Control of Viruses Infecting Crops and Plants
Plant viruses and virus diseases have been studied for more than 100 years and many measures have been given to their control. This has been difficult to achieve because of the lack of any effective means of curing virus infected plants. Chemotherapy, thermotherapy and meristem tip culture is successful, but they cannot be used on a large scale. The main approach has been to prevent or delay virus infection or to decrease its effects. Various means have been used to achieve these objectives, including quarantine measures, crop hygiene, use of virus-free planting material and eradication, changing cropping practices, use of pesticides, deploying a resistant or tolerant variety. Plant virus and viroid diseases are of vital importance and concern to the farmer, horticulturist, forester and gardener. It is well established that the virus and viroid diseases in different crops cause enormous losses in terms of quantity and quality of products.
Virus infection is a serious problem in agricultural sector. Mostly every plant species is prone to infection by at least one or more. There are more than 500 known plant viruses. These viruses create economic loss and affect the quality of crop. Farmers may need to use several control methods during the plant growth to prevent it from viral infection and dissemination. Viruses transmitted from vector can be controlled by chemical insecticides, fungicides and nematicides to stop from further infection to other crops, plants are infected with a mild dose of virus so that the plant can naturally produce resistance to the virus, this method has its limitations, developing conventional breeding technique could offer some level of resistance. At present many transgene have been designed to develop resistance to viral infection including viral replicase, nuclear inclusion gene, movement protein and nonviral sequences from many species.
Virus particles are extremely small and seen only with an electron microscope. Most plant viruses are either rod-shaped or isometric (polyhedral). TMV, potato virus Y (PVY), and cucumber mosaic virus (CMV) are examples of a short rigid rod-shaped, a long exuous rod shaped, and an isometric virus. Viruses contains of an inner core of nucleic acid (either ribonucleic acid [RNA] or deoxyribonucleic acid [DNA]) surrounded with outer sheath or coat of protein). The cell membrane in plants is surrounded by a rigid cell wall; plant viruses need a wound for their initial entrance into plant cell. Wounds in plants occur naturally, such as in the branching of lateral roots, it might also be the result of agronomic or horticultural practices, or other mechanical means; fungal, nematode, or parasitic plant infections; or by insects. Organisms which transmit pathogens are called vectors. Mechanical and insect vector transmission is the two most important means by which plant viruses spread.
Plant Virus Transmission
The plant viruses are taxonomically very diverse these can be found among arthropods, nematodes, fungi. Arthropod vectors transmit most plant virus like aphids, whiteflies, thrips, beetles, mealy bugs and mites. Aphides have more than 200 vector species among them; more than half of about 550 vectors are transmitted by aphids (55%), 11% by leafhoppers, 11% by beetles, 9% by whiteflies, 7% by nematodes, 5% by fungi and 7% by thrips.
Different modes of virus transmission have been characterized depending on the retention time, sites of retention and internalization of virions by vectors. Nonpersistent viruses are retained by their vectors for less than a few hours whereas semi-persistent viruses are retained for days, weeks, or even years. Viruses in these two categories are acquired from infected plants and inoculated within seconds or minutes to recipient plants. They do not require a latent period, that is time interval between acquisition and transmission, and do not replicate in the vector. Persistent viruses, once acquired from infected plants, are associated with the vector for the remainder of their lifetime. They require long acquisition times and long latent periods. Successful transmission of persistent viruses requires an internalization of the ingested viruses that are actively transported across several cell membranes.
Examples and Control of Some Plant Virus
Cucumber mosaic virus (CMV) genus Cucumovirus
Cucumber mosaic virus (CMV) genus Cucumovirus belongs to the family Bromoviridae. CMV consists of 30 nm icosahedral particles with a tripartite genome that is encapsulated in three distinctive particles. Numerous strains of CMV occur, often making the virus difficult to identify from symptoms alone. A number of strains have been assigned to two groups ToRS and DTL according to their antigenic properties. In Africa, CMV has been reported in eastern and southern African countries, in Sudan and in Tunisia.
Symptom are seen in cucumber, pumpkin, and melon, CMV causes severe leaf mosaic and deformed, stunted, or mottled fruits.CMV has one of the widest host ranges known among plant viruses. It attacks more than 750 plant species in 365 genera and 85 families. Other vegetable hosts of CMV include beet, celery, hot and sweet pepper, and tomato. CMV can be controlled by cultivars resistant, partially resistant, tolerant, are used for cucumber and melon crops.
