Health Library

Calcium deficiency shows up as brown or rust-colored spots on new growth, usually alongside distorted or crinkled young leaves. Because calcium is immobile — once it's deposited in cell walls, the plant can't move it — new growth always gets hit first. Typically that means the top of the canopy, though during flowering, symptoms can appear on lower fan leaves first as the plant prioritizes calcium toward developing buds (Gali et al. 2023). The transpiration dependence is what makes calcium tricky: even if your nutrient solution has plenty of Ca, low humidity, cold root zones, or still air can limit delivery. It's extremely common in coco coir (which naturally binds calcium), soft water areas, and RO water setups without supplementation. LED and HID growers often run into calcium issues because intense lighting drives faster growth without necessarily increasing transpiration to match. Unlike magnesium or iron deficiency which show clean interveinal yellowing, calcium produces brown spots combined with structural distortion — twisted leaves, cupped edges, and eventually hollow stems. That hollow stem symptom is unique to calcium and a reliable identifier.

Manganese is immobile, so deficiency always hits new upper growth first. You'll see a mottled, speckled interveinal chlorosis on young leaves — it's messier and less uniform than the clean vein-to-tissue pattern you get with iron. The key differentiator is spots: Mn produces tan or brown necrotic spots early on, while iron deficiency does not produce spots at all. As it progresses, Mn yellowing turns tan and brown, whereas Fe yellowing bleaches toward white or gray. You'll also notice a general size reduction in leaves, shoots, and buds. High pH (above 7.0 in soil, 6.5 in coco, 6.1 in hydro) is the most common trigger — Mn locks out quickly in alkaline conditions. Excess iron can also cause Mn deficiency through Fe–Mn antagonism, so heavy iron supplementation without balancing Mn is a common secondary cause.

Spider mites are tiny arachnids — not insects — that feed from the undersides of leaves by piercing individual cells and sucking out the contents. Each puncture destroys one cell's worth of chlorophyll, so the damage shows up on the leaf tops as tiny pale speckles called stippling. They thrive in hot, dry conditions and reproduce explosively — a handful of mites can become thousands in under two weeks. Two-spotted spider mites (Tetranychus urticae) are by far the most common species on cannabis. They're usually introduced on clones, clothing, or pets, and they're so small you won't notice them until the stippling gives them away. Don't confuse stippling with thrips damage — mites leave tiny dots in a scattered pattern, while thrips leave silver streaks along the veins. By the time you see webbing between branches, you're dealing with a serious colony that needs immediate action.

Thrips are tiny, slender insects that rasp leaf surfaces and suck out cell contents, creating distinctive silvery or bronze streaks and patches. The adults are fast-moving and hard to see without magnification, but the pale, worm-like nymphs are slow and easy to miss — they blend right into the leaf surface. Thrips can fly short distances and spread rapidly through grow rooms. They leave behind tiny black dots of frass (feces) alongside their feeding damage. Here's the critical differentiator: thrips create silver STREAKS and TRAILS — don't confuse this with spider mite stippling, which looks like scattered DOTS. The black frass dots alongside silver streaks are the giveaway. What makes thrips especially frustrating is their life cycle — eggs are laid INSIDE leaf tissue where sprays can't reach them, and pupae develop in the soil rather than on the plant. This means you're fighting them on two fronts at once. Silver scars from feeding damage are permanent, so the goal is always to prevent new damage rather than trying to fix what's already done. Some species also transmit plant viruses, making early intervention critical.

Leaf septoria — also called yellow leaf spot — is a fungal disease caused by Septoria cannabis that primarily hits outdoor plants during mild, humid weather (optimal 15-27°C / 59-80°F). It's one of the most common foliar diseases you'll see on cannabis, but the good news is it's rarely a serious threat. The fungus spreads through water splash from rain or overhead irrigation, carrying spores from infected soil or debris onto lower leaves first. The telltale sign is circular spots with dark borders and — critically — tiny black dots (pycnidia) in the center of mature spots. Those black dots are the fungal fruiting bodies and they're your definitive ID. Don't confuse septoria with calcium deficiency — Ca def spots appear on new upper growth, while septoria always starts on the oldest lower leaves near the soil. Spores overwinter in plant debris, so end-of-season cleanup is essential to break the cycle. While septoria reduces vigor through leaf loss, no confirmed cannabinoid yield losses have been documented in peer-reviewed studies — it's more of a nuisance than a killer.