Brown Spots on Cannabis Leaves: Causes and What Helps
Brown spots on cannabis leaves can result from a deficiency in calcium or potassium, from too much fertilizer and salt in the root zone, or from a leaf spot fungus. The first clue comes from where the spots appear on the plant: according to the e-GRO Cannabis diagnostic portal run by North Carolina State University, calcium deficiency starts on new growth, while potassium deficiency begins on the lower, older leaves.
Leaf spots caused by fungi such as Septoria leaf spot have a yellow halo, according to descriptions from several US universities, and develop in wet conditions with high humidity. The pH value and electrical conductivity (EC) of the substrate or nutrient solution help rule out fertilizing mistakes.
Where on the plant brown spots appear first
e-GRO Cannabis sorts nutrient disorders by where the first symptoms show up. Nitrogen, phosphorus and potassium deficiency, along with micronutrient excess, start on the lower leaves. Calcium, iron, manganese, boron, copper and zinc deficiency show up first at the top of the plant. The table below lines up the causes that come with brown spots or dead leaf tissue.
Cause | First symptoms on | Typical appearance |
|---|---|---|
Calcium deficiency | shoot tips and young leaves | stunted, irregularly shaped leaves, dead leaf margins, later the shoot tip dies |
Potassium deficiency | lower, older leaves | yellow leaf margins, especially at the serrations, then margins die back light brown |
Phosphorus deficiency | lower, older leaves | irregular olive-green spots that become sunken and look wet, later large dead areas |
Magnesium deficiency | lower, older leaves | yellowing between veins that stay green, later individual dead spots |
Septoria leaf spot | lower leaves inside the canopy | small brown spots with a yellow halo, later a gray to white center and dark specks |
Calcium deficiency on new growth
In the progression e-GRO describes, based on a study of nutrient disorders in Cannabis sativa published in 2019, plants are initially slightly stunted. Shoot tips and newly emerging leaves grow compressed and irregular. At the base of the leaflets, the blade stays narrower and lighter green, with a lightening between the veins starting there. Newly unfolded leaves are badly stunted and die back at the margin, producing irregular leaf shapes. In the advanced stage, the yellowed leaves and the shoot tip die, and the plant then produces more side shoots.
A fact sheet from Utah State University Extension on hemp places calcium deficiency on the lower leaves instead. It describes curled, twisted leaves with irregular yellow-brown spots while the rest of the leaf stays green, along with delayed flowering and dying root tips. On this point the two university sources disagree. Deformed young leaves are described only by e-GRO.
Potassium deficiency and rust spots on older leaves
Potassium deficiency shows up first on the lower, older leaves, according to e-GRO. The leaf margin turns yellow, especially at the serrations of the leaflets. The yellowing intensifies and moves inward toward the midrib until the margins die back light brown.
Utah State describes potassium deficiency on young leaves: rust-brown, dried-out margins and tips that curl upward, rust-colored spots that turn brown, plus weak stems and slower flowering. The same fact sheet also lists rust-brown spots for magnesium deficiency, there distributed irregularly on leaf margins, tips and between the veins of older leaves, along with interveinal yellowing. So rust spots appear with both deficiencies in this fact sheet. What differs is the location of the discoloration: at the margin for potassium, between the veins for magnesium.
Phosphorus and magnesium deficiency as a source of confusion
Phosphorus deficiency begins, according to e-GRO, with mild stunting and irregularly scattered olive-green spots on the lower leaves. The spots enlarge, look sunken and almost wet, the margins partly die back, and severe deficiency produces large dead areas. Utah State adds purple petioles, blue-green leaves, and dark copper to purple spots.
With magnesium deficiency, e-GRO describes a slight lightening between the veins of the lower leaves first. Later the green veins stand out sharply against the yellow tissue, and individual spots die off.
Over-fertilizing and high EC levels
Too much fertilizer or salt in the root zone shows up, according to the University of Maryland Extension's description for ornamental plants, as brown leaf margins and tips, wilting, yellow margins and stunted growth. The salts damage the roots and can build up in the leaves along with the water flow, particularly in hot, dry weather.
Trials with cannabis show that a high EC value does not automatically mean brown leaves. In a study published in Acta Horticulturae in 2023, the EC value was set to 1.8, 3, 6, 9, or 12 mS/cm, either through nutrient salts or through sodium chloride. Leaf necrosis occurred only in the sodium chloride treatments at 9 and 12 mS/cm. Shoot and flower weight, however, already dropped once the EC value exceeded 3 mS/cm with nutrient salts and 1.8 mS/cm with sodium chloride.
A study published in Frontiers in Plant Science in 2025, using a CBD-rich cultivar in hydroponics with 24 cuttings per run, found no leaf necrosis and no visible differences between treatments at EC values of 2 and 4 mS/cm and phosphorus levels of 15 to 90 mg/l. The pH value was kept between 5.8 and 6.2 throughout. Measured EC values can be converted to ppm with the EC/PPM converter.
Excess amounts of the micronutrients boron, copper, iron, manganese, molybdenum and zinc appear first on the lower, older leaves, according to e-GRO. The portal names mixing errors when preparing the nutrient solution, incorrectly set dosing equipment, and an unsuitable substrate pH as causes.
pH as a distinguishing factor
e-GRO gives a substrate pH range of 5.5 to 6.5. If the pH falls below 5, the availability of micronutrients rises, and the plant can take up iron and manganese in toxic amounts.
