Rust-brown spots on newer and middle leaves

Likely causes, most likely first

Ranked for this sign on its own. Your stage, medium and readings change the order, so run the checker for your plant.

1. PossibleCalcium deficiency (often a delivery problem)

   NutrientDeal with it this weekRead: Calcium (Ca) in cannabis: deficiency, transport, targets and sources

   Fix: Rule out mites with a 30-60x lens first. Then fix delivery: leaf VPD 0.8-1.5 kPa (lights-on RH not stuck high), airflow under the canopy, root-zone pH not below ~5.8 in coco. If supply is short (RO water, unbuffered coco), bring Ca to about 150-200 mg/L with calcium nitrate or a Cal-Mag at label rate. LEDs do not create a Cal-Mag need; that is a myth.

   Recovery: Leaves that open 7-10 days after the fix are flat and spot-free; old spots stay.

   How to confirm it

   • Magnifier check of leaf undersides and new growth. Use a 10-20x loupe for spider mites, thrips, aphids and whiteflies, and a 30-60x lens or clip-on phone macro for broad and russet mites. Check the undersides of 10 affected leaves across several plants and the newest shoot tips, in white light. Look for moving dots, eggs, shed skins, webbing, slender insects with black specks, and glassy specks on new growth.
   • Measure input pH and root-zone (runoff or reservoir) pH. Calibrate the meter with fresh pH 7 and pH 4 buffers, then measure the mixed feed and the runoff (containers) or reservoir (hydro). Soil and living soil should sit at 6.0-7.0, coco, rockwool and peat mixes at 5.5-6.5, hydro at 5.5-6.2. Track 3+ readings; one reading is not a trend.
   • Get a water report (or test your water). Ask your water utility for the annual report or test well water: alkalinity (mg/L CaCO3), pH, EC, Ca, Mg, Na, Cl, B, and whether it uses chlorine or chloramine. Alkalinity above ~100-150 mg/L as CaCO3 pushes root-zone pH up; sodium above ~50 mg/L and chloride above ~70 mg/L are a problem; softened water is high in sodium.
   • Check temperature and RH at canopy height with a calibrated sensor. Put a calibrated sensor (salt test: a sealed bag with damp table salt reads 75% RH after 8-12 hours) at canopy height, out of direct light, and log for 24 hours including lights-off. Look for lights-off RH spikes and dew point.
   • Watch new growth for 5-10 days after one change. Make one correction at a time, mark the top node with a twist tie and photograph the same leaves every 2-3 days. Damaged leaves never recover; judge only new growth and whether the symptom stops spreading. Mobile-nutrient symptoms stop climbing within 5-7 days; immobile ones show in new leaves within 7-10 days.

2. PossibleLow root-zone pH starving calcium and magnesium

   pHDeal with it this weekRead: pH and nutrient availability: what pH really controls, by medium

   Fix: Raise root-zone pH to range: coco 5.8-6.2, hydro 5.6-6.0, soil 6.2-6.8. Use pH up (potassium hydroxide or carbonate) in small steps; in coco, feed at 6.0-6.2 for a few waterings with Ca and Mg on board. A fast fall in hydro points to dying roots or microbes; check them.

   Recovery: pH holds in range for a week; new leaves spot-free within 7-10 days.

   How to confirm it

   • Measure input pH and root-zone (runoff or reservoir) pH. Calibrate the meter with fresh pH 7 and pH 4 buffers, then measure the mixed feed and the runoff (containers) or reservoir (hydro). Soil and living soil should sit at 6.0-7.0, coco, rockwool and peat mixes at 5.5-6.5, hydro at 5.5-6.2. Track 3+ readings; one reading is not a trend.
   • Check the meters against fresh standards. Put the pH probe in fresh pH 7.0 and 4.0 buffer and the EC probe in a 1.413 mS/cm standard. If pH reads more than 0.1-0.2 off, recalibrate; if it can't be calibrated or the slope is below ~90-95%, replace the probe. Many 'lockouts' are a drifting meter.
   • Pour-through extraction for container pH and EC. Water normally, wait 30 minutes to 2 hours, pour enough pH-adjusted water on top to collect about 50 mL of leachate (roughly 75 mL for a 15 cm pot, 150 mL for a 4 L pot, 350 mL for 11 L), and measure pH and EC of that 50 mL. Less disruptive than full runoff and more repeatable.

