Dissemination of Plant Diseases: 16 Mechanisms | Botany

The following points highlight the top sixteen mechanisms of dissemination of plant diseases. Some of the mechanisms are: 1. Dissemination by Seed 2. Dissemination by Propagating Stock 3. Dissemination with Plant Debris 4. Dissemination with Manure 5. Dissemination with Soil 6. Dissemination by Field Operations and Implements 7. Dissemination by Insects 8. Dissemination by Animals Other Than Insects and Others.

Dissemination of Plant Diseases:

1. Dissemination by Seed
2. Dissemination by Propagating Stock
3. Dissemination with Plant Debris
4. Dissemination with Manure
5. Dissemination with Soil
6. Dissemination by Field Operations and Implements
7. Dissemination by Insects
8. Dissemination by Animals Other Than Insects
9. Dissemination by Air Currents
10. Dissemination by Water
11. Dissemination by Exporting and Importing of Commodities
12. Dissemination by Natural Root Grafting
13. Dissemination by Shooting Out of Spores
14. Dissemination through Angiospermic Plant
15. Dissemination through Pollen Grains
16. Dissemination through Other Media

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Mechanism # 1. Dissemination by Seed:

Pathogens may be carried inside or outside the seeds or fruits functioning as seeds. The infective materials may be mycelium, spores or any other part of the pathogens.

One of the examples of externally seed-borne pathogen may be cited from Tilletia caries causes bunt of wheat; that of internally seed-borne is Ustilago nuda var. tritici, and of pathogen borne between the glumes and kernels in Ustilago hordei and U. avenae causing smuts of wheat, barley and oats respectively.

Mechanism # 2. Dissemination by Propagating Stock:

This is rather common in cases where plants are multiplied by using propagating stocks and is partly common with the virus diseases, as in Mosaic disease of Sugarcane. Here the propagating stocks behave as carriers of the pathogens.

Mechanism # 3. Dissemination with Plant Debris:

During cleaning of soil prior to sowing of seeds, previous year’s infected plant debris should be thoroughly removed and burnt. But very often the infected plant debris instead of being burnt off is carelessly transported from place to place resulting in the dissemination of pathogens and spread of the diseases.

One of the common examples is the late blight of potato where the pathogen perennates as oospore in the plant debris.

Mechanism # 4. Dissemination with Manure:

Pathogens are disseminated when manures like, compost and cowdung are used particularly in seed bed soil contaminated with various pathogens. This is rather common with the damping-off of seedling disease.

Mechanism # 5. Dissemination with Soil:

Transference of soil from one area to the other is a very common practice. When such soil is contaminated with pathagen knowingly or unknowingly contamination ‘through soil is spread. Clubroot of crucifers is an example.

Mechanism # 6. Dissemination by Field Operations and Implements:

Pathogens may be disseminated by field operations, such as, transplanting of seedling, watering, weeding, harvesting operations and through implements. Example may be cited from the dis­semination of the tobacco mosaic virus disease.

Mechanism # 7. Dissemination by Insects:

Insects play a very important role in the disse­mination of plant diseases. The mechanism of dissemination, however, is extremely variable which depends on the nature of insects and the nature of inocula of the dis­eases. Again the insects may play direct or indirect role.

As to direct role, the inoculum may be carried:

(i) On the body of the insects,

(ii) In the body passing through the alimentary tract when they feed on the inoculum along with other exudates, or

(iii) 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:

(i) 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.

(ii) 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.

(iii) 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 wild and domestic animals often help dissemination of plant diseases. Yellow ear rot of wheat disease caused by Corynebacterium tritici is dispersed by nematode Anguina tritici, seeds of Dendrophthae sp. are disseminated by birds, and cattle feeding with contaminated fodder often pass out viable fungal propagules (conidia, and sclerotia) in their dung are disseminated at different places.

Mechanism # 9. Dissemination by Air Currents:

The spores of many parasitic fungi are disse­minated by air currents from diseased to disease-free host. This may be illustrated from the spread of Powdery mildew disease. Besides this, the air currents may indirectly help dissemination of plant diseases. For example, diseased plant parts may be caught up by air currents and brought to distant places to spread disease.

Mechanism # 10. Dissemination by Water:

The dissemination of pathogens by water may be in two ways:

(i) Flagellate spores of pathogenic fungi are entirely dependent on the supply of water for their dissemination, example, and Wart disease of potato;

(ii) Pathogens like, bacteria are often disseminated by splashing of raindrops, as in case of Citrus canker disease

(iii) Dissemination of conidia of red rot fungus (Colletotrichum falcatum) takes place through flowing irrigation water or rain water

Mechanism # 11. Dissemination by Exporting and Importing of Commodities:

That dis­eases are disseminated during exporting and importing various commodities is very well-known.

This may happen in different ways:

(i) With contaminated commodities

(ii) With contaminated containers of the commodities

(iii) With both contaminated commodities and containers.

Some of the examples are the introduction of Dutch elm disease in the United States, the introduction of Wart disease of potato in India, etc. As a control measure against this quarantine laws, inspection, and health certificate system have been introduced.

Mechanism # 12. Dissemination by Natural Root Grafting:

This type of dissemination is en­countered in the Oak wilt disease caused by Endoconidiophora fagacearum. Where natural grafting between the roots of diseased and disease-free plants takes place resulting in the spread of the disease. The oak wilt fungus living in the sap-wood passes through the grafts from a diseased to a neighbouring disease-free tree.

Mechanism # 13. Dissemination by Shooting Out of Spores:

In Taphrina deformans, the causal organism of Peach leaf curl disease, the ascospores are shot out from the asci, come in contact with the host, and cause infection and thereby the disease is disseminated.

Mechanism # 14. Dissemination through Angiospermic Plant:

Pea mottle virus is transmitted by Cuscuta campeslris.

Mechanism # 15. Dissemination through Pollen Grains:

Tobacco ring spot virus and Potato spindle tuber virus are transmitted through pollen grains.

Mechanism # 16. Dissemination through Other Media:

Sclerotia of Claviceps purpurea unknow­ingly remain mixed with healthy rye seeds to help dissemination of disease being the source of inoculum. Besides this, mummified apples and pears also serve as source of inoculum for seasonal carry over and incidence of disease in orchards.