Septoria leaf spot is a fungal disease of cannabis leaves. It starts on the oldest leaves at the bottom of the plant and slowly moves upward. Spotted leaves turn yellow and drop, so the plant loses the leaf area it needs to feed itself. This article shows you how to recognise it, why wet weather makes it worse, and what to do about it in Thai and Southeast Asian conditions.
The first signs are small, roughly round spots with a pale centre and a darker rim. As more spots appear, they join together. The leaf then yellows and falls off. Growers usually notice the disease late, because the lower, inner part of the plant is the part they look at least.
What you can do now: once a week, bend the plant aside and look at the lowest, oldest leaves on the inside of the plant. That is where this disease shows up first.
This article draws on fact sheets from several American university extension services. These are the farm-advice departments of US state universities. They write free guides for local growers based on field trials and diagnostic lab cases. The main sources are the University of Kentucky, Rutgers University in New Jersey, and Utah State University. They are listed at the end.
What causes it on cannabis
The fungus is Septoria cannabis. The extension fact sheets describe it on field-grown hemp. Hemp and high-THC cannabis are the same species, Cannabis sativa.
Septoria fungi are usually tied to one host plant. The University of Kentucky fact sheet gives two examples: the Septoria on tomato and the Septoria on wheat are different species. The same sheet says Septoria cannabis is only known to cause disease in hemp.
This matters if you grow vegetables next to cannabis. The Septoria spots on your tomatoes are a different fungus, Septoria lycopersici. It is not a known source of infection for cannabis. But both fungi like the same wet weather. Spots on your tomatoes tell you that conditions are right for Septoria on your cannabis too.
The disease is common in the field. The Kentucky sheet calls Septoria the most commonly reported leaf spot of field-grown hemp in Kentucky. Rutgers lists it as one of the most frequently seen leaf spots in its New Jersey hemp trials.
What you can do now: if your tomatoes show Septoria spots, treat it as a weather warning. Check the lower leaves of your cannabis the same week.
Symptoms, step by step
The disease changes as it develops. Look for these signs in this order:
- Small, irregular brown spots with a bright yellow edge. They appear first on the lower leaves and inside the plant.
- The spots grow to about 6 mm (roughly a quarter inch). Their centres turn grey to white.
- Tiny black dots may be visible inside the spots. Kentucky compares them to flakes of ground pepper. Each dot is a pycnidium (plural pycnidia), a small fruiting body where the fungus makes its spores.
- The yellow rings around the spots spread and run together. Whole leaves then yellow quickly and drop.
- In severe cases early in the season, plants can stay small and lose vigour.
What you can do now: when you find a suspicious spot, look at it closely for the black pepper-like dots. They are the clearest sign of Septoria.
How to tell it from other leaf spots and deficiencies
Many things put spots on cannabis leaves. Utah State University names nutrient deficiencies and insect feeding as the two most common look-alikes.
Two simple checks settle most cases. First, look at the spot through a 10x hand lens, a small magnifying glass that enlarges ten times. Second, note where on the plant the problem started.
| What you see | Most likely cause | Key difference |
|---|---|---|
| Brown spots with bright yellow rings, grey-white centres and black specks inside; starts low and inside the plant | Septoria leaf spot | Black specks (pycnidia) inside the spot; the yellow ring spreads outward |
| Tiny dark spots with tan centres and dark brown edges | Bipolaris leaf spot (also called hemp leaf spot) | Rutgers describes tiny, dark spots of dead tissue from the start |
| Round to oval grey spots with dark brown edges | Cercospora leaf spot | Utah State describes grey spots with dark edges |
| Brown spots with rings inside, like a target (seen on tomato) | Early blight, caused by the fungus Alternaria solani | Ringed target pattern; Septoria spots have no rings |
| Yellowing between the veins or along the leaf edges, the same on many leaves, with no separate spots or specks | Nutrient problem | No black specks and no spreading ring around a spot |
| Fine pale speckles, sometimes with insects or webbing on the underside | Insect or mite feeding | Turn the leaf over and look underneath with the lens |
The black specks are the single most useful clue. If you cannot see them with a lens, a plant diagnostic lab can confirm the disease. The lab looks at the spores under a microscope.