A study on medical cannabis published in Frontiers in Plant Science in 2024 summarizes the literature similarly: 5.5 to 6.0 in hydroponics, 5.5 to 6.5 in soilless substrates. Below 5.5 there is a risk of excess manganese; above 6.5, phosphorus, iron and manganese become less available to the plant.
Brown spots from leaf spot fungi
Utah State University Extension names three fungi as causes of hemp leaf spot: Septoria cannabis, Cercospora cannabis and Bipolaris gigantea. With Septoria leaf spot, small, irregular spots with a bright yellow margin appear first, according to the University of Kentucky. They grow to about a quarter inch (roughly 6 mm), turn brown with a clear yellow halo, and develop a gray to white center. Rutgers University describes dark brown to black specks within the spots, the spore-bearing fruiting bodies of the fungus (pycnidia).
Septoria starts on the lower leaves inside the canopy, where leaves stay wet for long periods and humidity is high. In severe cases whole leaves turn yellow and drop; the University of Kentucky reports leaf losses of 50 to 90%. In New Jersey, the first symptoms appeared in late August. The disease is favored by prolonged leaf wetness after rain or dew and by high humidity, according to Utah State especially in cool weather.
Other pathogens look similar. Cercospora produces round to elliptical spots with a gray center and a dark brown to black margin, according to Rutgers, which later turn light brown to white and develop a yellow halo. Bipolaris begins with tiny dark spots that have a light brown center and a dark brown margin and is favored by warm, wet weather. In Florida, a Curvularia fungus has also been found on hemp, with small yellow spots on young and old leaves turning light brown to brown, likewise with a yellow halo. In Austria, AGES reported the first European detection of Phaeomycocentrospora cantuariensis on industrial hemp in 2019, with brown, roundish to irregular spots, a whitish-gray center, a dark brown margin and a diffuse yellow halo.
The yellow halo around the spot and dark fruiting bodies inside it show up in the descriptions of fungal diseases, but not in e-GRO's pictures of nutrient deficiencies. Brown, soft spots in the flowers rather than on the leaves are covered in the article on gray mold on cannabis flowers.
First steps when brown spots appear on cannabis leaves
Determine the location: whether the spots start on new growth or on the lower leaves, and whether they sit at the leaf margin, between the veins, or as individual spots with a yellow halo.
Measure the pH value in the substrate or nutrient solution and compare it with the range of 5.5 to 6.5 that e-GRO and the 2024 study give for soilless substrates.
Measure the EC value and compare it with the fertilizing schedule. In the 2023 study, yield already dropped above 3 mS/cm with nutrient salts.
Examine the spots closely for dark specks inside them, as found with Septoria and Cercospora.
Remove affected material: Utah State and Rutgers describe removing infected plants and plant debris, along with wider spacing for better air movement, for leaf spot fungi.
Flush out salts: for salt damage in ornamental plants, the University of Maryland Extension describes flushing the soil with several thorough waterings.
A photo of the affected leaves can be uploaded to the free plant check with AI-assisted assessment, which narrows down the likely cause.
Frequently asked questions
What do brown spots on cannabis leaves mean?
They can point to calcium or potassium deficiency, to too much fertilizer and salt in the root zone, or to a leaf spot fungus such as Septoria. The first clue is the location: calcium deficiency starts on new growth, according to e-GRO Cannabis, while potassium deficiency begins on the lower, older leaves.
Where do rust-colored spots on cannabis leaves come from?
Utah State University Extension describes rust-colored spots with potassium deficiency, there at the margins and tips, and with magnesium deficiency, there also between the veins along with interveinal yellowing.
How do you recognize calcium deficiency in cannabis?
According to e-GRO Cannabis, shoot tips and young leaves grow compressed and irregular, the margins of new leaves die back, and in the advanced stage the shoot tip dies too. Utah State University Extension, however, places the symptoms on the lower leaves instead.
How do fungal leaf spots differ from a nutrient deficiency?
Septoria spots are brown with a clear yellow halo, develop a gray to white center, and contain dark fruiting bodies. They start on the lower leaves inside the canopy and are favored by leaf wetness and high humidity.
What pH value should the substrate have for cannabis?
e-GRO Cannabis gives a range of 5.5 to 6.5 for the substrate; a 2024 study summarizes the literature as 5.5 to 6.0 for hydroponics and 5.5 to 6.5 for soilless substrates. Below pH 5, the plant can take up iron and manganese in toxic amounts, according to e-GRO.
Sources
e-GRO Cannabis: Micronutrient Toxicity, North Carolina State University, e-GRO Cannabis
Hemp: Nutrient Deficiencies, Utah State University Extension, Plant Health / IPM
Hemp Leaf Spot, Utah State University Extension, Plant Health / IPM
Septoria Leaf Spot (Hemp), University of Kentucky, Plant Pathology Extension (Nicole Gauthier)
Hemp Diseases, University of Florida, IFAS, Plant Pathology Department
Excess Fertilizer or Salt Damage on Flowers, University of Maryland Extension
As of: 2 November 2026
This article is for informational purposes only and is not a substitute for medical or therapeutic advice. If you have any health-related questions, please consult a doctor or another qualified professional.