Just guessing is totally fine – it's only about a first impression.
You know this from the diagnosis course: a damaged root can no longer take up water even when water is there. The plant dies of thirst with wet feet. Today we go behind that – why the root is damaged and who was involved.
Anyone who looks inside infected roots rarely finds a single culprit. In seven licensed facilities five species were found side by side – fungi and oomycetes mixed, within a single root.
A note on the naming confusion that will save you search time: some of the species formerly called Pythium are now called Globisporangium. That's not a new threat, it's a rename. When you research, query both names – otherwise half the literature escapes you.
More important than the names is the consequence: you don't fight a disease, you change an environment. Against a community of five species there is no remedy aimed at one of them.
In 2023 an outdoor stand in Canada was sampled, three genotypes side by side on the same ground. All five species were found in each of them – but in completely different proportions.
In one variety the oomycete accounted for two thirds of all finds. In another it had almost vanished, and a Fusarium dominated with half.
Same soil, same weather, same season – three different suspect profiles.
And in one of the three, something else entirely turned up: a viroid. That's the next lesson. That both sat in the same field isn't a narrative convenience, it's what the report said.
In the same study one species was the second most frequent of all in two of three varieties – a good quarter of all finds. A clear prime suspect, you'd think. The researchers then did something most people skip: they put all the isolates back onto healthy plants.
The second most frequent caused almost no disease.
Literally in the report: "minimal disease development". It was everywhere and did next to nothing. The real damagers were others – including one that made up just eight per cent of finds in one variety.
Anyone who only isolates and counts would have convicted the wrong one.
That's why Koch's postulates exist. Finding a suspect isn't enough – you have to give it back and see whether the disease returns. That's over a century of basic doctrine, and it's the reason you may distrust forum reports where somebody "found Pythium".
Found is not guilty.
A root disease reports upward. And what arrives up there looks like a nutrient deficiency: yellowing, stunting, drooping leaves.
But there is a distinguishing feature, and it's the distribution – the same logic as in the nutrient course, just rotated one axis further.
A deficiency follows the storey. Mobile nutrients migrate up into young tissue, so the bottom yellows first. The pattern is horizontal: a floor.
A vascular disease follows the vessel. The pathogen sits in the conducting tissue and blocks it – and vascular bundles don't supply a storey, they supply a sector. The literature has the sentence that captures it best: the plants were green on one side and yellow-tan on the other.
Half a plant, one branch, one side: that's not a deficiency pattern. That's a blocked pipe.
A root can be extensively colonised before anything is visible. In other crops this is well studied: pathogens were regularly isolated from entirely inconspicuous roots – and the affected plants still yielded less.
No browning, no smell, no wilting. Just less.
That's the most unpleasant property of this whole room: the damage has a preliminary stage without a picture. If you can see something at the root, you weren't early – you were late.
That's why the neighbouring clone from lesson 1 is so valuable here. It shows you differences long before a symptom appears.
In other greenhouse crops it is experimentally established that fungus gnats carry root fungi – on the body and through the gut. For cannabis this hasn't been shown specifically, so it's a transfer from neighbouring crops. It still changes the view: the visible gnat may be the taxi for the invisible fungus. Which makes controlling the gnat more than cosmetics.
The switch's first question is: is it crawling, or is it a coating? It works – except for one animal that happens to live where nobody looks.
The rice root aphid is an established indoor pest. It sits almost exclusively on the roots, feeds there, and what arrives up top looks like – correct – a nutrient deficiency. It is regularly misread as one and treated accordingly: with more fertiliser.
It often only becomes noticeable when winged individuals emerge from the substrate in numbers. Until then it crawls without anyone seeing it.
The consequence for the switch: the question "is it crawling?" needs a downward continuation. The tap test over white paper that you know for leaves also works with a root ball over a light surface.
There is a rule of thumb you'll find everywhere: above about 24 degrees in the root zone it gets critical. We checked it and had to strike it. For cannabis there is not a single measurement on this.
What does exist is a mechanism from another crop – and it's more interesting than the number.
Roots were warmed, cooled down again and only then brought into contact with the pathogen. The entire disease progression ran at normal temperature. Those roots still discoloured days earlier than the comparison group – and the effect persisted for over a week after the heat.
The heat itself is the damage. The pathogen comes afterwards.
The limits of this finding belong with it: it was pepper, not cannabis. There was only a single temperature step, well above anything a tent ever reaches. And the stimulus was days of continuous heat, not a midday peak.
What happens at 26 or 28 degrees, nobody knows. But the idea holds: a long-past heat period can explain why things tip now.
Under 10 per cent – and the authors expressly note that several other varieties on the same ground were considerably less affected than the three sampled.
That matters against the impression that doom looms everywhere. Root rot outdoors is real, but it's not mass death – and variety choice is the biggest lever.
As the only recommended outdoor measures the researchers name: choose tolerant varieties and remove diseased plants. That is literally the core message of the outdoor course in another subject: outdoors there are no controls, only decisions.
Of the three routes to proof this is the cheapest, and the least often taken – because it requires touching something.
You don't have to dig the plant up. At the pot's edge or through the drainage hole you see enough:
Colour. Healthy root tips are pale and creamy white. Brown is a finding, dark brown a clear one.
Firmness. Healthy root tissue is firm. If the outer layer strips off between your fingers and leaves a thread behind, that's unambiguous.
Smell. A healthy root zone smells of soil or of nothing. Foul, swampy or sweetish is not a normal state.
Three sensory impressions, no tools, no cost. And if you're unsure whether the brown is already rot: photograph it and submit it to the consultation.
Tap the steps in the correct order.
What stands out is that the first two steps cost nothing and need no equipment. The most expensive mistake in this room isn't the wrong treatment – it's never having looked at the root.
🎯 Hands-on mission
At the next repotting or clean-up: look at a root deliberately and smell it, even if nothing stands out. That's exactly the point – you need a picture of the normal state to recognise a deviation later. Photo into the grow diary, and if the finding is unclear, into the consultation.
Submit a photo →✓
Some causes you can expose. And some sit so deep in the tissue that digging won't help either.
Something still unclear?
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