Leaf Septoria

Leaf septoria is a fungal leaf spot disease, sometimes called yellow leaf spot. It works from the bottom of the plant upward, killing leaves one layer at a time, and it is far more common outdoors and in humid rooms than in a well ventilated tent.

What you are seeing#

Round or slightly irregular spots on the lowest fan leaves, roughly an eighth to a quarter inch across. Each spot has a yellow halo with a tan, gray or light brown center. Under magnification the centers can show tiny black dots, which are the fruiting bodies. Spots do not wipe off, which is the fastest way to separate this from powdery mildew.

As it progresses, spots multiply on the same leaf and merge into brown patches. The whole leaf yellows and dies, and the disease moves up to the next layer. On a badly affected plant the bottom third can be bare within two weeks.

Look on both sides of the lowest leaves, especially anything touching the soil surface or another leaf.

Why it happens#

The fungus survives in plant debris and soil and spreads by water splashing spores upward from the medium onto the lowest leaves. That is why it always starts low.

Conditions that let it establish: humidity above 60 percent, leaves that stay wet, overhead watering that splashes soil, poor airflow under the canopy, and temperatures around 68 to 77F.

Beginner causes are leaves resting directly on the soil, no fan reaching under the plant, dead leaves left on the medium surface, and reused unsterilized soil. Outdoors, a wet fall does it on its own.

How to fix it#

1. Remove every spotted leaf today. Bag them, seal the bag, carry it out. Do not compost them or leave them on the tent floor.
2. Clear the soil surface of fallen leaves and debris. That debris is the reservoir the spores come from.
3. Mulch or top-dress with an inch of clean material so water cannot splash soil onto leaves, and water at the base only, never over the canopy.
4. Remove the lowest leaves and branches so nothing is within 6 inches of the medium surface. This is a permanent fix rather than a treatment, and it is the most useful thing on this list.
5. Get humidity under 55 percent and put a fan under the canopy so lower leaves dry within an hour of any wetting.
6. Feed properly. Septoria hits nitrogen-starved plants harder, so make sure you are not also running a deficiency.
7. If it keeps spreading in vegetative growth, a copper based fungicide at label rate every 7 to 10 days for 2 to 3 rounds will slow it. One application does not work, because new spores land on new leaves continuously.

How to tell it apart#

Versus calcium deficiency. This is the confusion worth learning. Calcium spots are small rusty brown or reddish marks with ragged edges on new upper growth, because calcium does not move around inside the plant. Septoria spots are round with a yellow halo and a pale center, and they start on the oldest lowest leaves and climb. Location is the discriminator: new growth points to calcium, old growth points to septoria.

Versus spider mites. Mite damage is a fine even stipple of tiny pale dots, not discrete spots with centers and halos, and mites bring webbing and visible insects underneath. Check five undersides with a loupe. Nothing moving means it is fungal.

Versus powdery mildew. Mildew is a white raised dust that wipes off on your finger. Septoria is flat, yellow and tan, and stays put no matter how hard you rub it.

How long until it looks better#

New spots should stop appearing 7 to 10 days after you strip the affected leaves, clear the soil surface and get airflow underneath. The environment work matters more than any spray.

Spotted leaves never recover, so there is no point watching them. Remove them and judge progress on whether clean leaves higher up stay clean over the following two weeks.

This one rarely kills a plant or ruins a harvest indoors. What it does is steal lower canopy leaves and slow the plant down. If it keeps coming back across grows, the answer is prevention: strip the bottom 6 inches before flowering, water at the base, keep humidity under 55 percent, and start each grow with fresh medium and a clean tent.