More than 100 disease agents can cause infection in Cannabis species. Disease agents can infect plants in different biological stages and can be seen in different tissues. The extent and severity of the disease can vary depending on many factors such as the agent, the region, the biological period in which the plant is infected. Diseases can be of viral, bacterial, or fungal origin. Some diseases cause cannabis leaves to curl down.
Cannabis leaves curl down when they are overwatered, their roots are rotted, due to pests, because of too much fertilizer, due to heavy and muddy soil, or because of too hot or cold temperatures. Curling down may also be caused by a parasite named Pythium which can be activated by overwatering. So, you need to pay attention to your irrigation plan, temperature control, and soil&fertilizer quality and amount. There are also several more diseases that may cause cannabis leaves to curl. Let’s check the diseases that affect the cannabis plant in more detail!
Bacterial Diseases Causing Cannabis Leaves to Curl Down
There is confusion in the numbers given in the literature on bacterial diseases that are a problem in cannabis. Although many factors have been reported, many of them have been reported as misdiagnosis or synonymous names. Among the important bacterial pathogens that cause disease in cannabis plants, there are Pseudomonas Syringae PV. Cannabina, P.s. PV. Mori, Agrobacterium tumefaciens, Erwinia Tracheiphila, Xanthomonas Campestris PV. cannabis, P.s. PV. Tabaci, and P.s. PV. Mella.
- Bacterial blight (Pseudomonas Syringae PV. Cannabina)
Bacterial blight agent Pseudomonas Syringae PV. Cannabina is the most important bacterial problem in cannabis plants. This factor, which is also known as bacterial leaf spot and can be confused with fungal agents, has been determined in cannabis varieties in many countries in Europe (Italy, Germany, Hungary, Bulgaria, Romania, Yugoslavia). The agent can adversely affect cannabis in all biological development stages, from the seedling period to the flowering period.
In infected plants, the first symptoms begin as small wet leaf spots, the spots enlarge along the leaf veins and the leaf color turns brown. Dead tissues break down and cause holes to form in the leaves. The agent can spread by seed, the bacteria can overwinter in plant residues and infect seedlings during the growing season.
- Striatura ulcerosa (Pseudomonas syringae pv. mori)
This pathogen is the first bacterium isolated and identified from infected cannabis. It causes losses in cannabis varieties in many European countries, especially in Germany, Italy, and Russia. The pathogen causes waxy, dark gray lesions extending along the stem of mature plants. The length of the lesions can sometimes exceed 10 cm. Bacteria overwinter in plant residues and can be spread by wind and rain.
- Xanthomonas leaf spot (Xanthomonas campestris pv. cannabis)
The agent has been reported as blight in Japan and Korea, and brown leaf spot and wilt in Romania. Bacterial xylem causes clogging of tissues and inhibition of water and nutrient uptake. While many crucifer species are among its hosts, it can easily infect flax and hemp. The disease can become more severe during the summer months with high temperatures and humidity. The bacteria can survive in infected plant residues and seeds and can be spread by producers during the production season during sprinkler irrigation, wind, rain, insects, and cultural processes.
- Wildfire (Pseudomonas syringae pv. tabaci)
The manifestation of this factor is similar to the other bacterial pathogens mentioned (Pseudomonas and Xanthomonas blight) and it is very difficult to distinguish. While the agent mostly causes disease in Solanaceae family plants, it can also infect Cannabis species. In infected plants, the disease symptoms begin on the lower leaves and spread rapidly in humid weather. Wet spots turn into small necrotic lesions that have become chlorotic. Lesions coalesce to form irregular necrotic areas.
- Root tumor (Agrobacterium tumefaciens)
Another bacterial species that can infect Cannabis species is A. tumefaciens. 12-15 days after infection, abnormal cancer-like growth appears on plants. Beige galls form on the underground roots or crown of the plants. Urs rarely occur on the above-ground stem and rarely exceed 10 mm in diameter. While root tumor usually creates stunting in plants, it can rarely cause plant death. Bacteria can spread by seed and can stay in the soil for a long time. Bacteria in the soil can enter through the capillary roots of injured plants.
- Bacterial wilt (Erwinia tracheiphila)
This disease in cannabis plants has been diagnosed in Russia and Pakistan. Researchers reported that 1% of the cannabis plantation in Oxford, Mississippi-USA was infected with the bacterial wilt agent, Erwinia. Symptoms begin as dull green leaf spots, followed by sudden wilting and necrosis of leaves and stems. While some plants only wilt, some plants die. Generally, seedlings are more susceptible to disease. However, the symptoms can be confused with those caused by many other factors (drought, nutrient problems, soil diseases, and pests).