3. Less likelyUnbuffered or new coco pulling calcium and magnesium

   Root zoneDeal with it this weekRead: Buffering coco coir: why it matters and how to do it

   Fix: Feed with Ca and Mg on board: calcium nitrate and magnesium sulfate, or a Cal-Mag, bringing the feed to about 150-200 mg/L Ca and 40-60 mg/L Mg, at pH 5.8-6.2. For the next batch, rinse and buffer coco by soaking it in a calcium (and magnesium) nitrate solution before planting.

   Recovery: New leaves clean within 7-10 days; runoff Ca/Mg symptoms stop spreading.

   How to confirm it

   • Measure runoff EC against input EC. Water to 10-20% runoff, collect the runoff in a clean cup and measure EC of both with a calibrated meter (1.413 mS/cm standard). Runoff more than ~0.5 mS/cm above input means salt build-up; well below input means the plant is hungry or you are over-leaching. In hydro, compare reservoir EC with the EC you mixed.
   • Measure input pH and root-zone (runoff or reservoir) pH. Calibrate the meter with fresh pH 7 and pH 4 buffers, then measure the mixed feed and the runoff (containers) or reservoir (hydro). Soil and living soil should sit at 6.0-7.0, coco, rockwool and peat mixes at 5.5-6.5, hydro at 5.5-6.2. Track 3+ readings; one reading is not a trend.
   • Get a water report (or test your water). Ask your water utility for the annual report or test well water: alkalinity (mg/L CaCO3), pH, EC, Ca, Mg, Na, Cl, B, and whether it uses chlorine or chloramine. Alkalinity above ~100-150 mg/L as CaCO3 pushes root-zone pH up; sodium above ~50 mg/L and chloride above ~70 mg/L are a problem; softened water is high in sodium.

4. Less likelyUncalibrated or failing pH or EC meter

   Physical damageDeal with it this weekRead: pH and EC meter calibration: buffers, slope, storage and when to replace a probe

   Fix: Calibrate the pH meter with fresh pH 7 then pH 4 buffer before decisions and at least weekly; store the probe wet in storage solution, never in RO water. Check EC against 1.413 mS/cm. Replace probes that won't calibrate (typical life 1-2 years). Drops and strips are too coarse to diagnose with.

   Recovery: Readings match the buffers within 0.1 pH; the 'problem' pH disappears or is confirmed.

   How to confirm it

   • Check the meters against fresh standards. Put the pH probe in fresh pH 7.0 and 4.0 buffer and the EC probe in a 1.413 mS/cm standard. If pH reads more than 0.1-0.2 off, recalibrate; if it can't be calibrated or the slope is below ~90-95%, replace the probe. Many 'lockouts' are a drifting meter.

5. Less likelyTwospotted spider mites

   PestAct within a day or twoRead: Spider mites on cannabis: life tables, ID and IPM

   Fix: Act now: they breed fastest hot and dry. Remove the worst leaves into a sealed bag, cool the room and raise RH a little (in veg), release predatory mites (Phytoseiulus persimilis for hot spots, Neoseiulus californicus as a preventive) before numbers explode. Soaps or horticultural oils only in veg, never on buds. Quarantine new clones.

   Recovery: No new stippling on new leaves and no live mites at 10-20x after 7-14 days.

