Health Library

Powdery mildew is one of the most common fungal diseases in cannabis — and one of the sneakiest. It's caused by obligate parasitic fungi that thrive in humid air (above 55% RH) with poor circulation, but here's what trips people up: unlike most fungi, PM doesn't need wet leaves to infect. It actually prefers dry leaf surfaces with moisture in the surrounding air. Spores spread easily through air currents, on clothing, or via contaminated tools, and a single colony can spread through an entire grow room within a couple of weeks if left unchecked. The easiest way to tell PM apart from trichomes is the wipe test — PM rubs off cleanly with a finger, while trichomes are sticky and don't wipe away. If you see white patches that smear when you touch them, you've got powdery mildew. It's especially dangerous during flower because infected buds are unsafe to consume.

Bud rot (Botrytis cinerea) is one of the most destructive diseases you'll encounter during flowering. The critical thing to understand is that it starts from the inside of the bud and works outward — by the time you spot external signs, the interior is already compromised. It thrives in dense, tightly packed colas where humidity gets trapped, especially when airflow is poor and RH stays above 60%. Caterpillar damage is a major entry point — the wounds and frass they leave behind create perfect conditions for Botrytis spores to germinate, so if you're finding caterpillars, inspect those buds first. Unlike powdery mildew which sits on leaf surfaces and can sometimes be treated in place, bud rot penetrates deep into tissue and any infected material must be completely removed. Infected buds are unsafe to consume due to fungal toxins (botrydial, botcinic acid) and spore contamination — this applies to buds that were directly adjacent to infected ones too, since they likely harbor spores even if they look clean.

Root rot is caused by pathogens — primarily Pythium and Fusarium — that thrive in waterlogged, oxygen-depleted root zones. It's the end result of chronic overwatering, poor drainage, or root zone temperatures above 72F/22C. The key distinction from simple overwatering: overwatered roots are still white and firm (just sitting in too much water), while root rot roots are brown, slimy, and smell terrible. Fungus gnat larvae make things worse — they chew on roots and create entry points for Pythium, so if you're seeing gnats, check for rot too. Seedlings and freshly transplanted plants are especially vulnerable in the first three weeks — they can collapse at the soil line overnight (damping off). In hydro systems, root rot spreads through the reservoir fast and can take out an entire grow. Early intervention is critical because once the root mass is mostly brown and mushy, the plant rarely recovers.

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.