Plant Viruses And Disease Management

Viruses are obligate parasites that require a living host for their growth and multiplication. Viruses enter plant cells through wounds usually made mechanically or by vectors. Viral diseases are a significant cause of crop loss in India, affecting crops such as rice, wheat, maize, sugarcane, tomato and potato, among others. Infections caused by various viruses cause an average yield loss of about 70 – 80%. The extent of loss may also range 90 – 100% depending upon various factors like severity of disease, source of infection, duration of infection and environmental conditions.  

Symptoms of viral diseases in plants can vary depending on the virus and the host plant. However, some of the most common symptoms include stunted growth, yellowing of leaves, mottling or streaking of leaves, necrosis, distorted leaves or flowers, wilting and reduced yield. Therefore, prevention and control of viral diseases in plants is crucial for maintaining plant health and crop yield.     

Transmission of Plant viruses:  

Plant viruses are transmitted through various sources from an infected plant to a healthy plant; 

Vegetative Propagation

Vegetative parts such as tubers, bulbs, cuttings or rhizomes of the infected plants when used as mother plants for propagation will serve as source of viral infection. The new plant raised will also be infected by the viral disease. 

Seeds

Seeds can be a source of spreading viral infection in plants. Seeds can be affected by viruses through contaminated soil or water. Virus infected seed can transmit virus to plants through seed. This can result in a new generation of infected plants, which can then spread the virus to other plants through various means, such as insect vectors, contaminated soil, water etc. 

Mechanical transmission

Mechanical transmission of virus occurs when the virus is physically transmitted from one plant to another by rubbing of sap of infected plants over healthy plants or through contact with contaminated tools and equipments or through human handling during intercultural practices. 

Pollen transmission

When a pollinator visits an infected plant and collects pollen, the virus can be transferred to other plants as the pollinator moves from one plant to another. The virus can also be transferred through wind, which can spread the infected pollen over longer distances.   

Insect transmission

Plant viruses can also be spread through insect transmission, and this is known as vector transmission. Insects such as aphids, whiteflies, leaf hoppers and thrips are common vectors for many plant viruses. Other than this, nematodes and mites also act as vectors for viral infection.  

Major Viral Diseases in Plants: 

1. Tobacco Mosaic Virus:

Host range – Tobacco, Potato, Brinjal, Pepper, Cucumber 

Transmitted by – Sap, contaminated farm equipments, soil debris, hands of farm labours 

Symptoms – Mottling pattern of light and dark green patches on the leaf lamina, stunted growth, development of blisters/irregular crumbled swellings on leaves, small and misshapen infected leaves. 

2. Cauliflower Mosaic Virus (CaMV)

Host range – Tomato, Melons, Squash, Beet, Spinach, Broccoli, Cabbage  

Vector – Aphids 

Symptoms – Mosaic patterns on affected leaves, young leaves become twisted leading to stunted growth. 

3. Sugarcane Mosaic Virus

Host range – Sugarcane 

Vector – Aphids; Transmitted by – Infected canes used as seed/sett 

Symptoms – Yellowish/Chlorotic stripes with alternate normal green portion of the leaf, stunted growth. 

4. Maize Mosaic Virus

Host range – Maize 

Vector – Leaf hopper 

Symptoms – Chlorotic stripes and spots on leaf, leaf sheath, stalks & husks, moderate to severe rosette formation on new growth. 

5. Cucumber Mosaic virus (CMV)

Host range – Cucumbers, Tomato, Pepper, Squash, Melons, Beans, Peas, Spinach, Beetroot, Lettuce, Radish 

Vector – Aphids 

Symptoms – Mosaic patterns, stunted growth, distortion of leaves, yellowing of veins, necrotic spots or streaks on infected leaves. 

6. Groundnut Bud Necrosis

Host range – Groundnut, Tomato, Green gram 

Vector – Thrips 

Symptoms – Mottling or chlorotic spots on leaves, necrosis of terminal bud, distortion of lamina, reduction in leaflet size, reduced flowering, production of abnormally small and wrinkled seeds. 

7. Leaf Curl Virus

   

Host range – Tobacco, Cotton, Papaya, Guava, Chilli, Tomato  

Vector – Whitefly 

Symptoms – Stunted growth due to reduced nodes and internodes size, curling of leaf margins upward and downward, thickening of veins, crinkling and distortion of leaves. 

