Pests & diseases – recognise and understand

TL;DR

• Spider mites (Tetranychus urticae): tiny dots, fine webbing, explode at >26 °C and <50 % RH.
• Botrytis (grey mould): bud rot in dense flowers – prevention via climate & airflow.
• Powdery mildew (Podosphaera): white coating on leaves – high RH + stagnation.
• IPM > cure: prevention (climate, hygiene, beneficials) beats any late intervention.

Integrated Pest Management (IPM)

IPM = monitoring (sticky traps, loupe), cultural practice (climate, hygiene), biological control (predatory mites, nematodes) and minimal targeted chemistry. In flower, chemical fungicides/insecticides are largely off-limits – residues end up in the final product.

Pests and fungal diseases: prevention beats control

The relevant problems are manageable: spider mites, thrips, fungus gnats, and whiteflies among insects, powdery mildew and botrytis (grey mould) among fungi. All four insect species are almost always introduced — via cuttings, substrate, or clothing. Therefore, quarantine for new plant material is the most effective single measure of all, followed by climate control: spider mites love dry and warm conditions, botrytis loves damp and cool conditions.

Botrytis is the most dangerous infestation because it starts inside the bud and only becomes visible when the bud is already lost. Infested material cannot be saved and must be generously removed — drying or cutting it out eliminates neither spores nor the risk of mycotoxins. Mildew, on the other hand, is a surface problem and is easily manageable through air movement, lower plant density, and lower humidity.

• Keep new plant material separate for at least 10 days
• Yellow sticky traps as an early warning system, weekly inspection of the undersides of leaves
• Late flowering: no more foliar treatments — residues remain in the product
• Generously remove affected flowers instead of cutting them out locally

Residue issues and Swiss practice

In licensed Swiss production, the use of plant protection products during flowering is virtually not an option because residue analyses are part of the batch release. Facilities therefore work with beneficial organisms such as predatory mites and parasitic wasps, as well as strict climate control. For consumers, the residue test in the certificate of analysis is the only reliable proof that this approach was actually followed.

In Switzerland, the total THC value (THC + THCA × 0.877) alone determines whether a plant is considered industrial hemp or a narcotic. Below 1 %, cultivation is generally permitted; above that, only within the framework of an authorised pilot project under BetmG Art. 8b or with a medical prescription under Art. 8a. Anyone who cultivates themselves bears the burden of proof for the value — the analysis of the harvested batch is decisive, not the variety specification of the seed dealer.

Swiss approval regulations for plant protection products

In Switzerland, the use of plant protection products in cannabis cultivation is subject to strict legal requirements. For the commercial cultivation of medical cannabis or within the framework of pilot trials authorised by the Federal Office of Public Health (BAG), the guidelines on Good Agricultural and Collection Practice (GACP) apply. Since cannabis flowers are directly consumed or extracted, the list of approved active substances is extremely limited. The Federal Food Safety and Veterinary Office (BLV) and Swissmedic monitor compliance with strict limit values. Any illegal use of pesticides inevitably leads to the forfeiture of the entire batch and can result in legal consequences.

For the private cultivation of CBD hemp with a total THC value of less than one percent, the general provisions of chemicals and plant protection legislation also apply. Many agents approved in conventional horticulture are unsuitable for plants whose flowers are smoked or vaporised, as toxic pyrolysis products can be formed when heated. Swiss producers must therefore primarily rely on physical barriers, beneficial insects, and preparations approved by the BLV for organic farming. Systematic risk management is essential to guarantee that the end product is safe for health.

• Strict compliance with GACP guidelines for medical cannabis and in pilot projects.
• Monitoring of residues by Swissmedic and the Federal Food Safety and Veterinary Office (BLV).
• Ban on systemic pesticides due to unpredictable pyrolysis products during consumption.
• Focus on organic inputs approved for Swiss organic farming.

