The appearance of brown spots on cannabis leaves signals plant distress and requires immediate investigation. These spots, known as necrosis or localized tissue death, are symptoms of an underlying problem. Identifying the cause requires careful observation of the spot’s location, shape, size, and pattern of spread. Determining if the damage is focused on new or old growth, uniform or scattered, helps growers pinpoint the issue and begin corrective action.
Brown Spots Caused by Nutrient Imbalances
Nutrient deficiencies frequently cause brown spotting, often due to the plant’s inability to absorb minerals rather than their scarcity. The specific mineral involved is identified by whether symptoms appear on older or newer foliage, based on the nutrient’s mobility within the plant.
Calcium (Ca) is an immobile nutrient, so the plant cannot relocate it from older leaves. A deficiency first presents on newer, upper leaves as small, irregularly shaped, rusty brown spots. These necrotic patches may be accompanied by leaf curling or deformation.
Magnesium (Mg) is a mobile nutrient, meaning the plant sacrifices it from older, lower leaves to supply growing tips. The deficiency manifests as interveinal chlorosis, where tissue between the veins turns yellow while the veins remain dark green. This yellow area eventually develops into distinct, rusty or brown necrotic spots.
Potassium (K) deficiency also affects the oldest, lower leaves first because it is mobile. The characteristic sign is “tip burn,” where the edges and tips of the leaves turn yellow, then brown, and become dry and scorched. This necrotic progression moves inward along the leaf margins.
Nutrient lockout is a common complication causing deficiency symptoms, even when elements are present in the solution. This occurs because the pH level in the root zone is too high or too low for effective nutrient absorption. For soil, the ideal pH range is 6.0 to 7.0; soilless mediums like coco coir or hydroponics require 5.5 to 6.5. When the pH is outside this range, nutrients bind to the medium, making them chemically unavailable for root uptake.
Brown Spots Caused by Environmental Stress
Environmental conditions can cause brown spotting concentrated in specific areas. Light burn occurs when the light source is too intense or too close to the canopy. This damage is isolated to the closest leaves, causing them to bleach white or yellow before developing crispy, brown scorch marks on the tips and edges.
Heat stress often occurs with light burn, causing leaves to curl upward to minimize water loss. If temperatures remain excessively high, leaf edges dry out and develop brown, crunchy necrosis. This damage tends to be uniform across the plant’s upper canopy.
Water quality and feeding regimens can lead to salt burn or nutrient toxicity. Overfeeding or infrequent flushing causes mineral salts to accumulate in the medium, damaging the root system. This appears as immediate, dark brown “tip burn” on a wide array of leaves as the plant struggles to process the excess concentration.
Localized brown spots can also be caused by water droplets left on leaves under intense light. The surface tension of water on the hairy leaves can act like a rudimentary magnifying glass. This focuses light intensity onto a small area, resulting in distinct, circular burn spots.
Brown Spots Caused by Pests and Pathogens
When brown spots are caused by living organisms, the pattern is distinct from chemical imbalances. Pest damage results from the physical feeding behavior of small insects. Spider mites and thrips pierce leaf tissue to suck out cell contents, manifesting as tiny, scattered yellow or bronze dots known as stippling. If the infestation is severe, this stippling turns brown and necrotic, though the damage remains localized. Growers should inspect the underside of affected leaves for pests, eggs, or silky webbing.
Fungal and bacterial infections, or pathogens, leave a different signature on the leaves. Leaf Septoria is a common culprit that thrives in high humidity and poor air circulation, often starting on lower foliage. This infection creates distinct, circular brown lesions often surrounded by a yellow halo. These spots spread rapidly, causing the leaf to yellow and drop off, reducing photosynthetic capacity.
Steps for Immediate Recovery and Prevention
The most immediate step for non-pathogen-related brown spots is testing the pH of the root zone and nutrient solution runoff. Stabilizing the pH within the optimal range is the fastest way to restore the plant’s ability to absorb minerals.
If salt buildup is suspected due to overfeeding or poor drainage, a thorough flush of the growing medium is necessary to reset the nutrient concentration. This involves slowly running several gallons of pH-balanced, low-concentration water through the substrate to remove excess mineral salts. Once the medium is reset, nutrients should be reintroduced gradually at a lower strength.
For environmental issues, immediately adjust the distance of the grow light or increase air circulation to lower leaf surface temperature. Severely affected leaves displaying extensive brown necrosis will not recover, as the tissue is permanently damaged. Pruning these dead leaves is important to improve light penetration, increase air circulation, and remove potential entry points for pathogens.