Diseases of Cannabis are caused by organisms or abiotic sources. Organisms include fungi (first and foremost), nematodes, parasitic plants, bacteria, and viruses. Abiotic (non-living) causes include nutrient deficiencies, pollutants and genetic diseases. Different diseases prevail in different crops (e.g., drug cultivars versus fiber cultivars). Disease prevalence is also modulated by geography and climate. The claim that Cannabis has no diseases is not correct, Cannabis suffers over 100 diseases, but less than a dozen are serious. Serious diseases include gray mold, hemp canker, damping off, assorted leaf spots, blights, stem cankers, root rots, nematode diseases, broomrape, macro- and micronutrient deficiencies, and genetic diseases. Environmentally stressed plants become predisposed to diseases. Stress includes drought, insufficient light, untoward temperatures, or growing plants in monoculture.
(McPartland, J. M., 1996. A review of Cannabis diseases. Journal of the International Hemp Association 3(1): 19-23.)
That quote was taken from a great page on Cannabis diseases <
http://www.hempfood.com/IHA/iha03111.html> that everyone should read.
Grey Mold/Fungus (Botrytis) - Botrytis blight or gray mold is a fungus disease which infects a wide array of herbaceous annual and perennial plants. There are several species of the fungus Botrytis which can cause blights; the most common is Botrytis cinerea. Botrytis infections are favored by cool (60 degrees Fahrenheit or 15 degrees Celsius), rainy spring and summer weather. Gray mold can be particularly damaging when rainy, drizzly weather continues over several days. Look for masses of silver-gray spores on infected plant parts that are growing in humid areas. Tiny, black, shiny specks might also be seen embedded in diseased plant tissue. These are sclerotia of Botrytis: they allow the fungus to survive the winter. Botrytis blight can affect leaves, stems, crowns, flowers, flower buds, seeds, seedlings, bulbs, and just about any other part of a plant with the exception of the roots.
Solution - The best way to manage this disease is by inspection and sanitation. Remove infected flowers, leaves, or the entire plant if it's infected at the base, and take it away from your grow area before dispose if it. It is best not to do any sanitation when plants are wet since this could spread fungal spores during conditions which favor infection. Likewise avoid overhead watering, or misting plants especially if you have had trouble in the past. To promote rapid drying of plants space them to allow good air circulation. Sanitation alone is not sufficient to control this fungus. The fungus can produce 60,000 or more spores on a piece of plant tissue the size of your small finger nail. Even one spore can infect a plant and cause disease. So, avoid injuring plants in any way. Do not leave large stubs of stems when taking cuttings. Ventilate your grow space to prevent high humidity conditions. Even lowering the humidity slightly can have a significant effect on Botrytis. Outdoor planting should be planned to provide good air circulation patterns. This is the most important means of stopping this fungus. Added protection is available for many crops by applying a fungicide or combination of fungicides. However, Botrytis can develop resistance to certain chemicals. An ozone treatment is also an option, ozone is excellent for decimating spore counts in the grow room and a decent UV tube unit placed high in the room with a fan blowing through it can reduce dramatically the risk of botrytis.
Tobacco Mosaic virus - The tobacco mosaic virus can attack a wide range of plants, including tomato, pepper, eggplant, tobacco, spinach, petunia, marigold, and our beloved herb marijuana. On marijuana the virus infection causes light and dark green mottled areas on the leaves. The dark green areas tend to be somewhat thicker than the lighter portions of the leaf. The leaf mottling is seen more easily if the affected plant surface is partially shaded. Stunting of young plants is common and often is accompanied by a distortion and fern-like appearance of the leaves. Older leaves curl downward and may be slightly distorted. Certain strains of the virus can cause a mottling, streaking and necrosis of the buds. Infected plants are not killed, but they produce poor quality buds and low yields. Tobacco mosaic, is incited by a virus. The tobacco mosaic virus is very stable and can persist in contaminated soil, in infected plant debris, on or in the seed coat, and in manufactured tobacco products. The virus is transmitted readily from plant to plant by mechanical means. This may simply involve picking up the virus while working with infected plant material, then inoculating healthy plants by rubbing or brushing against them with contaminated tools, clothing, or hands. Aphids are not vectors of the virus, although certain chewing insects may transmit the pathogen.
Solution - Virus diseases cannot be controlled once the plant is infected. Therefore, every effort should be made to prevent introduction of virus diseases into the garden. Sanitation is the primary means of controlling virus diseases. Infected plants should be removed immediately to prevent spread of the pathogens. The use of tobacco products during cultural practices should be avoided to prevent inoculation of plants with the tobacco mosaic virus. Those people using tobacco or working with infected plant material should wash their hands thoroughly in soapy water before handling your plants.
