Modes of Infection and Dissemination | Botany Optional for UPSC PDF Download

Dissemination of Plant Diseases

In the intricate world of botany, the spread of plant diseases is a phenomenon of great concern. Understanding the mechanisms behind the dissemination of these diseases is crucial for managing and mitigating their impact on agriculture and ecosystems. In this article, we delve into the top sixteen mechanisms that facilitate the spread of plant diseases, shedding light on the intricacies of each process.

Mechanism #1: Dissemination by Seed

Pathogens can be carried within or outside seeds or fruits, serving as carriers of infective materials such as mycelium, spores, or other pathogenic components. Notable examples include Tilletia caries causing wheat bunt, Ustilago nuda var. tritici within wheat seeds, and Ustilago hordei and U. avenae causing smuts in wheat, barley, and oats, respectively.

Mechanism #2: Dissemination by Propagating Stock

This mechanism is particularly prevalent when plants are propagated using stocks. In cases like Mosaic disease of Sugarcane, propagating stocks act as carriers of pathogens.

Mechanism #3: Dissemination with Plant Debris

Neglecting the proper disposal of infected plant debris during soil preparation can lead to the inadvertent transportation of pathogens, as seen in the late blight of potato, where the pathogen persists as oospores in plant debris.

Mechanism #4: Dissemination with Manure

The use of manure, such as compost and cow dung, in soil contaminated with pathogens can facilitate the spread of diseases, as observed in damping-off of seedlings.

Mechanism #5: Dissemination with Soil

Transferring soil from one area to another is a common practice, often unknowingly spreading pathogens. Clubroot of crucifers is an example of this mode of transmission.

Mechanism #6: Dissemination by Field Operations and Implements

Field operations like transplanting, watering, weeding, and harvesting, as well as the use of implements, can disseminate pathogens. The tobacco mosaic virus disease is an illustrative example.

Mechanism #7: Dissemination by Insects

Insects play a pivotal role in disseminating plant diseases through various mechanisms. They may directly carry inoculum on their bodies, transmit pathogens through feeding, or create entry points for pathogens through wounds. Notable examples include Dutch elm disease, Ergot of rye, and virus diseases transmitted by leafhoppers and aphids.
As to direct role, the inoculum may be carried:

• On the body of the insects,
• In the body passing through the alimentary tract when they feed on the inoculum along with other exudates, or
• As they feed on diseased host tissue they carry the inoculum and spread infection.

Examples of direct role of insects for plant disease dissemination are as follows:

• The inoculum may get adhered to the body of the insects as they come and visit the diseased host and when the same insects visit disease-free plants the inoculum carried by them comes in contact with the healthy host plant. Best example is Dutch elm disease caused by Ceratocystis ulmi, disseminated by bark beetles.
• The spread of Ergot of rye disease is an example where the insects feed on the conidia along with honey during honey-dew stage, and the conidia pass through the alimentary tract of the insects and are brought in contact with the disease-free plants when they visit them.
• The transmission of virus diseases is a classical example of dissemination of plant diseases through insects. Insects like, leaf-hoppers and aphids feed on virus in­fected leaves and they go and feed on leaves of disease-free hosts and thereby spread infection behaving as vectors of virus diseases.

Insects indirectly help dissemination of plant diseases by causing wounds on the host surface through which the pathogen gains entrance in the host and induces disease. This may be illustrated from the dissemination of the Citrus canker. Another example of insects indirectly helping dissemination of plant diseases is the dissemina­tion of the rust disease.
Where insects transfer spermatia to the compatible receptive hyphae, dikaryotic condition is established and ultimately physiologic races are deve­loped which are disseminated through aeciospores from the barberry host.

Mechanism #8: Dissemination by Animals Other Than Insects

Nematodes, snails, birds, and domestic animals can also aid in the spread of plant diseases. Examples include yellow ear rot of wheat and the dissemination of fungal propagules through cattle dung.

Mechanism #9: Dissemination by Air Currents

Airborne spores of parasitic fungi, like those causing powdery mildew, can travel from diseased to healthy hosts. Additionally, diseased plant parts can be carried by air currents to new locations, further spreading diseases.

Mechanism #10: Dissemination by Water

Pathogens can be disseminated by water in various ways, including flagellate spores reliant on water, bacterial spread through raindrop splashing, and conidia transmission via irrigation or rainwater.

Mechanism #11: Dissemination by Exporting and Importing of Commodities

Plant diseases can be inadvertently introduced during the export and import of agricultural products. Contaminated commodities, containers, or both can facilitate the spread. Quarantine laws and inspection systems are essential for control.

Mechanism #12: Dissemination by Natural Root Grafting

In cases like Oak wilt disease, natural grafting between diseased and disease-free plant roots can enable disease transmission, as the pathogen passes through the grafts.

Mechanism #13: Dissemination by Shooting Out of Spores

Peach leaf curl disease, caused by Taphrina deformans, involves the ejection of ascospores from asci, initiating infection when they come into contact with hosts.

Mechanism #14: Dissemination through Angiospermic Plant

Cuscuta campeslris transmits Pea mottle virus through its parasitic interactions with host plants.

Mechanism #15: Dissemination through Pollen Grains

Tobacco ring spot virus and Potato spindle tuber virus can be transmitted through pollen grains, further complicating disease dynamics.

Mechanism #16: Dissemination through Other Media

Sclerotia of Claviceps purpurea, unwittingly mixed with healthy rye seeds, can serve as a source of inoculum for disease dissemination. Mummified apples and pears also contribute to seasonal disease carryover in orchards.

Conclusion

In conclusion, comprehending these sixteen mechanisms of plant disease dissemination is vital for the development of effective strategies to combat these agricultural and ecological threats. With this knowledge, scientists and agricultural experts can work towards safeguarding our plants and crops, ensuring a healthier and more resilient agricultural landscape.