Pests and Disease

The growers who never deal with serious pest or disease problems do not spray more frequently than other growers — they create conditions where pests and disease cannot get a foothold. The fundamentals: clean grow space (wipe down tent walls and floors regularly, remove dead organic matter), proper VPD and humidity, strong airflow through and under the canopy, and strict intake air management.

Intake air brings in everything the outside environment contains — dust, spores, insects, and mites. Filtered intake air is significantly safer than unfiltered. A HEPA filter on your intake fan adds meaningful protection, particularly in environments with outdoor plants nearby.

Never bring outdoor plants into an indoor grow space without quarantine and inspection. Pest colonies start from a single infected plant or branch, and they spread to everything in the same environment within days to weeks.

Spider mites are the most destructive common pest for cannabis. They thrive in hot, dry, low-humidity environments (which is why experienced growers maintain proper VPD rather than running excessively dry). They reproduce very rapidly — a population can double every three to five days under optimal conditions — and become increasingly difficult to control as resistance to treatments develops.

Identification: look for stippling (tiny yellow or white dots) on the tops of leaves, and webbing on the undersides of leaves and between branches. By the time you see visible webbing, the population is already significant. Early detection requires inspecting leaf undersides with a jeweler's loupe or magnifying glass — you will see tiny, slow-moving eight-legged specks.

Treatment: miticides (Avid, spinosad) are the most effective options. Insecticidal soap and neem oil provide some control but need to be applied every 2–3 days to break the reproduction cycle. In severe infestations, rotating miticide classes is important because resistance develops quickly. Never spray anything in the final 3–4 weeks before harvest.

Fungus gnats are the most common pest in soil grows. Adult gnats are a minor nuisance, but their larvae live in the top few inches of soil and feed on root hairs and organic matter, damaging the root zone and creating entry points for pathogens.

The root cause of fungus gnat infestations is consistently overwatering. The top layer of wet soil is the ideal environment for egg laying. Fix the watering first: allow the top 2–3cm of medium to dry completely between waterings. This alone eliminates most gnat problems. Fabric pots, which dry more evenly from the sides, are significantly less susceptible than plastic pots.

Direct treatments: Bacillus thuringiensis var. israelensis (BTi) — sold as Mosquito Dunks or Gnatrol — is a biological control that kills larvae in the soil without harming plants or beneficial microbes. Yellow sticky traps reduce adult populations. Diatomaceous earth (food grade) applied to the top of the medium dehydrates larvae on contact.

Powdery mildew appears as white, talcum-powder-like spots on leaves — most commonly on the tops. It is a fungal infection that spreads through airborne spores and thrives in conditions of low airflow, moderate humidity (40–70%), and moderate temperatures. Unlike many molds, PM can grow in relatively low humidity.

Early-stage PM can be treated with potassium bicarbonate, diluted hydrogen peroxide spray, or neem oil. The spray raises the leaf surface pH, making it inhospitable to the fungus. You need to treat every 3–5 days for several weeks to eliminate an outbreak, and you will likely not eradicate it entirely in an established plant — only manage it.

Prevention is vastly more effective: strong circulation fans that prevent stagnant air pockets, maintaining RH below 50% during flowering, removing dead leaves promptly, and keeping canopy density manageable so air can move freely through it.

Botrytis cinerea — bud rot — is the most serious disease risk for cannabis in late flower. It is a necrotrophic mold that grows inside dense buds, destroying them from within. By the time it is visible from outside, significant internal damage has already occurred. Affected material appears grayish-purple and falls apart at the touch.

Botrytis requires high humidity at the bud surface, which is difficult to prevent inside dense, tightly packed colas in late flower. Conditions that promote it: RH above 55% in a warm grow space, poor airflow through and under the canopy, physical damage to buds (mechanical, insect, or other), and water sitting on bud surfaces from condensation or watering splash.

There is no effective cure for botrytis in cannabis. Affected buds must be cut and removed immediately, well below the infected zone, and the infected material disposed of outside the grow space. Spores spread easily. After removal, increase airflow, reduce humidity aggressively (below 50% RH), and inspect remaining plants daily.

Root rot — typically caused by Pythium or similar water molds — is a hydro and coco grower's primary disease risk. It thrives in warm, stagnant, poorly oxygenated water. Symptoms: brown, slimy, mushy roots (vs white, firm healthy roots), rapid decline in plant health, and a distinctively bad smell from the reservoir.

Prevention in hydro: keep reservoir water below 68°F (20°C), maintain strong aeration (air stones, recirculating flow), keep reservoir and equipment clean and sanitized, and add beneficial microbes (Hydroguard, Cannazym, or similar Bacillus-based products) that outcompete pathogenic microbes.

Treatment once established is difficult. Hydrogen peroxide (3–5ml of 3% H2O2 per liter of reservoir water) kills Pythium but also kills beneficial microbes and can damage roots. Reapply beneficial bacteria after any H2O2 treatment. In severe cases, full system teardown and sterilization is the only reliable fix.