Oedema (edema, or intumescence) is a physiological disorder, not a disease. When the root zone is wet but the air is cool and humid, roots push in water faster than the leaves can lose it, and some leaf cells swell and rupture, leaving blisters that turn corky and tan. It is documented in many greenhouse crops (UMass Extension)being reviewed; cannabis-specific reports are anecdotal.
In practice
Look on the leaf underside for raised, water-soaked bumps that later turn brown and scabby, often on lower leaves. Fix the conditions: water less often, raise air temperature, lower RH and increase airflow so VPD rises toward 0.8-1.2 kPa. Damaged leaves stay marked; new growth will be clean.
Don't confuse with
- Mite or thrips damage: flat stipples or streaks, with pests visible under a lens.
- Septoria leaf spot: flat spots with dark margins and black fruiting bodies.
- Spray burn: spots where droplets dried in light.
The root zone is the volume of medium or solution where a plant's roots live, including its water, air, temperature, pH, EC and microbes.Relative humidity is the water vapour in air as a percentage of what it could hold at that temperature, so the same RH dries plants differently at different temperatures.Vapour pressure deficit: how much more water vapour the air could hold at its temperature, in kPa; it is the main environmental driver of transpiration.Slender insects about 1-2 mm long, chiefly western flower thrips (Frankliniella occidentalis), that rasp leaf surfaces, leaving silvery streaks dotted with black frass.A pattern of many tiny pale dots on leaves where a sucking pest such as a spider mite has emptied individual cells; the first visible sign of a mite infestation.A fungal leaf disease caused by Septoria cannabis that produces tan to grey spots with dark margins and yellow halos, starting on lower leaves.The evaporation of water from a plant, mainly through open stomata, which pulls water and dissolved nutrients up from the roots.Overwatering is watering so often that the medium stays saturated, starving roots of oxygen, which causes drooping, slow growth and root disease.
