Cannabis Pests and Diseases: Identification and Treatment

Common Insect Pests: Identification and Treatment

Spider mites are the most feared cannabis pest, and for good reason. These tiny arachnids (barely visible to the naked eye) colonize the undersides of leaves, piercing cells to feed on chlorophyll. Early signs include tiny yellow speckles on leaf surfaces (stippling) and microscopic webbing on the underside of leaves. By the time you see obvious webs, the infestation is severe. Spider mites thrive in hot, dry conditions (above 27 degrees Celsius with low humidity) and reproduce explosively — a single female can produce millions of offspring in a month. Treatment for early infestations includes neem oil sprays, insecticidal soap, or predatory mites (Phytoseiulus persimilis) that feed on spider mites. For severe infestations, products containing spinosad or pyrethrin are effective. Always spray the undersides of leaves and repeat treatment every 3-5 days for at least three rounds to break the reproductive cycle. Fungus gnats are annoying more than destructive in small numbers. The adults are tiny black flies that hover around the soil surface. Their larvae live in the top layer of moist soil and feed on organic matter and fine root hairs. Let the soil surface dry completely between waterings to break their lifecycle. Yellow sticky traps catch adults, and a top dressing of diatomaceous earth or sand prevents females from laying eggs. BTi (Bacillus thuringiensis israelensis), available as mosquito dunks or Gnatrol, kills larvae effectively when applied as a soil drench. Thrips are slender insects that scrape leaf surfaces, leaving silvery streaks and tiny black dots of excrement. They are fast-moving and hide in crevices. Spinosad sprays are the most effective organic treatment. Aphids cluster on stems and new growth, sucking sap and excreting sticky honeydew. Strong water sprays knock them off, and neem oil or ladybug releases provide ongoing control.

Fungal Diseases: Powdery Mildew and Bud Rot

Powdery mildew (PM) appears as white, powdery patches on leaf surfaces and stems. It thrives in moderate temperatures with high humidity and poor airflow — exactly the conditions found in crowded, poorly ventilated grow spaces. PM does not require free water on leaf surfaces to germinate, unlike most fungi; it grows in ambient humidity above 55 percent. Once established, it spreads rapidly through airborne spores. Prevention is far easier than treatment: maintain strong airflow with oscillating fans, keep humidity below 55 percent during flowering, space plants adequately, and defoliate to improve air circulation. If PM appears, remove affected leaves immediately and bag them rather than dropping them in the grow space. Organic treatments include potassium bicarbonate sprays, diluted milk sprays (1 part milk to 9 parts water), or neem oil applications. These work best as preventatives or for early infections. Severe PM infections during flowering may require you to cull affected buds to save the rest. Bud rot (Botrytis cinerea) is the grower's nightmare, capable of destroying an entire harvest in days. It starts inside dense buds where moisture gets trapped, appearing first as a discolored or wilting sugar leaf poking out of an otherwise healthy bud. Pull back the outer leaves and you will find gray, fuzzy mold consuming the interior. By the time bud rot is visible from outside, it has already destroyed the affected bud and potentially spread spores to nearby flowers. The only treatment is removal — cut out the affected bud plus at least an inch of healthy tissue on all sides, sterilize your scissors between cuts, and immediately increase airflow and reduce humidity. If bud rot is widespread, consider an early harvest of all remaining healthy buds to salvage what you can. Prevention means keeping flowering humidity below 50 percent, ensuring good airflow through the canopy, and avoiding getting water on buds.

Nutrient Deficiencies and Toxicities

Nutrient problems are the most common issues growers face, and they are almost always caused by incorrect pH rather than actually lacking nutrients in the soil. When pH drifts outside the optimal range, certain nutrients become chemically unavailable to roots even if they are present in abundance. For soil grows, maintain pH between 6.0 and 7.0; for coco and hydro, between 5.5 and 6.5. Always check and correct pH before diagnosing a nutrient deficiency. Nitrogen deficiency is characterized by yellowing that starts at the bottom of the plant and moves upward as the plant cannibalizes older leaves to feed new growth. This is normal during late flowering but problematic during veg. Increase nitrogen feeding or check pH. Phosphorus deficiency shows as dark green or purpling leaves with brown spots, often starting on lower leaves. The plant may appear stunted with slow bud development. Potassium deficiency presents as brown, crispy leaf edges and tips, moving from lower to upper leaves. Calcium deficiency causes brown spots on newer growth and can lead to blossom end rot in developing buds. It is common in coco grows and under LEDs. Magnesium deficiency appears as interveinal chlorosis — the areas between leaf veins turn yellow while the veins stay green, typically starting on middle-aged leaves. Cal-mag supplements resolve both calcium and magnesium issues. Nutrient toxicity (overfeeding) shows as dark green leaves with tips that curl downward and tips that turn brown and crispy. The universal fix is to flush the medium with plain pH-balanced water at three times the pot volume and resume feeding at a lower concentration. Keep a deficiency identification chart in your grow room for quick reference.

Root Zone Problems and Environmental Stress

Root rot is a devastating condition caused by Pythium and other water-borne pathogens that attack roots in overwatered or poorly oxygenated conditions. Symptoms include wilting despite wet soil, slow growth, and a foul smell from the root zone. If you can inspect the roots, healthy roots are white and firm while rotted roots are brown, slimy, and fragile. Prevention is the best approach: use well-draining media, avoid overwatering, maintain root zone temperatures below 22 degrees Celsius, and inoculate with beneficial microbes like Trichoderma. Treatment involves letting the medium dry out, removing any dead root material, and applying a hydrogen peroxide drench (3 percent solution at 1 tablespoon per gallon) or a beneficial microbe product like Hydroguard. In hydroponic systems, root rot can be prevented by maintaining dissolved oxygen levels with air stones and keeping reservoir temperatures below 20 degrees Celsius. Heat stress occurs when temperatures exceed 30 degrees Celsius. Leaves curl upward (taco) along the edges, and upper fan leaves may develop bleached or yellow spots. In extreme cases, foxtailing occurs during flowering, where new calyxes grow from the tops of mature buds in spire-like formations that reduce density and quality. Lower the temperature by increasing exhaust fan speed, adding air conditioning, or raising lights. Light stress shares similar symptoms and results from too much light intensity at the canopy. Some modern LEDs are powerful enough to overload cannabis even at moderate hanging heights. Use a PPFD meter to verify canopy light levels and dim or raise the light as needed. Wind burn from fans blowing too directly on plants causes leaf edges to curl and clamp downward. Redirect fans for gentle, indirect circulation rather than a constant direct blast.

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