(b) What are the various modes of infection and dissemination of plant diseases caused by viruses ? Describe.

Plant viruses employ various ingenious strategies to overcome plant defenses, infect host cells, and subsequently spread within a single plant and to new plants. These mechanisms can be broadly categorized into modes of infection and modes of dissemination.

Modes of Infection

Plant viruses generally cannot penetrate the intact plant cell wall directly. Therefore, they rely on pre-existing wounds or vectors for entry.

• Mechanical Wounding: Viruses often gain entry through physical damage to plant tissues.
  • Direct Contact: Wind, rain, or the rubbing of infected leaves against healthy ones can create microscopic abrasions, allowing viral particles from sap to enter. For example, Tobacco Mosaic Virus (TMV) and Tomato Mosaic Virus (ToMV) are highly stable and can remain infectious for extended periods in dry plant material and soil, easily spreading through mechanical contact.
  • Agricultural Practices: Pruning, harvesting, weeding, and other farming activities using contaminated tools or hands can inadvertently transfer viral sap from diseased to healthy plants.
  • Animal Feeding: Herbivorous animals, as they feed on plants, can create wounds and transfer viral particles.
• Vector Transmission: The most common and agriculturally significant mode of infection involves biological vectors that carry and transmit viruses.
  • Insects: A wide range of insects, particularly those with piercing-sucking mouthparts like aphids, whiteflies, leafhoppers, and thrips, are primary vectors. They acquire viruses while feeding on an infected plant and transmit them to healthy plants during subsequent feeding. The mode of transmission can be non-persistent, semi-persistent, or persistent, depending on how long the virus remains viable in the vector.
    • Non-persistent: Viruses attach to the insect's stylet (mouthpart) and are transmitted almost immediately upon feeding (e.g., Cucumber Mosaic Virus).
    • Semi-persistent: Viruses enter the insect's foregut and remain infectious for a few days (e.g., Beet Yellows Virus).
    • Persistent: Viruses circulate throughout the insect's body, sometimes even replicating within it, and can be transmitted for the insect's entire lifespan (e.g., Tomato Spotted Wilt Virus transmitted by thrips).
  • Nematodes: Soil-borne nematodes can transmit viruses by feeding on infected roots and then subsequently on healthy roots (e.g., Tobacco Ringspot Virus).
  • Fungi/Protozoa: Certain soil-borne zoosporic protozoa (e.g., Polymyxa graminis) can transmit viruses to plant roots, especially in cereal crops.
  • Mites: Some mite species are also known vectors for plant viruses.
• Grafting: When an infected plant part (scion) is grafted onto a healthy rootstock, the virus can readily move into the healthy plant, a method often used in horticultural practices for propagation.
• Parasitic Plants: Certain parasitic plants, like dodder (Cuscuta sp.), can establish connections between infected and healthy plants, facilitating virus transfer.

Modes of Dissemination

Dissemination refers to the spread of plant viruses from one plant to another or over longer distances, influencing the epidemiology of the disease.

• Vegetative Propagation: This is a highly efficient mode of long-distance dissemination, especially in vegetatively propagated crops.
  • Cuttings, Tubers, Bulbs, and Scions: If the parent plant is infected, any offspring derived from its vegetative parts (e.g., potato tubers, sugarcane setts, fruit tree scions) will also carry the virus. This is a major concern in crops like potato, rose, and sugarcane.
• Seed and Pollen Transmission:
  • Seed-borne: Some viruses can be transmitted through seeds, meaning the offspring from an infected mother plant can carry the virus. The virus can be present in the embryo, cotyledons, or even on the seed coat (e.g., Bean Common Mosaic Virus, Lettuce Mosaic Virus).
  • Pollen-borne: Viruses can also be transmitted via infected pollen, infecting the ovules of healthy plants or the plants pollinated by the infected pollen.
• Vector Movement: The movement of insect vectors, often aided by wind, can lead to the rapid spread of viruses across fields and even continents. For instance, whiteflies are notorious for transmitting geminiviruses over long distances.
• Human Activities: International trade of plants and plant products (seeds, cuttings, ornamental plants) serves as a significant pathway for the global dissemination of viruses. Contaminated farm equipment and clothing also contribute to local spread.
• Soil Transmission: Viruses that persist in the soil, often associated with nematode or fungal vectors, can infect new plants through their root systems.
• Root Grafts: Natural root grafts between adjacent plants can allow viruses to move from an infected plant to a healthy one underground.

The efficiency and prevalence of these transmission modes vary significantly among different virus species, influencing their epidemiology and the strategies required for their control.