Despite the often repeated quote, "hemp has no enemies," Cannabis suffers many diseases.
Cannabis is afflicted by more than one hundred diseases caused by fungi, bacteria, viruses, nematodes, plants, and abiotic challenges (genetics, nutrients, stress, pollutants) but only about a dozen cause serious problems.
Disease, by definition, is continued (irritation) damage by a causal factor (be it an organism or the environment). In contrast, injury is due to a transient causal factor. Insects are transients, they cause feeding injury, not disease. Never the less, insects do vector disease. (pass it along)
Organisms that cause continued irritation (disease) include viruses, bacteria, fungi, parasitic plants, and nematodes . Plant diseases are most often caused by fungi and rarely caused by bacteria. In people this fungi-to-bacteria ratio is reversed. Few Cannabis diseases can be transmitted to humans, but there are exceptions .
Some diseases prevail in Cannabis fiber and oil seed crops, other diseases predominate in drug crops. Disease prevalence varies geographically. For instance, many virus diseases are limited by the range of their insect vectors. Disease prevalence shifts between greenhouse crops and outdoor crops. Disease prevalence alters as plants grow from seedlings to flowering adults.
|Seedling||Flower/Leaf (outdoor)||Flower/Leaf (indoor)||Stems & Branches||Roots|
|Damping Off (fungi)||Gray Mold||Nutritional Diseases||Gray Mold||FusariumRootRot|
|Storage Fungi||Yellow/Brown Leaf Spots||Pink Rot||Hemp Canker||Root Knot Nematode|
|Genetic Sterility||Downy Mildew||Gray Mold||Fusarium Canker||Broomrape|
|Olive Leaf Spot||Powdery Mildew||Fusarium Wilt||Rhizoc Root Rot|
|Nutritional Diseases||Brown Blight||Stem Nematode||Sclerotium Rot|
|Bacterial Leaf Disease||Virus Disease||Charcoal Rot||Cyst Nematode|
As mentioned above, at least eighty eight species of fungi attack cannabis and more are being discovered every year. By far the most significant is gray mold, caused by Botrytis cinerea. B. cinerea thrives in temperate regions with high humidity and cool to moderate temperatures. Under these conditions gray mold can reach epidemic proportions and completely destroy a cannabis crop within a week . B. cinerea attacks many crop plants and weed species worldwide.
Gray mold presents in three scenarios, depending on plant maturity and cultivar. Seedlings succumb to damping off, discussed below. In fiber cultivars gray mold presents as a stem disease. It arises as a gray-brown mat of mycelium which becomes covered by masses of conidia (fungal spores). Stems become chlorotic at margins of the mat. Enzymes released by B. cinerea reduce stems to soft shredded cankers. Stems often snap at canker sites. Gray mold may encircle and girdle stems, wilting everything above the canker. Fiber varieties become more susceptible after canopy closure.
In drug cultivars, gray mold infests flowering tops. Large moisture-retaining female buds are most susceptible. Fan leaflets first turn yellow and wilt, then pistils begin to brown. Whole inflorescences soon become enveloped in a fuzzy gray mycelium then degrade into a gray-brown slime. Drug varieties are most susceptible during flowering near harvest time. Dense tightly-packed buds tend to hold moisture and easily rot. Afghan cultivars evolved in very arid conditions and have no resistance to gray mold. This unfavorable trait is often expressed in hybrids that have only a small percentage of C. afghanica heritage.
The second most important disease that attacks cannabis is hemp canker. This disease is caused by Sclerotinia sclerotiorum. The fungus primarily attacks fiber cultivars in Europe, but it has caused up to forty percent losses in North America and damaged hemp in Australia and Tasmania. Hemp canker has also appeared on drug cultivars in India. Symptoms begin as water soaked lesions on stems and branches of plants nearing maturity. The lesions collapse into cankers and become darkly discolored. Affected areas take on a shredded appearance and the pith becomes filled with a white cottony mycelium. Plants remain in this condition or wilt and fall over. Large black sclerotia develop on the stem surface or within pith of dead stalks.
