Plant diseases may be caused by microorganisms including fungi, bacteria, viruses, mycoplasmas, etc. or may be incited by physiological causes including high or low temperature, lack of excess of soil moistures and aeration deficiency or excess of plant nutrients, soil acidity or alkalinity etc.
The micro-organisms obtain their food either by breaking down dead plant and animal remains (saprophytes) or by attacking living plants and animals (parasites). In order to obtain nutrients, the parasitic organisms excrete enzymes or toxins and kill the cells of the tissues of host plant, as a result of which either the whole plant or a part of it is damaged or killed, or considerable disturbance takes place in its normal metabolic processes. Micro-organisms are reproduced and disseminated mainly by spores. Wind, water, seed, diseased plants, cuttings and tubers, animals, men and insects and soil help to disseminate diseases.
Viral diseases are spread as a result of direct contact, through seeds, diseased cuttings and grafts, and more commonly by insects and other vectors. Most of the fungi and bacteria erupt or ooze from the surface after the host tissues have been damaged, whereas, the viruses work altogether internally.
Therefore, control of the plant diseases usually lies in preventing infection, as it is very difficult to kill the fungi or bacteria or to inactivate the viruses inside the host plants.
However, as the result of recent studies made in the development of systemic fungicides, it appears that in near future the diseased plants may even be cured. The evolvement of disease resistant varieties is another important and cheap method of controlling diseases. However, this method of control has its limitations because of the appearance of new races of the pathogens from time to time.
Seed Borne Diseases:
In the case of seed borne diseases, the pathogens are carried either on the surface of the seed or within it. When a pathogen is external, it may be destroyed by treating the seed with a chemical, e.g. Formalin, TMTD, Copper carbonate, Captan, Organo-mercurials, like Agrosan and Ceresan. Except Formalin, all chemicals are available in the dust form. Systemic organic compounds have also been shown to be effective in eradicating both externally and internally seed-borne infections.
In the case of soil-borne diseases which attack the seeds or seedlings, the use of soil disinfectants or sterilizers is helpful in destroying the pathogens. The soil treatment involves the use of chemicals Formaldehyde, Captun, Thiram, Zineb, Organo-mercurials, PCNB, Ethylene dibromide, Vapam etc. They are used to drench the soil or are broadcast on it, or are applied to it in furrows, or are used to fumigate it.
Air Borne Diseases:
In the case of diseases caused by air-borne pathogens, the application of chemicals to the foliage is the most effective way of controlling them. Though many fungal and bacterial diseases can be controlled in this way, chemicals have not yet proved very successful in controlling virus diseases. In their case, protective measures have to be aimed at controlling the insect vectors by spraying insecticides on the host plants or by using disease- free material for propagation.