Leaf symptom troubleshooter: from symptom to diagnosis
Starting point for any troubleshooting: systematically match an observed leaf or plant symptom to a possible cause and jump directly to the matching diagnosis.
Diagnoses covered
29 deficiencies, toxicities, pests, diseases, stressors
First step, always
Determine symptom location (old/new, edge/center)
Second step
Single cause or combination (lockout/climate)?
Key points
- Symptom location and pattern (which leaves, which direction of discoloration/curling) often narrow down the cause more than the color alone.
- Multiple symptoms occurring at the same time point more toward a pH lockout or a climate problem than toward a single nutrient deficiency.
- This overview does not replace the detailed diagnosis — it only shortens the path to the right article, where the actual confirmation and correction takes place.
Note
- This overview is an entry point, not a final diagnosis — when in doubt, always consult the linked detail article for confirmation features and corrective actions before intervening.
How this troubleshooter works
First observe WHERE the symptom occurs (old vs. young leaves, leaf margin vs. leaf center, whole plant vs. single shoot) and HOW it behaves (direction of leaf curling, recovery after watering, spread rate) — this metadata often narrows down the cause more than color alone.
Then find the matching symptom category below and follow the link to the respective diagnostic article for confirmation, mechanism, and corrective actions.
Yellow/chlorotic discoloration
Interveinal chlorosis on OLDER, lower leaves, veins stay green → magnesium deficiency or potassium deficiency — potassium deficiency additionally shows necrotic leaf margins.
Uniform yellowing of whole OLDER leaves (not just between the veins) → nitrogen deficiency.
Interveinal chlorosis on YOUNG, upper leaves → iron deficiency.
Several of these patterns SIMULTANEOUSLY on the same plant → more likely pH lockout or nutrient lockout than a single deficiency.
Dark, purple, or burnt discoloration
Dark green, claw-shaped leaves curling downward on young shoots → nitrogen toxicity.
Dark green-bluish leaves with reddish-purple petioles → phosphorus deficiency — at low root temperature (< 18 °C), check for cold stress instead.
Purple stems/leaf undersides at normal root temperature → cold stress.
Brown, burnt leaf tips on the most vigorous leaves → nutrient burn/tip burn.
Mg deficiency symptoms despite nutrient feeding, plus rising pH with hard water → calcium toxicity.
Leaf shape, movement, and curling
Leaf edges rolled UPWARD (tacoing), upright posture close to the lamp → heat stress.
Leaves rolled DOWNWARD, dull/dusty appearance without webbing → suspected hemp russet mites.
Curled, claw-shaped leaves ONLY in the direct fan airstream → windburn.
Drooping leaves DESPITE wet substrate, no recovery after watering → overwatering/waterlogging, if the root is already brown and slimy → root rot.
Visible pests and their traces
Fine light stippling on top, webbing on the leaf underside → spider mites.
Silvery streaks with small black fecal dots, NO webbing → thrips.
Sticky honeydew, ant visits, sedentary colonies at shoot tips → aphids.
Cloud-like swarming of small white insects when touched → whiteflies.
Small black flies over the substrate → fungus gnats.
Fungal, rot, and vascular tissue symptoms
A single wilted, discolored leaflet deep inside a dense bud → suspected bud rot.
White, wipeable coating on leaf tops → powdery mildew.
Brown, slimy roots, wilting despite wet substrate → root rot (Pythium).
One-sided/sectoral wilting, brownish ring in the stem cross-section → suspected Fusarium.
Climate- and EC-related patterns
Wilting despite moist substrate, burned leaf edges → salt buildup/high EC.
Performance plateau despite optimal light, VPD, and nutrition → possibly a CO2 limit.
Symptoms shift with relative humidity, not with nutrition or watering rhythm → check humidity management.
Diagnostic protocol — step by step
1. Determine the symptom location: old vs. young leaves, whole plant vs. single shoot/sector.
2. Check the direction of movement/response: leaf rolling (up/down), recovery after watering, spread rate.
3. Watch for a combination of several patterns — this points to pH lockout, a climate problem, or root zone stress rather than an isolated deficiency.