Damping off fungi kill seeds in soil or seedlings shortly after they emerge from the soil. Fungi invade stems of seedlings at the soil line, causing a brown watery soft rot and then the plants topple over. Most damping off is caused by two Protoctistan pythium species (technically they are omycetes, not fungi). P. aphanidermatum and P. ultimum. Several fungi also cause damping off: Rhizoctonia solani, Botrytis cinerea, Macrophomina phaseolina, and several Fusarium species, (F. solani, F. oxysporum, F. sulphurem, F. avenaceum, F. graminearum). Together they make damping off a ubiquitous problem, attacking all cultivars of cannabis.
The two most common leaf spot diseases are yellow leaf spot caused by two Septoria species, and brown leaf spot caused by about eight Phoma and Ascochyta species. These diseases rarely kill plants but sharply reduce crop yields. Two common diseases of fiber varieties are downy mildew, caused by two Pseudoperonospora species, and olive leaf spot caused by a Pseudocercospora species and a Cercospora species. Pink rot, caused by Trichothecium roseum, has recently killed greenhouse-grown drug cultivars and seems to be on the rise. Less frequently seen but equally virulent diseases include brown blight (caused by two Alternaria and two Stemphylium species), anthracnose (caused by two Colletotrichum species) and white leaf spot (caused by Phomopsis ganjae). Powdery mildews, black mildews, and rusts are caused by high visibility fungi, but rarely cause serious problems.
Some “leaf disease” fungi also infest stems, especially Trichothecium roseum, Phoma, Stemphylium, Colletotrichum, and Phomopsis species. The most serious causes of stem cankers are Fusarium species—F. graminearum and F. avenaceum (in cooler climates), and F. sulphureum and F. sambucinum (in warmer climates).
Some root rots cause serious losses. Root rot caused by Fusarium solani is the worst disease of hemp in France. In 1967 a virulent strain of Rhizoctonia solani destroyed eighty percent of drug plants in northern India. Root rot by Sclerotium rolfsii predominates in southern temperate zones and the tropics, on both fiber and drug cultivars.
Above-ground symptoms of root rots are hard to distinguish from wilt diseases. Three wilt diseases are important—fusarium wilt caused by two forms of Fusarium oxysporum, verticillium wilt caused by two Verticillium species, and premature wilt (also called charcoal rot) caused by Macrophomina phaseolina. Fusarium wilt received attention as a potential biocontrol to eliminate illegal marijuana plantations in 1978. Wilt diseases are more severe in Cannabis fields harboring root-wounding nematodes or broomrape.
Nematodes are tiny roundworms, also called eelworms. Nematodes are not closely related to earthworms. Built on a much smaller and simpler scale, they have no respiratory nor circulatory systems. Their nervous system is so simple it can be described at the level of individual cells. Caenorhabditis elegans, for instance, has exactly three hundred and two neurons.
Crop damage by nematodes is underrated due to their small size and the unseen (mostly underground) nature of their pathology. Above-ground symptoms consist of stunting, reduced yield and insipient wilting (drooping of leaves during midday with recovery at night). Farmers may misinterpret symptoms as mineral deficiencies or drought, mysteriously arising despite adequate nutrients and moisture. These symptoms do not occur uniformly across a field, but in pockets of scattered infestation. Below ground symptoms are more distinctive, including root knots or galls. Six nematodes are known to infest Cannabis. All species attack roots except one.
Root knot nematodes embed themselves in roots and induce plants to form giant cells or syncytia. Syncytia swell into root galls and stimulate formation of adventitious rootlets, creating a bushy root. Compound galls arise on larger roots forming "root knots": hypertrophied roots with a rough surface. The southern root knot nematode, Meloidogyne incognita, has been reported on fiber cultivars in Europe, the former USSR, Brazil, and the southern United States. M. incognita is the most widely distributed worldwide, and attacks hundreds of hosts. Two other species are rarely reported: the northern root knot nematode Meliodogyne hapla and the Java root knot nematode Meloidogyne javanica.