8. Vein Clearing / Yellow vein Mosaic

Host range – Bhendi 

Vector – Whitefly 

Symptoms – Yellowing of veins in the leaf blade, reduced size of younger leaves and stunted growth, flowering and fruiting are restricted, if formed are small and hard. 

9. Rice Tungro Virus

Host range – Paddy 

Vector – Leaf hopper 

Symptoms – Stunted growth, yellow to orange discoloration of leaves, interveinal chlorosis, reduced tillering with poor root system. 

10. Tomato Spotted Wilt Virus

    

Host range – Tomato 

Vector – Thrips 

Symptoms – Appearance of streaks on leaves, stems and fruits. Leaves have small, dark, circular spots and may have bronzed appearance. Numerous concentric circular markings on fruits. Ripe fruits show alternate red and yellow markings. 

11. Papaya ringspot mosaic virus

    

Host range – Papaya, Guava 

Vector – Aphids 

Symptoms – Vein clearing, leaf margin roll downwards and inwards, mottling patterns, dark green blisters, fruits having circular concentric rings on them, stunted growth. 

12. Sterility Mosaic 

Host range – Red gram 

Vector – Eriophyid mite 

Symptoms – Bushy and pale green appearance of affected plants, excessive vegetative growth, mosaic pattern on leaves, reduction in leaf size, stunted growth, complete or partial termination of flowering leading to sterility. 

13. Bunchy Top Virus

Host range – Banana 

Vector – Aphids; Transmitted by – Infected suckers (Primary source of infection) 

Symptoms – Stunted plant growth, reduction in leaf size, leaf marginal chlorosis and curling, leaves crowded at the top giving “bunchy top” appearance and smaller branch size.  

14. Potato Leafroll Virus

Host range – Potato, Tomato, Capsicum 

Vector – Aphids 

Symptoms – Stunted growth, chlorotic spots, mottling patterns, leaves roll upwards, upright orientation of leaves. 

15. Citrus Tristeza Virus

Host range – Citrus 

Vector – Aphids 

Symptoms – Development of deficiency symptoms on leaves, dieback of twigs, decay of roots, reduced fruit set. 

16. Grassy Stunt Virus

Host range – Paddy 

Vector – Brown Plant Hopper 

Symptoms – Stunted plant growth with excessive tillering, pale green leaves with rusty spots, grassy and rosette appearance of plants. 

17. Ragged Stunt Virus

Host range – Paddy 

Vector – Brown Plant Hopper 

Symptoms – Dark green leaves with spiky or serrated edges, leaf edges are twisted into spirals giving ragged appearance, stunted plant growth, delayed flowering, incomplete panicle emergence. 

18. Mungbean Yellow Mosaic Virus

Host range – Black gram, Green gram, Moth bean, Soybean, Cow pea 

Vector – Whitefly 

Symptoms – Stunted growth, yellowing, presence of mosaic patterns, delayed maturity, reduced flower and pod formation if produced pods would be small and distorted. 

Preventive Measures to Control Plant Viral Diseases: 

• Use disease resistant plant varieties 
• Use certified virus free planting materials such as seeds or cuttings 
• Adopt crop rotation of host crops with non-host crops. This can help break the cycle of viral infections in the soil and reduce the risk of virus buildup 
• Cultivate trap crops to avoid disease causing insect vectors i.e., marigold in bhendi and cowpea in cucurbits field to control whitefly 
• Practice proper sanitation practices by removing and destroying infected plants and plant debris, cleaning and disinfecting equipment, to prevent spread of viral diseases 
• Isolate the plants that are known to be infected with viral diseases 
• Control the disease-causing insect vectors by using appropriate insecticides 

Management of White fly, Aphids, Thrips, Leafhoppers: 

Management of Mites: 

Conclusion: 

Plant viruses can have a significant impact on crop production and can cause a range of symptoms that vary depending on the host plant, virus strain and environmental factors. Plant viruses can be transmitted through vectors such as insects or through mechanical means, leading to significant economic loss of yield. Effective management strategies can help prevent the spread of plant viruses and reduce their impact on crops. Using virus-free planting materials, practicing good sanitation practices, controlling vectors through the use of insecticides and implementing crop rotation can help prevent the spread of viruses.