Pythium Root Rot (Pythium spp.) - Pythium root rot can be caused by several different species of the fungus Pythium. These fungi are common in field soil, sand or sediment of surface water supplies, and dead roots of previous crops. Pythium has also been found in some commercially available soilless potting mixes. Pythium is easily introduced into pasteurized soil or soilless mixes by using dirty tools, dirty pots or flats, walking on or allowing pets to walk on the mixes and by dumping the mixes on benches or potting shed floors that have not been thoroughly cleaned. When introduced into pasteurized soil or soilless mixes, Pythium can cause severe root rot because it has few competitors to check its activity. This fungus poses a threat to crops grown hydroponic systems. If the reservoir is heavily contaminated with debris or soil harboring Pythium, the fungus can spread to a large number of plants quickly. If the fungus infests a cutting bed or if contaminated water is used in propagation, large losses usually occur. Almost all plants are susceptible to Pythium root rot. Root tips which are very important in taking up nutrients and water are attacked and killed. Pythium also can rot the base of unrooted cuttings. Symptoms of Pythium include: Stunted plants, root tips are brown and dead, Plants yellow and die, Plants wilt at mid-day and may recover at night, rot may proceed up the stem, brown tissue on the outer portion of the root easily pulls off leaving a bare strand of vascular tissue exposed, and the cells of roots contain many microscopic thick-walled spores.
Solution - Pythium root rot is difficult to control once it has begun. Every effort should be directed toward preventing the disease before it begins. Pasteurize soil and sand with heat (a microwave) or chemical fumigant treatments. If the water supply is suspected of being the primary source of Pythium, it may be necessary to treat the water before use. Slow sand filtration has been shown to be an effective, simple, and inexpensive method for removing Pythium from water. Cover the treated soil and store it or the soilless mix in an area that will not be contaminated through the introduction of non-treated soil. Likewise, cover ebb and flow system reservoirs. Disinfest all surfaces, tools, and equipment that will contact the potting mix. We have also found that Hydrogen Peroxide (H2O2) is great for controlling Pythium in both dirt and hydro.
Damping-off - Damping-off generally refers to sudden plant death in the seedling stage due to the attack of fungi. These fungi are soil borne and are stimulated to grow and infect the seed or seedling by nutrients released from a germinating seed. However, seedlings may be injured or killed by something other than fungi, for example, toxic materials in the soil, excess or deficient soil moisture, seed defects, temperature extremes, toxic gases in the air, etc. A correct diagnosis is the key to effective control measures. Damping-off disease of seedlings is widely distributed and is a problem on a worldwide basis. It occurs in most soils, temperate and tropical climates, and in greenhouses. The disease affects seeds and seedlings of various crops. The amount of damage the disease causes to seedlings depends on the fungus, soil moisture, and temperature. Normally, however, cool wet soils favor development of the disease. Seedlings in seedbeds often are completely destroyed by damping-off, or they die after transplanting. Frequently, germinating seeds are killed by damping-off fungi before they emerge from the ground, which accounts for poor stands in many crops. Older plants are seldom killed by damping-off fungi mainly because the development of secondary stem tissue forms a protective barrier and limits fungal penetration. However, portions of the roots and stems still can be attacked, resulting in poor growth and reduced yields. When seeds are planted in infested soils, damping-off fungi may attack them at any stage. The damping-off fungi may attack the seed prior to germination, or they may attack after the seed has germinated but before the seedling has emerged above the soil line. Infected seed becomes soft and mushy turning a brown to black color, and it eventually disintegrates. Seeds that have germinated and become infected develop water-soaked spots that enlarge and turn brown. The infected tissue collapses, resulting in death of the seedling. Penetration and death of seeds before they emerge is termed preemergence damping-off. Seedlings that have emerged are usually attacked at or below the soil line. The organism can easily penetrate the young soft stem tissue. The infected stem portion becomes discolored and begins to shrink. As this occurs, the supportive strength of the stem's invaded portion is lost, and the seedling topples over. The fungi continue to invade the remaining portion of the seedling, resulting in death. This phase of the disease is termed postemergence damping-off. Older established plants also can be attacked by damping-off fungi. Usually the new developing rootlets are infected, resulting in root rot. Infected plants show symptoms of wilting and poor growth.
Solution - Proper conditions for seed germination and seedling emergence also favor vigorous growth of fungi that cause damping-off. Seed and roots must be kept moist and warm until the roots have penetrated the soil and the seedlings have emerged. As the seedlings continue to grow, moisture at the soil surface can be decreased, and the damping-off fungi then will have less of an advantage. When watering, thoroughly saturate the soil and then apply no more water until soil approaches the point at which plants wilt. This procedure will keep surface soil dry for a maximum time. Pasteurization of Soils is the best way to protect yourself. Soil for growing transplants in flats can be steam pasteurized. If steam is used, the entire soil mass should be maintained at a temperature of 160 degrees F for 30 minutes. The home gardener obviously does not have facilities to steam soil; however, pasteurized, packaged soil mix is available from many garden centers. To prevent soil recontamination, all items such as tools, pots, and flats, etc., must be clean. The items can be cleaned in hot water (160 degrees F for 30 minutes) or in a chlorine bleach solution (1 part bleach to 9 parts water; soak for 30 minutes). It is important to use fresh chlorine bleach-water solutions.