The stem nematode, Ditylenchus dipsaci, uniquely lives above the ground and does not infest roots. Initial symptoms arise in stems, branches and leaf petioles, which swell and become chlorotic. Stems subsequently become twisted and distorted with shortened internodes. Plants are stunted. D. dipsaci is found in North America, southern Africa, Australia, and temperate areas of Asia. But Cannabis disease has only been described from fiber varieties in Europe. Other nematodes are rarely reported: cyst nematodes (Heterodera schachtii, H. humuli), needle nematodes (Paralongidorus maximus), and root lesion nematodes (Pratylenchus penetrans).
Plants from two genera are genuine cannabis parasites. They sink specialized roots (haustoria) into the host’s xylem and phloem to withdraw fluids and nutrients. Broomrapes seem to be the worst. Dewey (1914) called branched broomrape (Orobanche ramosa) "the only really serious enemy to hemp." Barloy & Pelhate (1962) consider a combination of O. ramosa and Fusarium solani the greatest threat to Cannabis cultivation in southern France. Broomrapes do most of their damage underground, their haustoria provide portholes for root rot fungi. Only briefly do broomrapes send shoots above ground, which quickly flower and set seed. Rarely O. aegyptiaca and O. cernua have been cited on fiber and drug cultivars.
Dodder, in contrast to broomrape, sinks haustoria into above-ground parts. Five species have been reported, mostly Cuscuta campestris (on drug cultivars in the US and fiber varieties in Europe) and Cuscuta europea (on fiber cultivars in Europe). Dodder arises as conspicuous tangles of glabrous yellow filaments, bearing vernacular names such as "gold thread," "hair weed," "devil’s ringlet," and "love vine." They twine themselves around stems and branches. Robust specimens girdle branches and pull down hosts. Dodder, like broomrape, can vector viruses.
Viruses rarely kill Cannabis. They only exist and replicate in living plants. Viruses can seriously reduce yields. Once acquired, they are nearly impossible to eradicate. Viruses invade all parts of plants. Pollen and seed infections transmit viruses to subsequent generations.
The hemp streak virus (HSV) is frequently cited on fiber cultivars in Europe. Foliar symptoms begin as a pale green chlorosis (loss of green color). Chlorotic areas soon develop into a series of interveinal yellow streaks or chevron stripes. Sometimes brown necrotic flecks appear, each fleck surrounded by a pale green halo. Flecks appear along the margins and tips of older leaves and often coalesce. Streak symptoms predominate in moist weather, flecks appear during dry weather. Leaf margins become wrinkled and leaf tips roll upward, leaflets curl into spirals. Whole plants assume a "wavy wilt" appearance.
The hemp mozaic virus has been described on fiber cultivars in Europe and drug cultivars in Pakistan. Symptoms are described as a gray leaf mosaics. Three other viruses have been cited on European hemp—Alfalfa mozaic virus, cucumber mozaic virus, and arabis mozaic virus. Many insects transmit these viruses as they feed from plant to plant. The worst vectors of Cannabis viruses are aphids, whiteflies, onion thrips and green peach aphids.
Bacteria and MLOs
The Cannabis literature concerning bacteria is confused. Dozens of bacteria have been cited, a morass of misidentifications and taxonomic synonyms. Mutualistic species also appear. Kosslak & Bohlool (1983) isolated Azospirillum brasilense and A. lipoferum from the rhizosphere of marijuana plants growing in Hawaii. These diazotrophic bacteria live on the surface of plant roots where they fix nitrogen for their host. Diazotrophic bacteria have been sprayed on plants to serve as "biofertilizers".
Only four species of true pathogenic bacteria (with one species split into four pathovarieties) cause disease in living cannabis plants. Bacterial blight by Pseudomonas syringae pv. cannabina seems to be the most common problem. Symptoms resemble those caused by brown leaf spot, a fungal disease. Bacterial blight has only been described on fiber cultivars in Europe. Striatura ulcerosa produces similar symptoms on stems and is caused by a similar species, Pseudomonas syringae. It, too, is limited to fiber varieties in Europe. Uncommon diseases include crown gall by Agrobacterium tumefaciens, bacterial wilt by Erwinia tracheiphila, xanthomonas leaf spot by Xanthomonas campestris pv. cannabis, and a mycoplasma-like bacteria.
Diseases from abiotic (non-living) causes often arise suddenly. They usually resemble diseases caused by living organisms. Some abiotic diseases have unknown causes, such as "grandine" of hemp. Abiotic problems also predispose plants to other diseases. Drought-stressed plants, for instance, become much more susceptible to fungal cankers and mite infestations.
