Crop diseases and their control
The diseases in crop plants result in a heavy loss of crop yields and
cause considerable damage to crops year after year. To check the plant diseases, it becomes necessary to know about the cause of the diseases, of the life history of the causal organism and of the meterological conditions which influence the host and parasite interaction.
Control measures may be divided into two main groups – prophylaxis
and disease resistance. Prophylaxis includes the protection of the host from exposure to the pathogen, from infection or from environmental factors favourable to disease development.
Disease – resistance implies the improvement of resistance of the host to infection and to disease development.
Rice – Oryza sativa
Pathogen Disease incited by a fungus, Pyricularia oryzae.
Name of the disease Blast disease of rice.
Systematic position The fungus belongs to class: Deuteromycetes.
The symptoms are found on the leaf blades, leaf sheaths and rachis.
Characteristic isolated, bluish – green necrotic lesions with a water –
soaked appearance are formed on the leaf – blades.
The lesions are broad in the centre and possess narrow elongations on its top and bottom. The lesion – formation leads to ultimate drying of the leaves, and the seedlings wither and die.
After transplantation, the symptoms appear in the form of necrotic
lesions both on the leaf lamina and the leaf sheaths. The necrotic lesion is spindle shaped grey in the centre and remain surrounded by brown and yellowish zones. The leaves ultimately dry up.
The fungus Pyricularia oryzae when young possesses hyaline and
septate mycelium. On maturity, the colour of mycelium changes to olive brown. Conidia are produced terminally. Each conidium is obpyriform septate with a small basal appendage.
The most economic method of control is the cultivation of resistant,
high yielding varieties.
Immersion of the seeds in 0.2 per cent solution of Kalimat B for 24
hours controlled the disease and promoted the growth of seedlings. The seed protectants such as agrosan, cerasan and spergon have been proved responsible for the control of disease.
Plant debris should be collected and destroyed. The secondary host
plants such as Digitaria marginata should be collected from paddy fields and destroyed.
Spraying and dusting
Blast disease can be controlled effectively by spraying the fungicide,
Bordeaux mixture at least 4 times before and after flowering of the crop. Bordeaux mixture formula is as follows:
Copper sulphate 9 Kgs.
Quick lime 9 Kgs.
Water 250 litres.
The dusting of organomercuric compounds has been suggested for