4. Find the matching category above and go to the linked diagnosis article to read the confirmation and correction there.
Checklist
- Note the symptom location (old/young, edge/center, whole plant/sector)
- Check whether multiple symptom patterns occur simultaneously
- Only correct after confirmation in the linked article, not speculatively pre-emptively
Frequently asked questions
I see two different symptoms at the same time — what should I check first?
What if my symptom doesn't fit any category here?
Do I always have to come here first before reading a diagnosis article?
Sources
→ Full registerRelated articles
Diagnosis & Troubleshooting
Identify and fix magnesium deficiency in cannabis
Interveinal chlorosis on older leaves is the key symptom. This is how you distinguish a true deficiency from a pH-induced lockout and correct it specifically via substrate pH and Mg supply.
Diagnosis & Troubleshooting
Recognizing and fixing nitrogen deficiency in cannabis
Even yellowing of the lower leaves is the key symptom. This is how you distinguish true N deficiency from natural senescence in late flowering and dose accordingly for each phase.
Diagnosis & Troubleshooting
Identifying and correcting iron deficiency in cannabis
Bright yellow young leaves with sharply defined green veins are the key symptom. Almost always the true cause is not too little iron, but a pH that is too high.
Diagnosis & Troubleshooting
Identifying and correcting potassium deficiency in cannabis
Burnt, necrotic leaf margins on older leaves with otherwise green leaf area are the key symptom. Here's how you distinguish K deficiency from salt burn (nutrient burn).
Diagnosis & Troubleshooting
Identifying and fixing calcium deficiency in cannabis
Stunted, spotted new shoots and brown leaf margins reveal Ca deficiency. This shows you that the problem almost always lies in pH and water quality — not in the Ca dose.
Diagnosis & Troubleshooting
Recognizing and correcting phosphorus deficiency in cannabis
Dark green to bluish leaves with reddish-purple petioles and slowed growth are the key symptoms. Here's how to distinguish true P deficiency from cold stress and correct it specifically via pH and phosphorus supply.
Diagnosis & Troubleshooting
Detecting and fixing pH lockout in cannabis
Multiple seemingly independent deficiency symptoms occurring at the same time are the key signal for pH lockout, not for an actual nutrient deficiency. Here's how you recognize the lockout and correct it without blindly adding more nutrients.
Growing & Harvest
Nutrient lockouts and antagonisms
Why deficiency symptoms can occur despite adequate nutrient levels, and how to correctly classify lockouts.
Diagnosis & Troubleshooting
Recognizing and fixing nitrogen toxicity in cannabis
Dark green, claw-shaped downward-curled leaves ('The Claw') are the key symptom. Here's how to distinguish true N toxicity from overwatering and reduce nitrogen supply in a targeted way.
Diagnosis & Troubleshooting
Recognizing and correcting potassium toxicity in cannabis
Potassium toxicity is rarely directly toxic, but blocks the uptake of Mg, Ca and N via cation antagonism. Here's how you recognize the resulting secondary deficiencies and rebalance the ratio.
Diagnosis & Troubleshooting
Recognizing and correcting calcium toxicity in cannabis
Too much calcium is rarely directly toxic, but it raises the pH and displaces magnesium and potassium. Here's how to recognize the hard-water or overdosed Cal-Mag case and correct the ratio and pH.
Diagnosis & Troubleshooting
Recognizing and fixing overfertilization and nutrient burn (Nutrient Burn)
Burnt, brown leaf tips that eat their way inward are the hallmark symptom of overfertilization. This is how you distinguish nutrient burn from deficiency and light stress and correct the dose.
Diagnosis & Troubleshooting
Recognizing and fixing salt stress and high EC in cannabis
Excessive salt concentration in the root zone creates osmotic stress: the plant wilts despite moisture and burns at the leaf edges. Here's how to measure EC correctly and flush in a targeted way.
Diagnosis & Troubleshooting
Recognizing and fixing heat stress in cannabis
Upward-curled leaf margins (tacoing), an upright 'praying' posture, and marginal burns are the leading symptoms. This is how you distinguish heat stress from an actual nutrient deficiency and correct it via leaf temperature, VPD, and air movement.
