Leaf spot is a descriptive term for any disease that produces distinct necrotic spots on leaves. On cannabis the commonest cause is Septoria cannabis (Septoria leaf spot); Alternaria species, Cercospora species and some bacteria also cause leaf spots, mostly outdoors. Spots often have a darker margin, a yellow halo, or concentric rings, depending on the organism.
In practice
Most leaf spot pathogens need wet leaves, so they appear after rain, heavy dew or overhead watering, and on lower leaves first where splash and humidity are highest. Growers check spots with a hand lens for tiny black fruiting bodies, which point to a fungus, and send samples to a plant diagnostic lab when unsure. Management is the same across most types: remove infected leaves, clear debris, water at the base, improve airflow, and rotate outdoor sites. Many "spots" turn out to be nutrient or pH problems, so check the root zone before blaming a pathogen.
Don't confuse with
- Calcium or potassium deficiency: patterns linked to feeding and pH, no fruiting bodies.
- Light or spray burn: spots where droplets or hot spots hit, not spreading.
- Spider mite stippling: many tiny pale dots.
Death of plant tissue, seen as brown, tan or black patches, spots, tips or margins; it is permanent and the end point of many nutrient, salt, pest, disease and environmental problems.A fungal leaf disease caused by Septoria cannabis that produces tan to grey spots with dark margins and yellow halos, starting on lower leaves.pH measures how acidic or alkaline a solution is on a 0–14 scale; in growing it controls nutrient uptake, with soil best near 6.0–7.0 and hydro 5.5–6.5.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.Potassium (K) is a mobile macronutrient that controls water balance, stomata and enzyme activity; deficiency scorches the edges of older leaves.Tetranychus urticae, the commonest cannabis mite indoors: a 0.5 mm sap-feeder that stipples leaves pale and spins webbing, breeding fastest in hot, dry rooms.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.An oomycete leaf disease of cannabis, historically attributed to Pseudoperonospora cannabina, causing yellow patches above and grey-purple fuzz under leaves in cool, wet weather.Cleaning and disinfecting rooms, tools, containers, irrigation and people's clothing so that pests and pathogens are not carried from plant to plant or crop to crop.Systematic, regular inspection of plants, traps and roots to detect pests and disease early, identify them correctly and record numbers for decisions.
