Diseases of crops

A plant disease is a physiological or structural abnormality that is deleterious to the plant or to any of its parts or products, or reduces their economic value. It expresses itself in a definite series of symptoms.

Plant diseases may be temporary or permanent, depending on the speed of recovery.

Plant diseases may be caused by micro-organisms, including fungi, bacteria, viruses, mycoplasmas, etc or may be incited by physiological causes including high or low temperatures, lack or excess of soil moisture and aeration, deficiency or excess of plant nutrients, solid acidity or alkalinity etc. The micro-organism 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 the host plant, as a result of which either the whole part of it is damaged or killed, or considerable disturbance takes place in its normal metabolic processes.

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, e.g. Agrosan and Ceresan. Except Formalin, all chemicals are available in the dust form. The recommended dust is thoroughly mixed with the seed, preferably in a rotating-drum, so as to uniformly cover the seed surface with a layer of it. The grains are then heaped and kept covered with a tarpaulin overnight.

For internally seed borne diseases, hot water treatment and solar treatment are very effective. For the treatment of wheat seed with hot water to control loose smut, the seed is soaked in ordinary water for four to five hours and then dipped into hot water at 49C for one minute and then immersed again into hot water at 54 C for ten minutes. In the case of solar treatment, which is carried out on a bright sunny day during the summer months, the seed is soaked in water for 4 hours. It is then taken out and immediately spread in a thin layer on sheets of cloth to dry in the sun.

Soil-Borne disease:

 In the case of soil-borne diseases which attack the seeds of seedlings, the use of soil disinfectants or sterilizers is helpful in destroying the pathogens. The soil treatment involves the use of chemicals formaldehyde, Captan, Thiram, Zined, organo-mercurials, PCNB, ethylene dibromide, Vapam etc. They are used to drench the soil, or are broadcast on it, o are applied to it in furrows, or are used to fumigate it.

Air-Borne Diseases:

In case of diseases caused by air-borne pathogens, the application of chemicals to them 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.