Pests and pathogens threaten cannabis crops. Pests—larger organisms[1] like insects and mites—physically damage plants, whereas pathogens are microorganisms that infect and cause diseases within plants. A useful rule of thumb might be that while pests can typically be removed physically, pathogens cannot.
Both pests and pathogens pose distinct threats to plant health, reducing cannabis yield and quality. This blog post explores some of the most common pests and pathogens that cannabis growers may encounter, the specific plant sites they target, and the types of damage they cause.
[1] Note that some pests can be quite small—even microscopic—but are large in relation to pathogens.
Pests that attack cannabis
Numerous pests can damage cannabis plants, even within the protection of grow rooms. Fortunately, pests can be controlled if detected early enough; more serious problems arise if they have time to establish themselves.
Let’s look at some prevalent pests using Colorado State University’s classification[2] based on their attack strategies on plants.
Pests that chew leaves
- Caterpillars colonize stems or leaves, consuming foliage and leaving black droppings. Removing them promptly helps to preserve crop growth potential.
- Leaf miners are larvae of flies or moths that burrow within leaf tissue. Unfortunately, they’re hard to notice until their damage is done, evidenced by the trails they leave.
- Loopers consume buds, leaves, stalks and the main stem. Adequate air circulation in grow rooms prevents them from infesting.
Pests that suck plant fluids
- Aphids are oval-shaped, light-colored, sometimes winged insects (Figure 1) that suck plant phloem and secrete a sugary liquid called honeydew, which attracts a sooty mold, turning leaves and branches black. Aphids’ feeding causes little to no damage itself; it’s the loss of plant fluid that does the harm. An aphid infestation can reduce plant vigor enough to slow growth and cause wilting and yellowing of leaves.
- Leaf hoppers extract sap, leaving behind white, brown or yellow spots. They come in a variety of colors. Adults have winged, elongated bodies. If disturbed, they fly away.
- Thrips pierce surface cells of leaves and suck out the contents, leaving behind a light-colored stippling injury. Thrips strip leaves of chlorophyll, turning them brittle and dark before crumbling. Growers may see distorted leaves if they feed on expanding immature leaves. Thrips are a greater threat indoors; they have numerous predators outdoors.
- Two-spotted spider mites are common indoors, where they suck the liquid from plant cells. Adult spider mites are tiny and typically straw or green colored with a pair of large, darker spots on their sides. Their eggs are large relative to their size and usually found on the underside of leaves.
- Whiteflies look just as their name implies. These tiny pests lay eggs underneath leaves that stick like glue and are hard to remove. Preferring weak plants, they infest them from top to bottom.
Figure 1. Wingless form of cannabis aphid. Image source: Colorado State University. n.d. “Hemp Insect Fact Sheets: Cannabis Aphid.” Accessed July 21, 2024. https://webdoc.agsci.colostate.edu/hempinsects/PDFs/Cannabis%20aphid%20October%202018%20revision%20(1).pdf.
Pests that damage buds, flowers and seed
- Borers are the larval stage of a small moth. They tunnel into stems and buds, becoming evident when the latter wilt and die. They can cause stunting and stem and stalk distortion, which affect yield.
- Broad mites infest leaf crevices inside buds, where they lay their eggs. Invisible to the naked eye, infestations become apparent when new growth appears twisted, blistered and wet or plasticky. During the flowering stage, buds may turn brown and die.
- Corn earworms tunnel into buds and developing seeds, making them one of the most damaging insects to cannabis grown for CBD or pharmaceutical purposes.
- Stink bugs are one of the more conspicuous pests thanks to their half-inch length and broad bodies. Although they also feed on young leaves and flowers, their damage is primarily limited to developing seeds.
Other pests that damage cannabis
- Fungus gnats (Figure 2) mostly feed on fungi, algae and decaying plant matter in the growing medium, but they may chew on smaller plant roots in the process. While root damage is usually minimal, the root zone can become more susceptible to root-rotting fungi. Fungus gnats are common indoor pests, especially in potted plants in the winter and spring. They’re usually found within the top 2–3 inches of the growing medium.
- Barnacles are scale insects that may look like growths on the stem and leaves (hence their name). They remain stationary while sucking the life from the plant. Barnacles come in different colors and patterns and, like aphids, release honeydew. When infestations become severe, they weaken the plant.
Figure 2. Fungus-gnat larvae. Image source: Colorado State University. n.d. “Hemp Insect Fact Sheets: Fungus Gnats.” Accessed July 21, 2024. https://webdoc.agsci.colostate.edu/hempinsects/PDFs/Fungus%20gnats%20with%20photos.pdf.
