Circular yellow-brown spots with dark borders and black center dots
🌿Image coming soon
Leaf septoria — also called yellow leaf spot — is a fungal disease caused by Septoria cannabis that primarily hits outdoor plants during mild, humid weather (the sweet spot is 15–27°C / 59–80°F). It's one of the most common foliar diseases you'll see on outdoor cannabis. Good news: of all the things on this list, septoria is probably the least scary. It's unsightly and reduces vigor through leaf loss, but it's rarely a serious threat to overall plant health if you catch it before it climbs into the upper canopy.
The fungus spreads through water splash from rain or overhead irrigation, carrying spores from infected soil or debris onto the lower leaves first. The telltale sign is circular spots with dark borders and — critically — tiny black dots (pycnidia) in the center of mature spots. Those black dots are the fungal fruiting bodies, and they're the definitive ID. The most common diagnostic mix-up is with deficiency: Ca def spots appear on new upper growth, while septoria always starts on the oldest lower leaves near the soil. Spores overwinter in plant debris, so end-of-season cleanup is essential to break the cycle.
Solutions
✓Remove affected leaves immediately — bag and dispose of them, never compost infected material
✓Improve air circulation around the lower canopy by pruning dense lower growth and spacing plants adequately
✓Water at the base of the plant only — avoid overhead irrigation or anything that splashes soil onto leaves
✓Apply copper-based fungicide as a preventive or at the first sign of spots
✓Neem oil can help slow the spread early on, but avoid applying it near buds during flowering
✓Mulch around the base to create a barrier between soil spores and lower leaves
✓At the end of the season, remove ALL fallen leaves and plant debris from the grow area — spores overwinter in this material and will reinfect next year's crop
✓Check pH — optimal 6.3-6.8 in soil. Healthy plants resist fungal infection better than stressed ones
Plant Overview
How it appears across the plant
Septoria climbs the plant from the bottom up, splashed onto progressively higher leaves by rain, irrigation, or wind. From a distance, the affected leaves stand out as marked with brown circular spots against the surrounding green . The lowest leaves are hit first and most severely; the upper canopy stays clean until late in the infection.
Early. Small yellow or light brown spots appear on the lowest leaves closest to the soil. The spots are circular, starting around 1–3mm. Each has a distinct darker brown or purple border surrounding a flat, papery center. Spots stay isolated at first — the rest of the plant looks completely healthy. A loupe reveals tiny black dots in the center of mature spots (pycnidia) — that's the definitive septoria ID. Yellowing may appear in a halo around the spots but doesn't spread between like a nutrient issue would.
Mid. Spots now visible on dozens of leaves rather than just a handful. Individual spots have grown to 5–8mm with sharp dark borders. Pycnidia (black dots in spot centers) clearly visible to the naked eye on the largest spots. The infection has climbed from the bottom leaves into the lower-mid canopy. First leaves with merged spots show entire sections turning yellow-brown.
Late. Spots merge into larger irregular brown patches that kill whole leaf sections. The infection climbs upward to middle and upper leaves via splash and wind. Entire leaves yellow around the damaged areas, turn crispy, and drop off. Significant defoliation reduces plant vigor and photosynthetic capacity. Black pycnidia are clearly visible to the naked eye in older lesions. The infection may reach fan leaves near bud sites during — remove these promptly to protect the buds. Heavy leaf loss exposes buds to light stress and reduces the plant's ability to finish strong.
Early — how it first appears on a leaf
On a single lower leaf in the early stage, look for small yellow or light brown circular spots, each around 1–3mm across. Each spot has a distinct darker brown or purple border surrounding a flat, papery center. With a loupe, you can see tiny black dots in the center of mature spots — those are pycnidia, the fungal fruiting bodies, and they're the definitive septoria ID against nutrient deficiencies or other leaf-spot diseases. The yellowing may form a faint halo around each spot but doesn't spread between veins the way deficiency would. The rest of the looks green.
Mid — when spots multiply and grow
By the mid stage, that same leaf carries dozens of spots that have grown to 5–8mm. The dark borders are sharp, and the centers are clearly papery and flat. The pycnidia in the spot centers are now visible to the naked eye as small black dots. Where multiple spots have grown close together, they begin to merge, killing larger patches of tissue. Yellowing extends further from each spot. The leaf still has functional green tissue overall but is visibly disfigured.
Late — past the point of recovery
By the late stage, individual spots have merged into larger irregular brown patches that kill whole sections of the leaf. The yellowed areas surrounding the damage turn crispy and brittle. Black pycnidia are unmistakable in older lesions. The leaf eventually goes fully yellow and drops. A leaf this far gone doesn't recover, but removing it (along with any other infected material) plus improving airflow and avoiding overhead watering keeps the infection from climbing further.
Commonly Confused With
Calcium (Ca)
Both produce spots. Septoria spots are round with dark borders on lower/old leaves, with tiny black dots (pycnidia) visible inside mature spots. Calcium spots are irregular rust-brown marks on upper/new growth.
Both cause brown marks. Nutrient burn produces uniform tip browning across the entire plant. Septoria produces random circular bordered spots concentrated on lower leaves.