Fungal Diseases Causing Cannabis Leaves to Curl Down
It has been reported that there are many fungal agents that can infect Cannabis species. It is reported that at least 88 species can infect this group and new agents have been detected. Some important fungal diseases that can be seen in cannabis plants include Gray mold, fusarium, leaf spots, Alternaria, powdery mildew, and slump.
- Gray Mold
It has been reported that gray mold (Botrytis cinerea) is one of the most important diseases of cannabis. The agent can infect plants in both controlled and open field conditions. B. cinerea can thrive in moist, warm, and temperate regions with moderate temperatures. Under these conditions, gray mold can cause an epidemic and completely destroy a cannabis crop within a week. Gray mold is a seed-borne disease that usually causes seedling infection.
Female flower buds are more susceptible to infection and may produce typical symptoms as a gray-white layer around the inflorescences. The symptoms of the disease and the rate of loss may vary depending on the variety and plant period. Stem rot is more common in fiber hemp varieties. The fungus produces enzymes that can reduce the hardness of the stem, then the stem becomes soft and chlorotic.
The genus Fusarium includes species that cause wilt, blight, rot, and cankers in many cultivated and wild plants. Fusarium oxysporum f.sp. cannabis and F. oxysporum f.sp. Vasinfectum is among the Fusarium species that cause disease in cannabis. The morphology of these two species is similar, but the first one infects only cannabis, while the latter can infect cotton, beans, pigeon peas, rubber trees, clover, soybeans, coffee, tobacco, and many other plants, apart from hemp.
Symptoms in the infected plant are chlorosis on lower leaves presents as irregular small dark spots, followed by upward curling of the leaf tips and complete wilting of the plant. Infected stems show yellowing and a reddish-brown discoloration in the xylem. Symptoms caused by F. oxysporum are similar to other root rot wilts.
- Leaf Spots
Many fungal agents, especially septoria leaf spot, can cause different stains on the leaves and cause yield losses. Two Septoria species (Septoria cannabis, S. Neocannabina) have been reported to cause yellow spots on cannabis leaves. The sign of infection is seen as small white-yellow lesions on the lower surface of the dried leaf and holes appear in the leaves.
Apart from Septoria, there are many leaf spot agents. Brown leaf spot species (Phoma and Ascochyta), pink rot (Trichothecium Roseum), brown blight (Stemphylium spp.), anthracnose (Colletotrichum spp.), white leaf spot (Phomopsis Ganjae), Cercospora leaf spot (Cercospora spp.), and mildew. (Pseudoperonospora spp.) is one of the important leaf spot diseases in cannabis. Due to these diseases, plants can sometimes dry out completely.
- Alternaria Blight
Alternaria species from cannabis varieties have been isolated in Italy, Poland, Czechia, the USA, India, and many countries. Brown blight usually occurs late in the growing season. Irregular dry spots on cannabis leaves appear as leaf necrosis. Lesions can reach petiole, shoot, and stem. The most common species, A. Alternata, infects mainly female flowers. Therefore, it can reduce hemp seed production by up to 46%.
- Powdery Mildew
Powdery mildew is one of the most common fungal diseases that can be seen in many cultivated plants. Among the powdery mildew agents that cause disease in cannabis plants, Leveillula Taurica f.sp. cannabis and Sphaerotheca Macularis have been reported. The disease can be seen in hot and humid areas grown in the open field. The first sign of infection begins with the appearance of white mycelium on the top of the cannabis leaves. It then turns into a necrotic state before yellowing and drying.
- Damping Off
Many species in the genus Pythium can cause damping off. Again, many species such as Botrytis cinerea, Macrophomina Phaseolina, and Rhizoctonia Solani cause damping-off in cannabis plants. The scattering agents can cause disease in plants before emergence (seed origin) and after emergence (seedling period).
The stem of the infected seedlings turns brown from the part in contact with the soil, the plants are wilted, and the seedlings are affected by overturning and drying. It is very difficult to distinguish the symptoms caused by the factors of precipitating from the symptoms caused by the wilting agents. Some root rot diseases can cause significant yield losses in hemp. It has been suggested that F. Solani is the most important disease of cannabis in France, a root rot factor.
- White Rot
One of the important fungal diseases of cannabis is white rot (Sclerotinia Sclerotiorum), which is known by many different names (soft, wet, stem, white mold, gray rot). The agent can infect different cannabis varieties. The agent has been determined in cannabis varieties in Europe, USA, India, and Australia, and it has been reported to cause yield losses of up to 40% in North America.