Beneficial insects
Not all insects are pests. Some are pest predators that protect plants. Spiders—particularly crab spiders, which lie in wait to ambush prey—are one of the most common plant-beneficial bugs.
Coco coir is naturally rich in sodium, potassium and chloride, so it may be high in salt if not washed or buffered. However, salts are relatively soluble and leachable in coir.
Pathogens that infect cannabis
Pathogens that cause diseases in cannabis include bacteria, fungi, viruses and nematodes.
Bacteria
Xanthomonas and Pseudomonas are the most concerning bacterial pathogens for cannabis. They cause wilting, leaf spots and stem cankers. They enter plants through natural openings and wounds, proliferating under high humidity and poor ventilation.
Fungi
Thriving in moist environments, fungal pathogens can spread rapidly. Fungi cause diseases such as damping off, powdery mildew, and root or crown rot. Botrytis cinerea, which causes bud rot, and various species of Fusarium, which cause root and stem rot, are two of the most common fungal pathogens to afflict cannabis grows.
Viruses
Viral pathogens like hop latent viroid stunt plant growth, discolor foliage and reduce plant vigor. These viruses spread through contaminated tools, infected plant matter or insects. Proper sanitation and hygiene are crucial for keeping them out of grow rooms.
Nematodes
These microscopic roundworms parasitize cannabis roots, causing stunted growth, wilting and reduced nutrient uptake. Species like Meloidogyne incognita (root-knot nematodes) are particularly damaging, affecting root development and compromising plant health.
Identifying cannabis diseases
Cannabis diseases are categorized by the tissue they infect (Figure 3).
Figure 3. Emerging pathogens on cannabis and hemp. Pathogens in bold are the most damaging. Image source: Punja, Zamir K. 2021. “Emerging diseases of Cannabis sativa and sustainable management.” Pest Management Science 77(9): 3857-3870. https://doi.org/10.1002/ps.6307.
Table 1 lists the different pathogens that infect cannabis by the plant part they infect and the diseases they cause. Some pathogens, like Botrytis cinerea, can infect different plant tissues and cause multiple diseases. Figure 4 illustrates what some of those symptoms look like on cannabis.
Common diseases
Infested plant part
Pathogens
Damping off
Stem
Botrytis cinerea
Fusarium oxysporum
Fusarium proliferatum
Fusarium solani
Root and crown rot
Roots
Fusarium oxysporum
Fusarium proliferatum
Fusarium solani
Pythium myriotylum
Pythium dissotocum
Pythium aphanidermatum
Stem canker
Stem
Fusarium oxysporum
Fusarium solani
Stem rot
Stem
Fusarium & Pythium
Powdery mildew
Leaves and stem
Golovinomyces spp.
Bud rot
Flowers
Botrytis cinerea
Fusarium spp.
Mold (post-harvest)
Flowers
Botrytis cinerea
Penicillium species
Dudding
Flowers
Hop latent viroid
Leaf spot and blight
Leaves
Alternaria alternata
Cercospora spp
Septoria spp
Wilting and leaf spots
Leaves
Xanthomonas
Pseudomonas
Table 1. Common cannabis diseases categorized by the pathogens that cause them and the plant part they infect.
Figure 4. Disease symptoms on cannabis. (A)–(C) Fusarium causes yellowing foliage and internal stem necrosis. (D)–(F) Pythium causes wilting and crown necrosis. (G)–(I) Golovinomyces causes powdery mildew on leaves and flowers. (J)–(L) Botrytis causes bud rot. Respective pathogen cultures are shown in (C), (F) and (L). Image source: Punja, Zamir K. 2021. “Emerging diseases of Cannabis sativa and sustainable management.” Pest Management Science 77(9): 3857-3870. https://doi.org/10.1002/ps.6307.
Without the right (or wrong) conditions in place, a pathogen may never develop into a disease. A combination of factors can contribute to an infection and help determine the severity of the disease, ranging from the plant’s genetic makeup to growing conditions such as temperature and humidity and cultivation practices like irrigation and fertilization.
In our next blog post, we’ll discuss how growers can prevent and control both pests and pathogens with integrated pest management (IPM) techniques.
Emerald Harvest Team
[2] Colorado State University. n.d. “Hemp Insect Fact Sheets.” Accessed July 21, 2024. https://agsci.colostate.edu/hempinsects/hemp-insects-text/
Michael Fowlkee
All that great info but NO suggestions for solutions or treatments.
Samantha White
Hello! Please check out our latest blog post “Managing Cannabis Pests and Pathogens” for more information.
Ernest Davis
Great reading. Thank you for the in-depth analysis. Looking forward to the next article.