Viral Diseases of Leaf - Virology - The Biology Notes

Why do we study viral diseases of the leaf?

• that infect humans can infect plants too.

• Leaves are important parts because they are the primary source of photosynthesis Respiration, Transpiration, Guttation (in some plants only).

• A leaf is a plant organ, exposed to the external environment and is the primary way plants interact with the atmosphere and take care of their basic needs.

Viruses Green leaves contain chlorophyll, which is not only how the leaves get their green color but also a major ingredient in the photosynthesis process.

• The main reason that we study plant viruses is the negative impact that viral diseases have on crop production, economy and human health.

• In 1886 German-born Adolph Mayer, a Dutch investigator in Holland was first to point out that tobacco mosaic is readily transmissible and infectious.

• The field of virology really began to develop after the invention of bacterial filters by Charles Chamberland in 1884.

• It was started in the late 19th century, (1892) when Dutch microbiologist Martinus Beijerinck and Russian researcher Dmitrii Iwanowski were investigating the cause of a mysterious disease of tobacco.

• These researchers independently described an unusual agent that caused mosaic disease in tobacco.

• Its size distinguished this agent from other disease-causing agents which was much smaller sized compared to that of other microbes

• The current estimate of recognized viruses is approaching 4000 of which about 1000 are plant viruses.

Image Sources: BigHaat and Ohio State University

List of leaf virus

• Tobacco Mosaic Virus:- Attacks members of the nightshade, or Solanaceae family. These include tobacco, pepper, potato, tomato, eggplant, cucumber, and petunia. The virus enters through breaks into the cell wall by insects or other physical damage.

• Cucumber Mosaic Virus: It infects cucumber, tomato, beet, pepper, melons, celery, and other plants. Aphid spread it. The virus causes twisting in young leaves. Stunt the growth of plants.

• Sugarcane Mosaic Virus:- It discolors the leaves of the sugarcane plants, stunts the growth of young plants. Aphid and infected seed spread the virus.

• Cauliflower Mosaic Virus: – virus infects members of the brassica, or mustard family, which includes cabbage, brussels sprouts, cauliflower, broccoli, and rapeseed. Causes mosaic on leaves and stunt the growth of the plant.

• Maize Mosaic Virus: – virus causes yellow spots and stripes on the leave of corn, stunting its growth. Leafhoppers spread the virus.

• Lettuce Mosaic Virus: – virus mottles the leaves of almost all types of lettuce,

• Cotton leaf curl Virus:- cause vein swelling, cup-shaped outgrowth on the inner side of leaves, plant looks like a dwarf  and wilted. whiteflies spread the virus.

• stunting its growth. Aphid and infected seed spread this disease.

• Leaf Roll of Potato

• Peanut stunt virus: – virus causes discoloration and distortion of the leaves of peanuts and some other rhizomes, stunting their growth. Aphid spread it.

Symptoms due to viral infections

• Mosaic formation

• Vein clearing and vein banding

• Ring spots

• Necrosis

• Wilt

• Stunting

• Changes in leaf form

• Flower breaking

• Fruit abnormalities

Transmission of plant viruses

• Graft transmission

• Mechanical transmission

• Infected seed

• Vectors which include:-

• Aphids

• Leaf Hoppers and Plant Hoppers

• Whiteflies

• Mites

• Beetles

• Grasshoppers

• Nematodes

Cotton Leaf Curl

• Cotton (Gossypium hirsutum) is the most important cash crop having a great agroeconomic significance throughout the world. Cotton is grown around sixty countries of the world.

• Cotton leaf curl virus (CLCuV) is the most devastating pathogen of cotton, which is responsible for causing huge economic yield losses especially in agricultural countries like Pakistan, where cotton gives 10% GDP.

• The virus is usually prevalent in Pakistan and North India.

• It was 1st time reported in 1912 in Nigeria.

• In Pakistan, it was reported in 1967. In Pakistan, cotton is grown on an area of 2.806 million hectares with a production of 13.983 million bales

• Also observed in Multan in 1985.

• ssDNA Virus, Icosahedral

• Belongs to Geminiviridae

• It has 4 genera of viruses but most prevalent is begomovirus.

• Transmit via vector named Whitefly.