Translator’s note: hand lenses are cheap and easy to buy in Thailand. A clip-on macro lens for a phone camera also works, and it lets you post a sharp photo when you ask for help. For sending a sample, see the forum guide on getting a sick plant diagnosed before you treat it. The Department of Agriculture (กรมวิชาการเกษตร) runs a plant clinic for disease diagnosis, and Kasetsart University’s Department of Plant Pathology has a plant disease clinic. We could not confirm whether they accept cannabis or hemp leaf samples, so contact them first and ask.
What you can do now: before you change anything, look at three spotted leaves under the lens and note whether the specks are there. Then decide which row of the table fits.
Life cycle: why wet weather drives it
Knowing how the fungus spreads tells you where to cut it off.
The pycnidia produce large numbers of spores called conidia. When the leaf is wet, the spores ooze out. Splashing water carries them a short distance, for example from a lower leaf to the one above. Rain blown by the wind carries them further.
According to Kentucky, about 7 to 10 days pass between infection and visible spots. When humidity stays high, each cycle runs faster and makes more spores. As the plants grow and the leaves close over, the air inside the plant stays more humid. The disease then gets worse. That is why it often takes off suddenly in the middle of the season.
Between crops, the fungus survives on infected plants and on dead leaves and stems. Kentucky reports these survival times for the pycnidia:
- In cool, dry plant debris: up to nine months.
- In hot weather: survival drops sharply.
- Buried in the soil: less than a month.
No other host plant has been found for Septoria cannabis. Still, both Kentucky and Rutgers suggest that weeds may carry the fungus from one season to the next. Plant pathologists call this a “green bridge”: living plants that keep a disease alive while the field is empty.
Rutgers sums up the climate risk plainly. In regions with wet, humid summers, these leaf diseases will keep coming back. How bad they get depends on two things. One is how long the leaves stay wet from rain or dew. The other is how easily the cultivar catches the disease.
Translator’s note: much of Thailand and Southeast Asia has exactly the climate Rutgers describes. Through the rainy season, leaves outdoors can stay wet for hours from rain, and again overnight from dew. Outdoor and greenhouse crops in that season face the highest risk. The heat may work in your favour between crops, because Kentucky found survival drops sharply in hot weather. But that was measured in Kentucky, not in Thailand. Debris kept dry under a roof in the cooler months may last longer, so do not rely on the heat alone.
What you can do now: during the rainy season, write down how many hours the lower leaves stay wet after rain or watering. If they are still wet the next morning, the plant has the conditions Septoria needs. Open up spacing and airflow first.
How much damage it does
Septoria looks alarming, and in bad years it strips a lot of leaves. Kentucky reports that under extreme conditions, plants can lose 50% to 90% of the leaves in the lower and middle canopy.
The same sheet notes that no direct loss of biomass or cannabinoid yield has been seen. However, processors have sometimes rejected shipments of affected hemp. Utah State rates the threat as very low for the same reason.
In practice, Septoria is mainly a problem of quality and plant vigour, not a crop killer. It is also a sign that the leaves are staying wet for too long.
Translator’s note: a buyer who sees spotted, yellow leaf in a batch may judge the whole batch as poorly grown, even if the cannabinoid content is fine. If you sell to a processor or an exporter, ask in advance whether visible leaf spot is a reason to reject a lot.
What you can do now: do not panic or throw out plants because of Septoria. Treat it as a warning about humidity and airflow, and fix those first.
Prevention and integrated pest management
Integrated pest management means combining several methods in a set order: prevention first, then regular checking, then removal by hand, and products only as a last step. For Septoria, this order works well.
1. Change the growing conditions first. Both Kentucky and Rutgers put cultural measures like these first:
- Space plants and rows wider. Leaves then dry quickly after rain or watering.
- Thin out crowded plants and remove the lowest, shaded growth that stays wet.
- Cover the soil surface with mulch. It cuts the splash of soil and water up onto the leaves.
- After harvest, remove infected plant debris. In a field, you can instead bury it deeply by ploughing. Move the crop to a different site in the next season if you can.
