Leaf Spot Disease in Hydroponics — Identify & Fix
Brown or yellow spots with dark margins signal fungal or bacterial leaf spot. Drop humidity below 70%, prune, and treat in 5 days.
BY ROOTLESS FARM
Quick answer
Brown or yellow circular spots with dark margins, sometimes ringed concentrically = leaf spot disease, usually Septoria, Alternaria, or bacterial Xanthomonas. The cause is humidity above 80% with weak airflow. Drop RH to 60–65%, prune all visibly infected leaves, increase canopy airflow, and treat with copper soap if spread continues. Outbreak contained within five days.
Symptoms
- Circular brown or yellow spots, 2–10 mm
- Spots often have darker rim or concentric rings
- Yellow halo around the lesion in bacterial cases
- Spots merge as disease progresses
- Lower, older leaves affected first
- Spots may have small black dots (fruiting bodies) in fungal cases [RHS-HYDRO-01]
Cause
Leaf spot is caused by several pathogens that all share environmental requirements: leaf wetness, high humidity, and weak airflow. Septoria and Alternaria are fungal and spread by spores that need free water on the leaf surface for at least 6 hours to germinate. Xanthomonas is bacterial and enters through stomata and wounds. All three sporulate or multiply most aggressively at 20–28 °C and RH above 80% [RHS-HYDRO-01]. Closed canopies in indoor systems trap humidity and create exactly the conditions these pathogens need; outdoor crops escape it through wind.
Diagnose
| Check | Target | Disease signal |
|---|---|---|
| RH | 60–65% | > 80% |
| Airflow | leaves trembling | still air at canopy level |
| Spot shape | none | circular with halo or concentric |
| Spot edges | none | sharp, bounded by leaf veins (bacterial) |
| Leaf wetness | dry by morning | wet > 6 hours overnight |
| Lower vs upper | none | lower leaves first |
Differentiate from nutrient deficiency: leaf spot has well-defined edges, often with concentric rings or yellow halo; deficiency yellowing is diffuse and follows leaf venation patterns.
Fix
- Remove all visibly infected leaves with clean snips, then sterilize the snips with isopropyl alcohol.
- Drop RH to 60–65% with a dehumidifier or increased ventilation.
- Run horizontal airflow across the canopy — leaves should tremble slightly.
- Avoid overhead watering that leaves leaf surfaces wet overnight.
- Spray copper soap at 1.5 g/L if spread continues after environmental fixes — evening application, full coverage including leaf undersides [RHS-HYDRO-01].
- Quarantine the affected plant if in a multi-plant system; spread happens fast through shared canopy.
Prevention
Keep RH below 70% at all times — install a dehumidifier if your room runs humid. Run a horizontal fan at canopy level. Space plants so the canopy never fully closes; air must move between plants. Never water late in the day; morning irrigation lets leaf surfaces dry before the night-time temperature drop that triggers condensation [GROWER-LOGS]. Sterilize tools between plants and between cuts.
FAQ
4 entries- Q01How do I tell fungal from bacterial leaf spot?
- Fungal spots usually have a clear concentric ring pattern and may show dark fruiting bodies. Bacterial spots are water-soaked, angular (bounded by leaf veins), and yellow-haloed.
- Q02What humidity prevents leaf spot?
- Below 70% RH with steady airflow. Septoria and Alternaria sporulate above 80% RH; below 65%, infection rates drop sharply.
- Q03Should I spray fungicide for leaf spot?
- Only after removing infected leaves and fixing the environment. Copper-based sprays at 1.5 g/L work on many bacterial and fungal spots, but environment fixes the recurrence.
- Q04Can leaf spot kill my plant?
- A single spot won't, but unchecked spread across 30% of canopy in a week is common, and severe defoliation collapses yield. Address within 48 hours of first signs.