Diagnostic procedures and sources of error in practice

A precise analysis of damage symptoms is the basic prerequisite for any successful intervention in cannabis cultivation. Inexperienced growers frequently mistake physiological disorders for pest infestation or fungal disease. For example, the visual symptoms of over-fertilisation or an incorrect pH value in the substrate often resemble the damage patterns of thrips or spider mites in the early stages. Similarly, burns caused by overly intense LED lighting or insufficient air circulation are sometimes falsely interpreted as powdery mildew. A misdiagnosis usually leads to incorrect countermeasures, which put additional stress on the weakened plant and reduce the yield.

To reliably identify pests, the use of a magnifying glass with at least forty-fold magnification or a digital pocket microscope is absolutely essential. Only upon close inspection can the tiny eggs of spider mites on the undersides of leaves or the larvae of thrips in the tissue be reliably identified. Yellow and blue sticky traps do not serve as a control method, but rather as indicators of the population density in the cultivation room. Anyone who rushes the diagnosis and initiates plant protection measures without microscopic confirmation risks wasted investment and a prolongation of the actual problem.

• Use of pocket microscopes for the unambiguous identification of eggs and larvae.
• Distinguishing nutrient deficiencies and light stress from actual pest infestation.
• Targeted use of blue sticky traps for the early detection of thrips, and yellow sticky traps for fungus gnats.
• Regular documentation of the upper and lower leaf surfaces on a weekly basis.

Beneficial organisms in detail

The use of beneficial organisms represents the primary method of pest control in professional Swiss cannabis cultivation. Predatory mites such as Phytoseiulus persimilis are highly specialised natural enemies of the two-spotted spider mite and can completely contain an initial infestation under optimal conditions. It is crucial to pay attention to climatic parameters here, as these beneficials require a relative humidity of over sixty percent and moderate temperatures to actively hunt. If the indoor climate is too dry, the predatory mite population collapses, while the spider mites multiply rapidly.

For controlling fungus gnats in the substrate, beneficial nematodes of the genus Steinernema and predatory mites of the species Hypoaspis miles have proven successful. These organisms penetrate the soil and parasitise the larvae of the pests without damaging the roots of the cannabis plant. Since beneficial organisms work most effectively preventatively or under low infestation pressure, they must be released early. In the case of an already advanced, massive infestation, biological control reaches its limits, which is why continuous monitoring of the plants remains indispensable.

• Targeted use of Phytoseiulus persimilis against spider mites under high humidity conditions.
• Nematodes for the biological eradication of fungus gnat larvae in the substrate.
• Amblyseius californicus as a more resilient predatory mite species in drier climates.
• Apply beneficial insects exclusively as a preventive measure or at the first signs of pests.

Sanitation and disinfection after an infestation

After the successful completion of a cycle that was affected by pests or fungi, a thorough sanitation of the cultivation room is absolutely essential. Spores of botrytis and powdery mildew, as well as spider mite eggs, can survive for months in cracks, ventilation ducts, and on surfaces. Without complete disinfection, the probability is extremely high that the subsequent generation of cannabis plants will be immediately reinfected. All technical equipment, pots, hoses, and the walls of the grow room must be mechanically cleaned and subsequently disinfected.

Biodegradable agents based on hydrogen peroxide or diluted alcohol solutions, which leave no harmful residues for the next generation, are suitable for disinfection. Activated carbon filters should be replaced after a severe fungal infestation, as the spores settle in the fleece and can be continuously released into the room. The intake air ducts must also be equipped with appropriate filters to effectively prevent the entry of new pests and fungal spores from the environment. Only a sterile restart guarantees healthy plants in the long term.

• Complete disassembly and cleaning of all fans, reflectors, and irrigation systems.
• Use of hydrogen peroxide for the residue-free destruction of fungal spores.
• Replacement of the pre-filter and, if necessary, the activated carbon filter after spore infestation.
• Installation of fine-mesh insect screens on all air intake openings.

FAQ