   How to confirm it

   • Magnifier check of leaf undersides and new growth. Use a 10-20x loupe for spider mites, thrips, aphids and whiteflies, and a 30-60x lens or clip-on phone macro for broad and russet mites. Check the undersides of 10 affected leaves across several plants and the newest shoot tips, in white light. Look for moving dots, eggs, shed skins, webbing, slender insects with black specks, and glassy specks on new growth.
   • Tap a branch over white paper. Hold white paper under a branch or bud and tap it sharply 3-4 times. Watch the paper for 30 seconds: moving specks are mites or thrips (smear them; spider mites leave red-brown or green streaks), larger fliers are whiteflies. A quick whole-room screen before the magnifier.

6. Less likelyHumidity too high / VPD too low

   EnvironmentAct within a day or twoRead: Humidity by stage: RH targets from clone to cure

   Fix: Target about 55-70% RH in veg, 50-60% early flower, 40-50% late flower (leaf VPD 1.0-1.5 kPa in flower). Dehumidify, raise exhaust, keep air moving under the canopy and watch the lights-off spike (dew point). High RH in flower raised bud-rot risk and in one study delayed flowering and lowered cannabinoids.

   Recovery: RH holds in band through lights-off for 3+ days; no new mould signs.

   How to confirm it

   • Check temperature and RH at canopy height with a calibrated sensor. Put a calibrated sensor (salt test: a sealed bag with damp table salt reads 75% RH after 8-12 hours) at canopy height, out of direct light, and log for 24 hours including lights-off. Look for lights-off RH spikes and dew point.

7. Less likelySpray or foliar burn (pesticide, oil, soap, foliar feed)

   Physical damageLow urgencyRead: What can I safely spray on plants in flower?

   Fix: Stop spraying; rinse residue off with plain water at lights-off if damage is fresh. In future spray only at lights-off or with lights raised, test one leaf 24-48 h first, never mix sulfur with oils, and never spray buds.

   Recovery: No new damage on leaves that grow after the spray.

   How to confirm it

   • Magnifier check of leaf undersides and new growth. Use a 10-20x loupe for spider mites, thrips, aphids and whiteflies, and a 30-60x lens or clip-on phone macro for broad and russet mites. Check the undersides of 10 affected leaves across several plants and the newest shoot tips, in white light. Look for moving dots, eggs, shed skins, webbing, slender insects with black specks, and glassy specks on new growth.
   • Wipe test for white coatings. Wipe a white spot gently with a damp finger or cotton bud and mark the leaf. Powdery mildew comes off as powder and reappears in 2-5 days on the same and nearby leaves; spray or hard-water residue wipes off and stays gone; trichomes and thrips scars do not wipe off. Wash hands afterwards: spores travel on fingers.

8. Less likelyAmmonium toxicity (too much NH4 in the feed)

   Too much of somethingDeal with it this weekRead: Nitrogen (N) in cannabis: nitrate vs ammonium, deficiency, toxicity and targets

   Fix: Keep ammonium at or below about 10-30% of total N (cannabis trials showed toxicity and lost yield above ~30%); switch to a mostly nitrate feed and correct root-zone pH.

   Recovery: pH stops falling and new leaves lose the dark claw within a week.

   How to confirm it

   • Measure input pH and root-zone (runoff or reservoir) pH. Calibrate the meter with fresh pH 7 and pH 4 buffers, then measure the mixed feed and the runoff (containers) or reservoir (hydro). Soil and living soil should sit at 6.0-7.0, coco, rockwool and peat mixes at 5.5-6.5, hydro at 5.5-6.2. Track 3+ readings; one reading is not a trend.
   • Look at the roots. Hydro: lift the net pot and look at the root mass in good white light. Containers: tip the plant out with a hand over the medium and look at the edges and bottom of the root ball. Healthy roots are white to cream with fine fuzzy tips. Brown roots whose outer layer slides off between the fingers leave a bare thread: rot. Firm, evenly tan roots with white tips in coloured solution are staining.

Look for

Irregular brown or rust spots with yellowish halo between veins on upper-middle leaves; check the underside for mites..