Management of plant virus

                                                    MANAGEMENT OF PLANT VIRUS

 • Many plant pathogens, particularly fungi, can be controlled by the application of chemicals which interfere in some way with the metabolism of the invading pathogen, and so prevent disease.

 • Unfortunately, these methods cannot be used so extensively to control plant viruses.

 Prevention, or at least measures, for the control of viruses, involves : 

1. Elimination of sources of virus. 

2. Elimination of the virus from infected plants.

3. Control of vectors. 

4. Breeding for resistance and the use of cross-protection methods. 

1. Elimination/Removal of source of virus

 ❖ Living host for the virus 

            Removal of perennial and annual weed host

 ❖ Plant remains Plant remains in the soil, or attached to structures such as greenhouses, may act as a mechanically transmitted virus and act as source of infection for the next crop(TMV)

 ❖ Rouging Removal of diseased plants from the field. If the spread is occuring rapidly from the sources outside the crop, rouging the crop will have no beneficial effect. If the spread is within the crop it is effective. 

❖ Virus free seed The selected seed should be free from virus infection. 

❖ Virus free vegetative stocks The selected vegetative stocks should be free from virus. 

 Methods for Obtaining virus free plants 

1. Naturally occuring virus free material

      • Budwood can be taken from the uninfected parts of tree.

      • The shoot tips of rapidly growing stems may sometimes be free of virus. (Eg. Holmes was able to obtain dahlia cuttings free of TSWV.)

 2. Heat theraphy

       • Temperature of 35-45 degree celcius for minutes or hours (Eg. PVX-culture of shoot tips from heat treated sprouting potatoes gives a useful portion of plantlets free of pvx) 

       • Potato leaf roll virus can be free from potato tubers, if they are stored during summers and in temperature upto 36 degree celcius.

       • Sugarcane cuttings stored in water at temperaure of 52 degree celcius for 30 min. 

 3. Cryotheraphy

         • Cuttings of sweet clover are stored at 14 degree celcius control Wound tumour virus

         • Potato and chrysanthemum plants free from four different viruses were regenerated form meristem tip culture that had been held for 6 months in the dark at 6-7degree celcius. 

 4. Thermotherapy 

            High temperature treatment has been widely used in production of virus free plants(30-40degree). Some of the list of host, virus eliminated, and on which temperature the viruses are eliminated are - Chrysanthemum, Chyrsanthemum B virus, 35-38.  Carnation, Carnation ringspot virus, 25-40.  Banana, Cucumber mosaic virus, 35-43.  Gooseberry, Gooseberry vein banding virus,  33.  Potato, Potato virus x,y, 33-38.

  5 .Meristem tip culture 

 Used to produce virus-free stock 

        • Apical meristem region normally free from virus are excised (short pieces 0.1mm to 0.5mm) and grown in vitro in tissue culture medium to raise virus free plants. 

        • The basic ingredients are an appropriate selection of mineral salts (macro and micronutrients), sucrose, and one or more growth stimulating factors such as IAA or GA, sometimes in agar.

         • Culture of single cells or small clumps of cells from virus-infected plants may sometimes give rise to virus-free plants 

         • The technique works well for chrysanthemums, carnations and potatoes

 6. Tissue culture 

        • Culture of single cells or small clumps of cells from virus-infected plants may sometimes give rise to virus-free plants. 

        • Plants regenerated from shake subculture of small pieces of tobacco tissue originally from TMV infected plants were free of TMV. 

        • A significant proportion of calli obtained from yellow green areas of TMV infected Nicotiana tomentosa gave rise to regenerated plants that were free of virus. 

Management by cultural/Agronomic practices 

 1.Breaks in infection cycle

            A break during the year where no susceptible plants are grown has proved effective in the control of certain other viruses (Eg – WSMV planting in winter)

 2.Changed planting dates 

            The best time to sow will depend on the time of migration of the vector. If it migrates early in the growing season, late sowing may be advisable. If is late migration, early sowing should be done. (Eg- RTV which is transmitted by GLH, the farmers are encouraged to plant synchronously, to allow a break between crops and to avoid very late planting). 

 3.Plant spacing

             The high percentage of more closely space tend to escape infection than widely spaced ones Eg-Groundnut rosette virus and RTV.

 2.Elimination of the virus from infected plants 

Possible chemical control

        • Virazole (ribavirin, a ribofuranosy ; thiazole carbonate compound) has been tried on plant viruses. 

