the nutrient solution. That is standard practice, not a rate from the study. The per-hectare field rates above cannot be converted directly into a fertigation tank, the tank that mixes fertiliser into irrigation water.
Products sold for this purpose include Rhino Skin (Advanced Nutrients), Silica Blast (Botanicare) and Power Si. Silicon prevents disease. It does not rescue a sick plant. Apply it through vegetative growth and into early flower.
Translator’s note: To check whether these products are sold in Thailand, ask the official distributor in Thailand. Also ask to see each product’s Thai fertiliser registration. How to check a fertiliser’s Thai registration is explained in Verifying a Fertiliser Is Legally in Thailand. Start small. For one run, give silicon at the label rate to only part of your plants. Then count spotted leaves on treated and untreated plants at the same canopy height. This tells you whether silicon helps in your own room before you buy it for the whole farm.
4.5 Biological products that prevent infection
The Oregon handbook [2] lists products registered against hemp powdery mildew in the Pacific Northwest of the United States. They are Actinovate AG, Double Nickel LC, Fungisei, LifeGard WG and Serenade Opti. The handbook itself adds that their effect in that region is unknown. Registration means a product may be sold. It does not prove that the product works in your room.
These products work in different ways:
- Bacillus subtilis and Bacillus amyloliquefaciens are beneficial bacteria. They colonise the leaf surface and take up the space the fungus needs. These microbes are explained in Mycorrhizae, Trichoderma, Bacillus….
- Trichoderma is a beneficial fungus that lives in the root zone. It prepares the whole plant’s defences (“induced systemic resistance”), so the plant reacts faster when the fungus attacks.
All of them are preventive. Apply them before infection. None of them cures colonies that are already established.
Translator’s note: The registrations above are US registrations. In Thailand, you may use only pesticides registered with the Department of Agriculture (กรมวิชาการเกษตร), and only according to the label. Before you buy, ask the official distributor in Thailand to show you the Thai registration. The Thai rules are explained in What You May Legally Spray Under Thai GACP. If you find a registered product, start applying it in vegetative growth on one batch, before any mildew appears. Record the results so you can see whether it is worth using across the farm.
4.6 Resistant genetics and the PM1 gene
Resistance to powdery mildew is not only “more” or “less”. Researchers have found and mapped a resistance gene in Cannabis sativa called PM1 [1]. It sits at a single location on the chromosomes (a locus). In the study, it gave complete resistance to one isolate of Golovinomyces ambrosiae from Washington State, in the US. The authors note that it may not work against isolates from other regions. The gene is dominant, which means a plant needs only one copy, from one parent, to be resistant.
This matters when you buy seed or clones. Breeders can easily move a single dominant gene into their lines and select for it. So ask your genetics supplier a precise question, not a general one. Do not ask “Is this line mildew-resistant?” Ask: “Has this line been screened against Golovinomyces ambrosiae, and do you select for PM1?” A vague answer sounds very different from a real one.
If mildew comes back run after run, resistant genetics are still the cheapest long-term fix.
Translator’s note: Ask for written proof of PM1 screening. A vendor who resells seed may not know the answer, so ask them to pass the question to the breeder. How to check a vendor is covered in How to Vet a Thai Seed Vendor.
You can also test cultivars yourself. Grow a small batch of several cultivars in the same room under the same conditions. Record which ones show spots first and how fast the spots spread. Then keep a mother plant of the cleanest one (see Phenotype Hunting). This shows how a plant performs in your conditions, but it does not prove that the plant carries PM1.
Buying clones from another farm brings its own risks. Clones can carry powdery mildew spores, pests or hop latent viroid (see Hop Latent Viroid), and they may be the wrong cultivar. Check every clone’s leaves with a loupe. Keep new clones away from your main room until you are sure they are clean.
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.
Continued in Part 2: Powdery Mildew on Cannabis: Treatment, Late-Flower Decisions, Lab Testing and the Tropical Climate (Part 2 of 2)