In infected plants, symptoms begin as watery, wet lesions on the stems and shoots of mature plants. The cortical tissues beneath the lesions collapse, forming light brown thrush-like structures. In moist conditions, the outer surface of the stem is covered with cottony-white mycelium. Plants may wilt at this stage, and death may occur in advanced stages. Black sclerotia develop on the stem surface or on drying shoots.
Viral Diseases Causing Cannabis Leaves to Curl Down
Plant virus agents are very common in cultivated plants worldwide and pose a serious threat to crop production. Cannabis is usually infected by viruses with single-stranded RNA. After infection, it becomes almost impossible to prevent viruses in the plant. Viruses can spread systemically to all plant tissues including leaves, stems, pollen, and seeds, causing severe symptoms and yield losses in cannabis plants.
Rarely, it can also cause plant death. The vast majority of viruses that cause infections in plants are transmitted from host to host by some insect vectors. While many viruses can infect cannabis plants, five viral agents are common. Hemp streak virus (HSV), Hemp mosaic virus (HMV), Alfalfa mosaic virus (AMV), Cucumber mosaic virus (CMV), and Arabis mosaic virus (ArMV) are among the more common viruses in cannabis plants.
- Hemp streak virus (HSV)
After HSV was first diagnosed in Germany, it was detected by different researchers in different countries (Italy, Czechia, Russia, Hungary, etc.). Leaf symptoms of the virus begin as pale green chlorosis. Leaf symptoms appear as yellow streaks between veins, then leaf margins wrinkle, leaf tips curl upwards. It is reported by producers that such symptoms are common in cannabis varieties.
Vector species play an important role in the spread of the virus. The most important vector of HSV is the cannabis aphid Phorodon cannabis. The virus can be transmitted from generation to generation through the seeds of infected plants. Within six days after infection or inoculation, signs of virus can be observed in plants. It has been reported that male cannabis plants are more susceptible to the virus than female cannabis plants. Diseased plants are stunted and cause reductions in fiber yield and seed production.
- Alfalfa mosaic virus (AMV)
AMV, first reported in 1931, is among the most important viral agents. AMV is one of the most common viruses worldwide and has an extremely wide host range. This virus can naturally infect many herbaceous and woody plant species. It has been reported that the virus infects more than 700 plant species from 71 families. AMV was found naturally in cannabis plants in Germany by researchers.
AMV, which is in the Bromoviridae family Alphamovirus genus, has 18 nm wide and 18-60 nm long bacilliform particles. The AMV genome contains positive-sense single-stranded RNA. The virus has a large number of natural variants with different pathogenicity. Although the symptoms of the virus vary according to the host plant species, it usually causes mosaic on the leaves, chlorotic lines along with the leaves or chlorotic spots between the veins, and slight curling of the young leaves.
AMV can be transmitted by vector, seed, graft, dodder, and mechanically. However, aphids play the most important role in the transmission and spread of the virus. The virus can be transmitted non-persistently with many aphid species. It can be carried by at least 14 aphid species.
- Cucumber mosaic virus (CMV)
CMV is a type member of the Cucumovirus group of the Bromoviridae family. CMV is recognized as a worldwide virus after it was first reported in the cucumber plant. CMV is capable of infecting more than 1,200 plant species in more than 100 families. Cannabis plants are among the hosts of the virus. CMV has been reported to cause mottle symptoms in cannabis plant leaves.
While CMV seeds, vegetative propagation materials, dodder can be transported mechanically, vector insects are at the beginning of the most important transportation and spread ways. CMV can be transmitted non-persistently with more than 80 aphid species. Yield losses caused by CMV in plants can vary from plant to plant, from year to year, and from region to region.
- Arabis mosaic virus (ArMV)
ArMV is a virus with 30 nm diameter isometrically shaped particles belonging to the Nepovirus genus, Comoviridae family. The virus can infect many plant species. It has been reported that the virus can infect 93 species from 28 families. It can cause severe symptoms such as leaf curl, mottled mosaic, deformity, and dwarfism in virus-infected plants. Cannabis plants are among the hosts of the virus.
ArMV causes symptoms in the form of light green/yellow spots and lines on cannabis plant leaves. ArMV has been detected in many countries in Europe, Asia, Africa, America, and Oceania. While ArMV is carried over short distances by nematodes (Xiphinema spp.) in the soil, its transport over long distances can be by contaminated production materials, especially seeds. For this reason, it will be very important to use clean production/propagation materials in aquaculture and to grow in production areas that do not contain vector nematodes.
- Hemp mosaic virus (HMV)
Ancient sources report that HMV can infect cannabis plants. However, there is not much detailed information about this virus. It has been reported that the causative agent may be Cucumovirus such as CMV or Nepovirus such as ArMV, TRSV, and TomRSV. HMV symptoms can vary widely in plants and can be confused with other biotic (virus, bacteria, viroid) and abiotic factors. Therefore, it is very difficult to detect and combat.