- Choose cultivars with a record of catching the disease less easily. Rutgers points to a list from the University of Tennessee Extension that rates hemp cultivars by their leaf spot response.
- Water at the base of the plant, early in the day. The leaves should not stay wet overnight.
Translator’s note: the Tennessee list covers hemp cultivars sold in the United States. Most of them are not grown in Thailand, and we found no similar Thai list. The workable alternative is your own records. Note which cultivars spotted first and worst in each season, and share it in the Strain Reviews board. Outdoors in the rainy season, a simple clear plastic roof over the rows keeps rain off the leaves. Rice straw is a cheap, widely available mulch. If you compost infected debris, keep the pile far from next season’s beds.
2. Check the plants on a schedule. Utah State suggests starting about a month after planting. Focus on three places: the older leaves inside the plant, crowded parts of the planting, and plants that have been wetted again and again.
3. Remove early spots by hand. Pick off the first spotted lower leaves and take them out of the growing area. This lowers the number of spores that can reach new leaves. Do it when the leaves are dry, so you do not spread spores on wet hands.
4. Products, as a last step. Pesticide registration is local, and labels for use on cannabis are strict.
- When Kentucky wrote its fact sheet, no conventional fungicides were labelled for hemp.
- Biological products based on Bacillus bacteria had shown suppression of the disease only in lab and greenhouse tests, not in the field.
- Utah State lists a few products registered in the state of Utah. Examples are sulfur, hydrogen peroxide, Bacillus strains and potassium soaps. Several come with limits, such as greenhouse use only, outdoor use only, or use only before final transplant.
Treat any such list as valid only where it was written. Nothing should go onto flowers unless it is clearly allowed for that stage. And no spray repairs leaves that are already spotted.
Translator’s note: the Utah list does not apply in Thailand. In Thailand, use only products registered with the Department of Agriculture (กรมวิชาการเกษตร), and use them exactly as the label says. Before you buy anything, read the forum guide on what you may legally spray on cannabis under Thai GACP. If your farm is certified or audited, ask your auditor too.
What you can do now: set a weekly reminder to check the lower inside leaves, starting one month after planting. Keep a bag ready to collect every spotted leaf you remove.
Common mistakes
- Calling it a deficiency and feeding more. Nutrient problems do not cause spots with black specks inside. Check with a lens before you change the feed.
- Stripping healthy upper leaves in a panic. The disease starts low. Improve airflow and remove the infected lower leaves. The upper leaves are the ones feeding the plant.
- Leaving fallen leaves on the ground. Fallen leaves carry pycnidia. Collect them.
- Watering from above in the evening. Leaves that stay wet for a long time are exactly what the fungus needs.
- Composting infected debris next to next season’s plot. The fungus survives a long time in cool, dry debris. Bury it, or take it off the site.
If the spots do not match any pattern above, send a sample to a plant diagnostic lab before you spend money on products. Before applying anything, check the current local rules on which products may be used on cannabis.
If you are unsure what you are looking at, post a sharp close-up of the spots and a photo of the whole plant below this topic. Say where on the plant the spots started and how often the leaves get wet.
References
- Gauthier, N., Leonberger, K. and Rahnama, M. Septoria Leaf Spot of Field Hemp (PPFS-AG-H-01). University of Kentucky Cooperative Extension, September 2021. https://plantpathology.mgcafe.uky.edu/files/ppfs-ag-h-01.pdf
- Rajmohan, N. et al. Emerging Diseases of Hemp in New Jersey (Cooperative Extension Bulletin E374). Rutgers New Jersey Agricultural Experiment Station, March 2023. Emerging Diseases of Hemp in New Jersey | Rutgers Cooperative Extension
- Utah State University Extension, Utah Pests IPM. Leaf Spot (hemp). No date given on the page. Leaf Spot | USU
- University of Tennessee Extension. Hemp Variety Trials in Tennessee 2020 (W1016), as cited by Rutgers E374. The link given by Rutgers (https://extension.tennessee.edu/publications/Documents/W1016.pdf) no longer opens.