Diseases of plants caused by viruses and their prevention
Some of the same types of viruses that infect humans can also infect plants. Plants and humans do not transmit viruses to each other, but humans can spread plant viruses through physical contact. Most frequently, plants catch a cold when an insect or other small animal invades the cellulose armor that protects them from the environment. Viruses also spread through infected seeds, grafting, wind, splashing, pollination and dripping sap.
Tobacco Mosaic Virus
This is the best known of all virus diseases. The tobacco mosaic virus affects all dicotyledonous plants of which most important are tobacco and tomato. But it does not affect any monocotyledonous plants.
Although Adolph Mayer in 1886 first pointed out the mosaic pattern on leaves of affected tobacco plants, it was not until 1898 the first scientific proof of the existence of a virus was given by Beijerinck. Earlier than this, in 1892 Iwanowski demonstrated that tobacco mosaic virus would pass through a bacteria-proof filter.
The typical tobacco mosaic virus is Tobacco mosaic virus 1, Marmor tabaci Holmes. 
The virus remains active in extracted host plant juice even up to 25 years. It is a very resistant virus, can stand desiccation for 25 years or more. It occurs in very high concentration in plant and its dilution end point is 10-6. The thermal inactivation point of the virus is 90°C.
Prevention of Tobacco Mosaic Virus
- Seed beds should be located at a great distance from the tobacco warehouses.
- Seed beds should be free from any tobacco refuse.
- Seed bed soil should be sterilized by steam.
- Care should be taken to avoid contamination through hands and cultivation implements.
- Since pipe tobacco, cigarettes and chewing tobacco are all sources of primary inoculum, smoking or chewing of any kind of tobacco should be avoided.
- Susceptible hosts, weed or otherwise in which virus may harbour, should be destroyed.
Cucumber Mosaic Virus
Historically, Cucumber mosaic virus (CMV) was first described in detail in 1916 on cucumber.
CMV infects 1200 species in over 100 plant families and can cause significant economic losses in many vegetable and horticultural crops. CMV causes a systemic infection in most host plants, but may remain symptomless in some crops like alfalfa. Symptoms of cucumber mosaic can vary greatly depending on the crop infected and the age of the plant when infection occurs.
Prevention
Nowadays, there is not any chemical capable of removing this virus from an infected plant. Therefore, the best control in this case is prevention of the infection and eradication. To achieve this, it is crucial to remove weeds and diseased plants from the field, as well as use clean and sanitized tools. Another option consists of the use of resistant varieties or the so-called “trap crops”.
Barley Yellow Dwarf
Barley yellow dwarf virus (BYDV), a member of the Luteoviruses, is a group of five closely related virus strains. Strains of BYDV differ serologically and in virulence, host range and vector specificity. Virus particles are spherical. BYDV is transmitted by more than 20 aphid species. The most important are the oat bird-cherry aphid, the corn leaf aphid, the English grain aphid and the green bug. Aphids acquire BYDV by feeding on infected plants and transmit the virus in subsequent feedings.
Symptoms of barley yellow dwarf are highly variable and can be confused with those of wheat streak mosaic, nutrient deficiency, root and crown diseases, and environmental stress. Barley yellow dwarf is tentatively diagnosed from the presence of aphid vectors and the occurrence of yellowed, stunted plants grouped singly or in small patches among normal plants. Leaf discoloration in shades of yellow, red or purple from the tip down and from the margin to midrib is typical. In wheat symptoms start to become obvious at about the jointing stage of growth. Barley yellow dwarf does not produce a distinct mosaic pattern as do wheat streak mosaic or soil-borne mosaic.
Prevention of Barley Yellow Dwarf
- An important strategy to manage BYDV is to plant resistant or tolerant cultivars. High levels of resistance or tolerance are not available in wheat; however, commercial cultivars vary in their susceptibility to barley yellow dwarf. A good strategy is to grow diverse varieties to minimize the potential that a single variety will be severely affected by the disease.
- Cultural methods of managing barley yellow dwarf include controlling grassy weeds, including volunteer cereals, within and near wheat production fields. In addition, small grains should not be planted in midsummer as cover or companion crops in wheat-producing areas. These practices will minimize the virus and vector reservoirs in wheat growing areas.
- Delaying fall seeding of wheat until aphid populations decline minimizes the risk of barley yellow dwarf. Optimum seeding dates for winter wheat have been established for the various growing areas of Nebraska. These can be obtained from your local extension office. Avoid planting wheat earlier than the recommended date for your area.
Bud Blight
Bud blight, otherwise known as bud rot or bud mold, is the presence of moldy buds developing on cannabis plants. It is a common but devastating occurrence that can occur for a variety of reasons. Bud blight affects many different types of plants, including tomatoes and soybeans, in addition to marijuana of all types.
Bud blight is not a symptom of a specific disease; rather, it can be caused by a number of different issues affecting marijuana plants, such as overwatering, overcrowding, and too much humidity. Unlike other cannabis plant infections, such as powdery mildew, which is caused by a fungal pathogen, bud blight can be caused by either a fungus or a bacterium. Even plants weakened by a pest infestation can begin to develop bud blight.
Peanut Stunt Virus
The peanut stunt virus causes discoloration and distortion of the leaves of peanuts and some other rhizomes, stunting their growth. Aphids and sap spread the virus.
PSV is transmitted from plant to plant by several species of aphids (Aphis craccivora, A. spiraecola and Myzus persicae) in a stylet-borne manner. It can also be transmitted by mechanical inoculation.
It has been shown to be transmitted by seeds in peanuts at a very low level but this is not considered to be very important to the spread of this virus. The virus can be introduced into a susceptible field crop by aphids from a nearby reservoir (infected perennial hosts like clover, alfalfa or perennial peanuts) and then is spread further into the field by aphids. It can be spread in perennial crops by harvesting (mechanical transmission) and possibly by root grafts.
Prevention
Because PSV is not an economically important disease in groundnuts, no control methods are available. However, it is advisable to use cultural practices such as eliminating the source of infection and using good quality seed to prevent the disease spreading. In the case of forage legumes, such as clover in perennial pastures, the use of resistant or tolerant varieties is recommended.
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