Root rot: brown roots and a plant that keeps declining

Root rot is easy to mistake for a watering or feeding problem, and more nutrients won’t help roots that have stopped working.

What it looks like

Above the soil, plants become stunted and yellow, and some die. Greenhouse plants near harvest may collapse (Punja et al. 2019). In hydroponic plants, researchers saw stunted growth, brown root lesions, rot and slight yellowing of the leaves (Punja & Rodriguez 2018).

Below the soil, the roots brown and decay. The crown, where the stem meets the roots, and the pith inside the stem can also discolor (Punja et al. 2019).

Why it happens

Root rot on cannabis is linked to two groups of organisms: Pythium, a water mold, and Fusarium, a fungus. Several species of each have been found on diseased plants, and they can cause overlapping symptoms (Punja et al. 2019).

In hydroponics, spores can move through the water. If nutrient solution is recirculated without treatment to destroy them, it can carry disease into the propagation area, killing cuttings and infecting roots and crowns (Punja et al. 2019). Tools, equipment and tubing that haven’t been cleaned properly can spread it too (Punja & Rodriguez 2018).

Too much water and poor drainage are the usual conditions growers blame. That matches general plant advice, and our mold and root rot guide, but the cannabis studies we cite identified the organisms, not how much overwatering raises the risk.

How to confirm it

1. Look at the roots. Gently slide the plant from its pot, or lift it from the hydro system. Roots that are brown and decaying are the sign to look for (Punja et al. 2019).
2. Check the crown. Look for discoloration or sunken lesions at the base of the stem.
3. Check the water. Is the mix staying wet, or is drainage poor? Is the hydro system recirculating?
4. Get a lab diagnosis if it matters. Brown roots alone don’t tell you which organism it is. For persistent wilting or a problem that’s spreading, ask a plant diagnostic lab or your county extension office. Rules on sending them cannabis vary, so call first.

What else it could be: plain overwatering or underwatering can look the same from above, and so can fungus gnat damage in seedlings. A plant with healthy roots probably has something else wrong, so check pH and watering next.

What to do

1. Separate the plant from healthy ones, and in hydro take it off any shared water.
2. Stop feeding more to fix it. Damaged roots can’t use it.
3. Correct the watering. Let the mix get air between waterings, and make sure pots drain. In hydro, keep the water cool and oxygenated (see our mold and root rot guide).
4. If a plant dies or is clearly failing, throw out the plant and its medium, and clean the pot.
5. Treat or replace recirculating solution so it doesn’t carry spores to other plants (Punja et al. 2019).
6. Clean tools, tubing and equipment before they touch healthy plants, using a labeled disinfectant or a validated procedure. Never mix cleaning chemicals.
7. Start the next round clean. Use healthy cuttings or seed and fresh or pasteurized media. Unpasteurized coco fiber was identified as a possible source of mold (Punja et al. 2019).

About root treatments: beneficial microbes such as Trichoderma are widely sold for this. We couldn’t verify a controlled trial showing that any treatment rescues an infected cannabis plant, so don’t assume a product will. Prevention has the better case.

How much to trust this

The list of organisms and symptoms is well supported by cannabis research, though mostly from one research group. We read the 2019 paper in full, but only the abstract and press coverage of the 2018 hydroponics study.

The treatment advice is much weaker. Those papers identify the problem, not the fix, and the steps above are sensible hygiene and water management rather than tested treatments on cannabis. We also found nothing on how well plants recover, so our warning that a badly affected plant may not recover is our judgment.