It has been reported that the virus causes signs of enation in cannabis in Czechia, and different symptoms in Pakistan such as mosaic, leaf curl, apical curl, and shrinkage of the tip leaves. The most important vector of HSV, the cannabis aphid P. cannabis, is effective in the transmission and spread of the virus.
- Other Viruses
While it has been reported that the five viral agents mentioned above (HSV, HMV, AMV, CMV, and ArMV) generally infect cannabis plants, it has been reported that a large number of viruses can infect cannabis plants naturally or by mechanical inoculation method such as Tobacco mosaic virus (TMV), Tobacco ringspot virus (TRSV), Tobacco streak virus (TSV), Tomato ringspot virus (TomRSV), Eunoymous ringspot virus (ERSV), Elm mosaic virus (EMV), Foxtail mosaic virus (FMV).
Two new viruses that infect cannabis strains have been reported for the first time in recent years. Beet curly top virus (BCTV) in industrial cannabis production areas in the USA/Colorado and Lettuce chlorosis virus (LCV) in cannabis production areas in Israel was reported as the first record.
Fighting Against Diseases Causing Cannabis Leaves to Curl Down
It has been reported that an estimated 11% of the annual average loss of crops in agricultural products is due to diseases, and in addition, crop losses continue in the post-harvest period and are approximately 9%. For this reason, it is extremely important to know the disease-causing factors in plant products during and after the cultivation process and to take preventive measures accordingly.
It can cause many pathogen (bacterial, fungal, viral, phytoplasma) diseases in cannabis plants. Some of the important bacterial diseases seen in cannabis include; bacterial blight, wilt, root rot, and Xanthomonas leaf spot. Again, gray mold, white rot, various leaf spot diseases, blight, root rot are among the important fungal diseases in hemp.
HSV, HMV, AMV, CMV, ArMV are among the important viral agents that can cause disease in Cannabis species and varieties. Likewise, phytoplasma-borne diseases can cause yield losses in cannabis plants. In order to prevent yield losses that may occur in crop production, it is necessary to know and fight against disease factors. Some points to be considered in the fight against diseases seen in cannabis are listed below:
- Clean Seed
Seeds should not be taken from diseased plants, and diseased seeds should not be used in production. The seeds to be used must be disinfected with hot water. In the production and reproduction phase of the seed, which is the starting material of many products in agricultural production, the infection rate due to viral diseases increases. Many viral, bacterial, and fungal agents that can infect cannabis can be transmitted by seed. For this reason, it is extremely important that the seeds used in cannabis cultivation are healthy, not contaminated with disease factors, and seeds are not taken from infected plants.
Disease agents can be transmitted from year to year with infected plant residues. It is extremely important to clean the end-harvest residues and collect the stems and roots. It requires special tillage equipment, especially for shredding the fiber-containing hemp stalks. Seeing the plant residues deeply by tillage, destroying the inoculum sources for the disease can cause the death of many bacterial and fungal disease agents and pests. Care should be taken not to open wounds on plants that may become suitable for the entry of disease agents. During the growing season, diseased plant parts or plants should be removed from the production area and destroyed.
- Environmental Conditions
In particular, care should be taken not to create suitable environments for the development of fungal and bacterial diseases. Ambient humidity causes the growth of microorganisms. While over-watering makes plants more susceptible, it creates a favorable environment for pathogens to thrive.
- Soil structure
Cultivation should be done in soil structure with suitable nutrient content and pH structure for plants. Excessive irrigation promotes the development of pathogens (fungi, bacteria). For soil and leaf pathogens, moisture reduction and excessive moisture should be avoided. Likewise, drought makes plants more susceptible to diseases.
Care should be taken to ensure that the tools, equipment, clothes, hands, etc. used in aquaculture are sterile. Many soil pathogens can be inactivated by soil sterilization. Again, with the solarization method, many soil-borne fungi, bacteria, virus vectors can cause the death of harmful nematodes and weed seeds.
The same products should not be grown continuously in the field, rotation should be applied. Rotation with non-host crops can reduce losses from some diseases and pests.
- Fighting Vector
Disease carrier vectors should be kept away from plants. Prevention of vectors (insect, mite, nematode) in the production area will reduce both the losses caused by the pests and the losses that may arise from the diseases they can carry. Almost all of the viruses that can infect cannabis can be transmitted by vectors (aphids, whiteflies, nematodes, etc.). For this reason, taking measures to prevent the entry and spread of vectors into production areas is also very important in terms of preventing viruses.
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