Common Pests and Diseases: Identification and Treatment

Pest and disease pressure is a reality of cannabis cultivation regardless of how carefully you set up your grow environment. The key is early detection — problems identified and addressed at their onset are manageable; problems allowed to develop undetected often become catastrophic.

SPIDER MITES

The most common pest in indoor cannabis cultivation. Spider mites are tiny arachnids (not insects) that feed on plant cells, causing stippling damage visible as tiny white or yellow dots on leaf surfaces. Advanced infestations produce visible webbing between leaves and stems. Check the underside of leaves — this is where mites and their eggs accumulate.

Treatment: miticides specific to spider mites (spinosad, neem oil, predatory mites such as Phytoseiulus persimilis), combined with environmental control (mites thrive in hot, dry conditions — raising humidity above 60% and reducing temperature inhibits reproduction).

FUNGUS GNATS

Fungus gnats are small flies whose larvae live in growing medium and feed on roots, causing growth stunting and increasing vulnerability to root disease. Adults are visible flying near the soil surface. Yellow sticky traps capture adults and help monitor population levels.

Treatment: allow growing medium to dry between waterings (larvae cannot survive in dry conditions), apply beneficial nematodes (Steinernema feltiae) to the medium, use yellow sticky traps. Bacillus thuringiensis israelensis (BTi) in irrigation water targets larvae specifically.

POWDERY MILDEW

A fungal pathogen that appears as white, powdery patches on leaf surfaces. Thrives in conditions of high humidity, low air circulation, and moderate temperatures. Once established, it spreads rapidly through an indoor environment.

Treatment: remove affected leaves immediately, improve air circulation, reduce humidity below 50%, apply potassium bicarbonate solution or diluted hydrogen peroxide spray to affected areas. In serious infestations, fungicide sprays (sulfur, copper-based) may be needed. Avoid spraying in late flowering — spray residue affects aroma and flavour.

BOTRYTIS (BUD ROT)

The grower's worst nightmare in flowering: Botrytis cinerea ("grey mould") develops inside dense bud structures in humid conditions, turning inner bud tissue grey-brown before becoming visible externally. By the time it is spotted, the infection is usually advanced. Prevention is everything: maintain flowering humidity below 50%, ensure strong air circulation through and under the canopy, and physically separate touching buds when possible.

NUTRIENT DEFICIENCIES

Often confused with pests, nutrient deficiencies produce characteristic yellowing, browning, or spotting patterns. Nitrogen deficiency: progressive yellowing of lower leaves moving upward. Iron/manganese deficiency: interveinal chlorosis (yellowing between veins) on new growth. Calcium deficiency: brown spots on leaves, tip burn, weak stems. Always check and correct pH before adding more nutrients — most apparent deficiencies are lock-out caused by incorrect pH.