• In Pakistan, its epidemic was reported in 1993 when it infected 0.9 Million hectares of cotton in southern Punjab.

• White Fly feeds in Plant phloem sap by inserting its stylets in plant tissue and locating the vascular tissue.

• Threshold period of whitefly and virus transmission.

• Latent Period

• The disease is affected by temperature, disease infestation is maximum at temperature ranges from 33-45 degrees Celsius.

• Maximum disease incidence are recorded in 6 weeks old seedlings.

• Incidence decrease as plant age increases.

• Rainfall prior to the seedling increase vector population

Symptoms

1. Cup-shaped outgrowth on the lower side of curl leaves-Enation.

2. The appearance of disease at seedling stage retards flowering and maturation.

3. Vein Swelling.

4. Plant becomes Dwarf

5. Leaves may look Wilted

6. Young leaves become yellowish

Management of disease

1. Use resistant or tolerant cultivars.

2. Protect seedlings from vector whiteflies.

3. Use only good seeds and healthy transplants.

4. Control whiteflies

5. Immediately remove infected looking plants and bury them

6. Control weeds

7. Do not plant cotton near tomato or other crops susceptible to whiteflies

8. Practice crop rotation by planting crops that are not susceptible to whitefly

Tobacco Mosaic Virus (TMV)

• Positive sense ssRNA Virus belongs to the genus Tobamovirus that infects a wide range of plants, especially tobacco and other members of the family Solanaceae or dicotyledonous plants.

• Well known to cause mosaic on Tobacco.

• Rod-shaped.

• The TMV genome consists of a 6.3–6.5 kb.

• The protein coat consists of helically arranged capsomeres.

• The disease is caused by RNA Penetration into the cell.

1. Penetration: – life cycles start by penetration of the virion into the cell**.** Plant viruses are unable to penetrate the plant cuticle and cell wall. It is believed that the virion enters the cytoplasm of the cell passively through wounds caused by mechanical damage to the cuticle and cell wall.  Tobacco mosaic virus (TMV) and some other plant viruses are carried by the wind, raindrops, gardening equipment, and grazing animals to host plants. They enter into leaves damaged by mechanical injuries, like abrasion.

2. Uncoating: – The next phase of virus infection is the partial or complete removal of the coat protein shell of the virion in the cytoplasm.

3. Transcription:- Next the cell mediates expression of the viral genome by providing a transcription apparatus (for DNA viruses) and a translation apparatus (for all viruses). The DNA viruses must be transported to the nucleus for transcription in order to gain access to the cell proteins required for the production of messenger RNA from viral DNA.

4. Translation:- Translation of viral RNA in the cytoplasm produces viral proteins that are required for the completion of the virus life cycle. All viruses must direct the formation of at least three types of proteins: replication proteins that are essential for nucleic acid production, structural proteins that form the protein shell and other components contained in the virions, and movement proteins that mediate virus transport between plant cell.

5. Replication: The viral replication proteins combine with cellular proteins to produce a complex of proteins that manufactures multiple copies of the virus genome. These newly made genomes interact with the structural proteins to form new virions. Replication occurs in the cytoplasm of plants.

6. Assembly: – Assembly of virions of TMV occurs in the cytoplasm of host cells. When a large number of capsomer proteins and ss-RNA have accumulated in the infected cell, the virions are assembled spontaneously in a highly organized manner. The capsomers are assembled to form disks made up of two layers of helically arranged protomers.

7. Release of the virus through plasmodesmata: – The next step in the virus reproduction cycle is the movement of the virus into neighboring cells. Depending on the virus, the viral genomes or the virions are transported into neighboring cells through small channels called plasmodesmata that form connections between cells. Many viruses produce movement proteins that modify the plasmodesmata channels and facilitate viral movement into neighboring cells.

Transmission

• Direct Contact

• Soil

• Seed

• Air and water

• Vectors (aphids and White Flies)

Symptoms

• Vein Clearing

• Black green Spots

• Lightning of leaf color after infection

• Make leaves chlorophyll deficient

• Yellow spots on green background

• Formation of Concentric Rings

Management of disease

• Avoid infected soil.

• The seedbed should sterilized.

• Avoid hand contamination, use soap.

• Diseased plants should be removed and burnt to stop further spread of the disease.