The most common abiotic diseases are nutrient deficiencies. Generally, deficiencies of mobile nutrients (N, P, K, Mg, B, Mb) begin in large leaves at the bottom of plants. Shortages of less mobile nutrients (Mn, Zn, Ca, S, Fe, Cu) usually begin in young leaves near the top.
Pollutants take their toll. Sulfur dioxide causes interveinal leaf chlorosis and hydrogen fluoride causes a complete chlorosis in Cannabis. A Cannabis indica found near a Himalayan highway was suffering from chlorosis and necrosis. Automobile polluted plants produced fewer stomates but more trichomes per leaf area. Because of increased trichome density, (Sharma & Mann) hypothesized that auto pollution increased THC production.
Genetic diseases are common. There are often negative consequences with inbred hemp.
An easily preventable form of disease is deficiency or deprivation illness. The plants lack some necessary ingredient in their feeding. A shortage of iron produces yellowed (and falling) leaves. The pH value plays an important role in the prevention of deficiency disease. Keep the pH value around 5.8. If this value is too low, the plants can't absorb calcium as well. Consequence: the osmotic processes are impeded. Too low a pH number causes less iron intake, with the well known results. A second form of deficiency disease is caused by a shortage of the primary nutrients (NPK). It often involves a lack of nitrogen (N). A nitrogen shortage delays growth, and makes the lower leaves turn yellow and drop off.
Less often, we see a shortage of phosphate (P). With a phosphate shortage, the leaves turn deep green, and they remain small. Yellowing and dying lower leaves happen here, also. Potassium shortage is another seldom occurring problem. The noticeable feature is first the yellowing of the point of the leaf, after which the whole leaf turns yellow and brown, and dies off. A lack of potassium is more often caused by an acidic soil than by an actual potassium shortage. Maintain an optimal pH! The remedy advised for these kinds of deprivation sicknesses: use NPK fertilizer. Deficiency disease as a consequence of a shortage of the secondary nutrients is rarely seen. This usually involves a lack of magnesium and/or calcium. It can usually be remedied by using an NPK mixture containing trace elements.
The same counts for the micro-elements. We must make an exception for iron, since there is rarely too little iron. In that case, the pH value is usually too high. Moulds (molds) can completely destroy a garden in a short time. If the climate in the grow room is well-controlled, moulds, in general, have little chance. Moulds and fungi thrive very well under humid conditions, preferably without much air circulation. Under these circumstances, mould spores, which are always present in the air, search for a spot to grow into mould cultures. If you don't succeed in preventing mould growth, then you must do something about it as quickly as possible. With light mould growth, immediately remove the affected plant parts, and then create a climate in which cannabis does well, and moulds do not (good ventilation, control of humidity and temperature, and make sure your soil is not too wet).
If there is already too much mould present, you don't have much choice but to spray with poison (fungicide). Repeat the treatment after a few days, even if you think the first application has definitely helped. Improve ventilation (air movement). Fungicide treatment should always be a last resort. Prevention is always better than having to apply poisons to your infected plants.
An often ocurring mould affecting cannabis is pythium. This mould causes root-rot, and rot in the lowest part of the stem. It appears most in young plants, and in cuttings. Larger, healthy plants are less sensitive to pythium. Plants get 'falling-over disease' with a serious pythium attack. Pythium is recognizable by the bark at the base of the stem turning brown. In the beginning, the 'brown attack' is easily removable. Later, the rotting process eats deeper into the base of the plant. Pythium is a fungus which flourishes best in wet and humid environments. Pythium spores spread only with water. Two kinds of spores are formed, swarming ones and stable ones. The swarming spores germinate best at a temperature of approximately 15 degrees Celsius, while the stable spores germinate in relatively warm temperatures 28 degrees celcius. To prevent a pythium attack, a constant temperature of the soil or rockwool is needed. Large fluctuations in temperature should be avoided. Pythium can only be fought in a limited manner with chemicals. A proper relative humidity must also be maintained (not too high). Leaf moulds, such as mildew, and thread moulds occur less frequently than pythium.
