Recognizing and fixing root rot (Pythium) in cannabis

Root rot is usually triggered by Pythium (an egg-fungus-like oomycete) that colonizes and decomposes weakened, oxygen-poor roots.

Pythium is an opportunistic pathogen: healthy, well-aerated roots fend it off; only oxygen deficiency or root damage opens the door for it.

Pythium forms motile zoospores that actively swim to the root in warm, stagnant water. Warm nutrient solution (> 22 °C) and low oxygen are ideal conditions.

Infested roots lose their outer cortex ('sleeving'): the white tip sloughs off, leaving a brown, slimy core.

Stage 1: Root tips discolor beige/brown, slight odor, plant already looks limp in the morning.

Stage 2: Brown, slimy roots with a foul odor, wilting despite moist substrate, leaves yellow/wilt on the upper side.

Stage 3: Widespread root necrosis, stagnation, plant collapse; in hydro the infestation spreads through the reservoir.

Wilting + wet substrate + brown, slimy roots → root rot (Pythium).

Wilting + dry substrate + white, firm roots → drought stress, not rot (opposite corrective action!).

The smell is a strong indicator: healthy roots smell earthy-neutral, rotting ones smell musty.

1. Stop/reduce watering and let the substrate dry out; eliminate waterlogging immediately.

2. Increase oxygen: cool the nutrient solution (< 20 °C), boost aeration/air stones, clean the reservoir.

3. In hydro: fully replace the reservoir, clean the system; carefully remove rotting roots.

4. Use beneficial microorganisms (e.g. Trichoderma, Bacillus) to recolonize the root zone.

Don't overwater: tie feedings to demand and drying; ensure good drainage.

Keep the nutrient solution cool (< 20 °C) and aerate it vigorously — oxygen is the best brake on Pythium.

Hygiene: clean systems, darken reservoirs (against algae), control fungus gnats (larvae damage roots).

Warm, stagnant, low-oxygen nutrient solution is the central risk factor — temperature and aeration are the most important levers.

Fungus gnat larvae and mechanical root damage pave the way for Pythium.

Iron and other micronutrient deficiencies that persist despite a correct solution are often a consequence of diseased roots, not of the nutrients.

Misreading wilting as drought and watering MORE — this massively worsens Pythium.

Ignoring warm nutrient solution in summer.

Only 'treating' infected roots without fixing the oxygen/water cause.

An established population of beneficial microorganisms in the root zone acts prophylactically and actively outcompetes Pythium.

In recirculating systems, solution temperature monitoring and reservoir hygiene are the most effective ongoing measures; root tissue that has already died does not regenerate.