        • Callus of gladiolus infected with CMV and Bean yellow mosaic virus, grown in tissue culture, was freed of the viruses when treated with virazole. 

        • Virus inhibitory substances are present in many plants and animal products like skimmed milk. 

        • Bruceine-D, isolated from seeds of Brucae javanica (Simarcoubaceae) is reported to significantly inhibit infection and replication of tobacco mosaic virus (TMV). It also shows a strong inhibitory effect on the infectivity of potato virus Y and cucumber mosaic virus (PVY and CMV) 

        • Many higher plants contain antiviral factors such as Pelargonium hortorum, Chenopodium album, Azdiracta indica, vitis vinifera contains alkaloids, terpenoids, lignans and ribosome inactivating specific proteins such as in Mirabilis jalapa 

        • Some plant growth promoting rhizobacteria are reported to delay appearance of virus symptoms • In tobacco cucumber mosaic virus is controlled by Pesudomonas chlororaphis 

3. Control of vectors 

 1. Air borne vector

 2. Soil borne vector

  1.Air borne vector 

It includes insects such as Aphids, white flies etc.., 

Control of insects by various methods like

  • Insecticidal control

 • Biological control with fungi 

• Biological control with predators and parasitoids 

• Modifications in cultural practices. 

Insecticides control 

• Use of Nicotinoids(Imidachloprid) are systemic neurotxins that quickly incapacitate the insects.

 • The Insect growth regulators include buprofezin(chitin synthesis inhibitor) and pyriproxyfen. 

• Mineral oil, rapeseed oil and soya oil have been used against aphids with success.

 Biological control with fungi 

• Lecanicillium(verticilium) lacani , Paecilomyces fumosoroseus and Beauveria bassiana are fungi that parasitize aphids and whiteflies. 

• Lecanicillium muscarium is highly effective against immature stages of Bemisia tabaci. 

• Lecanicillium lacani is reported as an effective parasite of Macrosiphum euphorbiae (potato aphid) and Myzus persicae (green peach aphid) 

• Pandora delphacis (Emtomophthorales) used as the effective for the control of aphids. 

Biological control with predators and parasitoids 

• Coccinelid predator of Aphis craccivora are often common cultivars resistant to aphids. 

• Nephaspis oculatus (coloeptera) a beetle is predator of Bemisia tabaci and Bemisia argentifolii(silver leaf whitefly) 

Cultural practices 

It includes crop-free period, alteration in date of planting , crop rotation, intercropping, Sticky traps and crop residual disposal perform well against insect populations. 

2. Soil borne vector

 It mainly includes nematodes and fungi 

1. Exclusion or avoidance usually by quarantine. 

 2. Reduction of the initial population density by cultural approaches such as use of clean planting stock or crop rotation with a break crop of a species that is not a host for the target nematode, by chemical nematicides, by biological tactics such as introducing biological agents antagonistic to nematodes and organic amendments, or by the use of nematode-resistant crop varieties that will reduce nematode populations. 

 3. Suppression of nematode reproduction by chemicals, organic amendments, and certain natural and transgenic resistance traits.

 4. Restriction of the current or future crop damage by nematode resistance. However, tolerant cultivars will reduce crop damage due to nematode feeding but will not reduce the chances of virus infection. 

Soil borne vector – Fungi 

• In general, attempts to control infection with viruses having fungal vectors by application of fungicides

 • The zoospores of Olpidium brassicae transmits Tobacco necrosis virus, Tobacco stunt virus and lettuce big vein virus control by fungicides . 

4. Breeding for resistance and the use of cross-protection methods

 Protecting the Plant pathogen 

1. By immunization 

2. Satellite virus as biocontrol of plant viruses.

 Immunization

 By use of the mild strain the severe strain should by controlled 

Satellite virus

• Satellite viruses and RNAs are described in and, as far as potential bio control agents, fall into three categories: those that enhance the helper virus symptoms, those that have no effect, and those that reduce the helper virus symptoms. 

• The last one has potential as a control agent. Most of the work has focused on the satellites of CMV with some successes in field application, especially against the satCMV 

Virus resistance can be obtained by

 Expression of viral proteins in plants 

1.Coat protein (Eg-TMV, Potato leaf roll virus)

2.Replicase Interactions between transgenic replicase proteins and other virus-encoded proteins may affect the processes of replication and cell to cell movement. (RdRps on TMV) 

3.Movement protein Trangenic plant expressing Mutant Movement protein show resistance through competition for plasmodesmatal binding sites.(Eg-TSWV,Caulifower mosiac virus)