Mildew can cause tops to rot, among other things. Also, ensure optimal climate control. Contrary to other moulds, mildew flourishes well at a low relative humidity. Mildew can be more easily fought with chemicals, and fortunately, is not often found with cannabis. Rotting tops occurs mainly at the end of the flowering phase. The more compact the plant, the bigger the chance for tops to rot. You can identify “toprot” by the sudden yellowing of the top leaves. These yellow leaves are fairly loose on the plant, and can be easily removed. To prevent the whole plant from being affected and infected, you must remove the whole top. The appearance of toprot can be prevented in some cases, by lowering the relative humidity during the dark period.
Plagues and Pests
The most frequently ocurring plague in cannabis cultivation is spider mite. A spider mite is not an insect, as many people think, but actually a tiny spider. A spider mite is small, and difficult to discover for the inexperienced eye. The damage caused is certainly visible. The mite feeds on the sap of the plant, mostly underneath leaves. White (silvery) specks appear on the upper side of the leaf. After that, you can find spider mites on the undersides of the leaves, and on the stem of the plant. Spider mites make small webs, which you can detect by spraying with water. If there are not too many spider mites, you can try to get rid of them by removing them by hand. A tedious job.
Treating with insecticide generally gives a better result. In any case, repeat the application after a few days, otherwise, you risk the chance that the whole garden will be eaten. Spider mites can also be controlled with their natural enemy Phytoseiulus persimilis; a predator mite which feeds on spider mites. White flies are also a formidible opponent of cannabis. It can not be repeated enough, control the climate, and have strong, disease resistant strains. Then, insects will have the least chance to propagate.
Assassinator wasp & White Fly
White flies behave just like spider mites. The insect hides underneath the leaf, pierces the leaf and sucks out its dinner. Soon little white spots appear on the top side of the leaf. White flies are easily spotted with the naked eye. If you shake the plant a little, they will fly around. They look like little white moths (2 mm.) in size. A sizeable infestation can be combatted with insecticide. You can also use sticky cards (yellow). White flies are attracted to the color yellow. If you are not so anxious to see immediate results, you can purchase a certain type of 'assassinator' wasp: the ichneumon fly (the Latin name is Encarsia formosa). This natural enemy does not sting people, but works well at eliminating white flies. Since it is only a small wasp (smaller than the white fly itself), it takes a while before all the white flies are gone. Additionally, you have to put new assassinator wasps out approximately every two weeks.
Thrips & Lice
Another common herbivore is thrips. They are small, fast-moving insects with wings. They rasp, or grate the leaves open, and then suck the sap out. Thrips prefer bloom tops, and fresh, young leaves. Affected leaves have shiny, silvery spots. This is caused by the thrips, which suck the chlorophyll out of the leaves. In spite of the fact that they're small, you can see thrips marching in columns on an infected plant. Thrips can be fought with insecticide. It is more environmentally friendly however to unleash the thrips' natural enemy: the predator Amblyseius cucumeris. Lice are found inside as well as outside. During the summer, when lice do the best outside, they also do as well inside. Lice are the most interested in plants with questionable health. There are two methods to kill lice: spraying with insecticide, and setting out assassinator wasps. The problem with most flying pest- destroyers is that they're attracted by the high-pressure gas lamps, which draw them to a firey death.
The starting point for cultivating cannabis is successful climate control. This goes two ways: the plants do well and produce the greatest possible yield, and diseases and pests get the least possible chance to get started. Create a good climate, and do not forget to keep everything meticulously clean.
If you are bothered by diseases and/or insects, preferably use natural methods of control rather than chemical remedies. You can fight your pests by releasing their natural enemies, or by spraying with organic solutions that you can even make yourself. Use chemical pesticides only if nothing else works. Stop using pesticides several (at least three) weeks before harvest, otherwise, you will be smoking some of those poisons . Ultimately, fighting diseases and pests works best only if you know how to optimally control the climate. Along with climate control, the prevention of deficiency disease demands an optimal mixture of fertilizers, and the correct pH of your medium, nutrient solution and your water!
Botrytis (fungus) Damping-Off (fungus) Moldy Marijuana (fungus)
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