Diagnosis & Troubleshooting
Recognizing and fixing cold stress in cannabis
Violet-purple stems and leaf undersides, slow growth, and downward-curled leaves are the key symptoms. This is how you distinguish cold stress from true phosphorus deficiency and stabilize air and, above all, root temperature.
Diagnosis & Troubleshooting
Recognizing and fixing windburn in cannabis
Curled, claw-like leaves bent downward directly in the airflow of a fan — without pests and without the color pattern of a deficiency. Here's how you recognize windburn and set up air movement correctly.
Diagnosis & Troubleshooting
Recognizing and fixing overwatering and waterlogging in cannabis
Drooping leaves despite wet substrate and no recovery after watering are the leading symptoms. This is how you tell waterlogging apart from drought stress and save the root zone in time.
Diagnosis & Troubleshooting
Recognizing and fixing root rot (Pythium) in cannabis
Brown, slimy roots and wilting plants despite moist substrate are the key symptoms. Here's how to tell root rot apart from drought stress and save the root zone.
Diagnosis & Troubleshooting
Recognizing and treating Fusarium in cannabis
Sudden, one-sided wilting despite moist substrate and brownish discolored vascular tissue in the stem cross-section are the key symptoms. This is how you distinguish Fusarium from root rot and limit the damage.
Diagnosis & Troubleshooting
Identifying and controlling spider mites on cannabis
Fine stippling on the upper leaf surface and webbing on the underside reveal spider mites. Here's how to spot the infestation early and stop the explosive reproduction with staggered IPM.
Diagnosis & Troubleshooting
Identifying and controlling thrips on cannabis
Silvery streaks with tiny black fecal specks are the key symptom. Here's how you distinguish thrips from spider mites and control the flying larvae with blue sticky traps and beneficial insects.
Diagnosis & Troubleshooting
Identifying and controlling fungus gnats on cannabis
Small black flies over the substrate and glossy larvae in moist soil are the leading symptoms. Here's how to fight them at the root of the problem: through your watering regime.
Diagnosis & Troubleshooting
Recognizing and controlling aphids on cannabis
Soft, often green insects in colonies on shoot tips as well as sticky honeydew reveal aphid infestation. This is how you recognize the symbiosis with ants and stop the reproduction early.
Diagnosis & Troubleshooting
Identifying and controlling whiteflies on cannabis
Small white insects that fly up in a cloud when the plant is touched are the key symptom. This lets you recognize the infestation early and prevent it from spreading via honeydew and sooty mold.
Diagnosis & Troubleshooting
Identifying and controlling hemp russet mites on cannabis
Yellowed, downward-curled leaves and a dull, dusty appearance of the upper canopy indicate russet mite infestation. Here's how to recognize the tiny pests before the infestation becomes irreversible.
Diagnosis & Troubleshooting
Recognizing and preventing bud rot (Botrytis) in cannabis
Gray, cottony rot inside the bud with wilted individual leaflets is the warning sign. Bud rot is not curable — this climate strategy prevents it before it breaks out.
Diagnosis & Troubleshooting
Identifying and controlling powdery mildew on cannabis
White, powdery spots on the upper leaf surface are the key symptom. Unlike most fungi, mildew doesn't need moisture — this climate and hygiene strategy stops it.
Diagnosis & Troubleshooting
Relative humidity problems in cannabis: diagnosing too high and too low
Too high RH opens a window for mold and mildew, too low RH forces stomatal closure and transpiration-related deficiencies. Here's how you read relative humidity through the VPD window of the respective phase and correct it in a targeted way.
Diagnosis & Troubleshooting
Recognizing and managing CO₂ deficiency and excess in cannabis
Under strong lighting, CO₂ becomes the limiting factor: stagnating photosynthesis despite optimal light, climate, and nutrients points to CO₂ deficiency. This is how you recognize the limit and use supplementation safely and sensibly.
Growing & Harvest
Using Cal-Mag supplementation correctly in cannabis
Not every setup needs Cal-Mag — but with reverse osmosis water or coco coir, an additive is almost always necessary. This is how you recognize the real need and dose correctly instead of supplementing across the board.