Watermelon mosaic virus-2 (WMV-2) genus Potyvirus
Watermelon mosaic virus-2 (WMV-2) genus Potyvirus has flexuous rod-shaped particles 730-760 nm in length. WMV-2 contains a single-stranded positive-sense RNA.WMV-2 has worldwide spread. The virus has been reported in eastern and southern African countries and in Morocco. Foliar symptoms in many cucurbit crops include chlorotic mottle and mosaic. Fruits are often stunted and distorted. WMV-2 has a narrow host range. It infects cucurbit crops. However, it can also infect some noncucurbitaceae such as pea (Pisum sativum) .WMV-2 is transmitted in the nonpersistent manner by many aphid species. The virus is easily sap-transmissible. It is not seed borne. Resistance to WMV-2 has been reported in melon and in pumpkin. When resistant varieties are not available, overlapping of old and new cucurbit crops should be avoided.
Zucchini yellow mosaic virus (ZYMV) genus Potyvirus
Zucchini yellow mosaic virus (ZYMV) genus Potyvirus has particles which are flexuous filaments with a modal length of 750-800 nm, containing a single-stranded positive-sense RNA. The virus was first described in zucchini squash in Italy. ZYMV is also known as Muskmelon yellow stunt virus. ZYMV is very variable. Twenty-two isolates have been grouped into three pathotypes according to the reaction on muskmelon. ZYMV is found in many areas of the world where cucurbits crops are grown. In Africa, ZYMV has been recorded in Algeria, Egypt, Mauritius, Morocco, Kenya, and Zambia. ZYMV causes severe damage on marrow or zucchini squash (Cucurbita maxima), muskmelon (Cucumis melo), cucumber (Cucumis sativus), and watermelon (Citrullus lanatus). The leaf symptoms include mosaic, yellowing, shoestring, and stunting. Fruits are deformed, twisted, and covered with protuberances. ZYMV has a narrow host range that includes mainly cucurbit species. ZYMV is transmitted in a nonpersistent manner by aphids. Myzus persicae, Aphis gossypii, and Macrosiphum euphorbia are common vectors. The virus is mechanically transmitted and seed transmission, although at a low rate, has been reported in zucchini squash. Resistant varieties of cucumber and melon are available. Mild cross-protection has been used successfully in marrow and melon crops.
Tomato spotted wilt virus (TSWV) genus Tospovirus
Tomato spotted wilt virus (TSWV) genus Tospovirus belongs to the family Bunyaviridae. TSWV consists of roughly spherical, enveloped particles, ranging in diameter from 70 to 110 nm. The genome of TSWV consists of three single stranded RNA segments. TSWV is a cosmopolitan virus which was first found in South Africa in 1905 many other countries in subtropical as well as in temperate climate zones. TSWV causes severe symptoms in pepper and tomato crops. Infected plants are stunted and old leaves turn yellow. Systemic necrotic patterns may occur. Infected fruits usually show characteristic green, yellow, and red, slightly raised bulls eye rings. TSWV has a broad host range. At least 250 plant species are susceptible to the virus. TSWV is transmitted in a persistent manner by several thrips species that include Frankliniella fusca, F. occidentalis, F. schultzei, Scirtothrips dorsalis, and Thrips tabaci. It has been shown that the virus replicates within the thrips vector. Non vector transmission of TSWV is by mechanical inoculation. Sources of resistance to TSWV have been identifed in close relatives of pepper and tomato and have been used to develop resistant varieties. Other efficient control methods include the elimination of thrips and host plants.
Rice yellow mottle virus (RYMV) genus Sobemovirus
This virus is a single-stranded, positive sense RNA (SSRNA) measuring about 28nm in diameter. It is specific to the African continent. The disease is identifed by mottled and yellowing symptoms of varying intensities depending on the genotype. Additional symptoms include stunting, grain or spikelet sterility, and grain discoloration. In severe cases, diseased plants may die. It is the only rice disease of economic importance in Africa.
The management strategies of the virus disease involve the use of varietal resistance and chemical treatment, cultural practices, training for disease management and diagnosis, and Task Force mechanisms.
Genetic Host Resistance: -
Since different cultivars and species show different degrees of resistance to some viruses, resistant types should be planted whenever they are available. Recent advances in plant cell molecular biology and virology have lead to the development of genetically modified plants with superior resistance to some viruses.
There are numerous cultural practices that can be used to reduce plant losses due to virus infection. Scouting and removal of symptomatic plants or known alternative weed or volunteer plants that may serve as a reservoir for a given virus , use of clean or sanitized tools and equipment use of disposable over garments Rotations to non-host crops. Geographic isolation of production facilities may also help avoid losses caused by plant viruses. Some viruses are permanently inactivated by prolonged exposure of infected tissue to relatively high temperatures for example, 20 to 30 days at 38 degrees C (100 degrees F). This procedure, called heat therapy, frees individual plants or cuttings of the virus. The clean tissue is then used as a propagative source, allowing large-scale production of virus-free plants.
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