Area under cultivation, production and yield per hectare of rice indicates its importance. It is one of the leading food crops of the world and a cash crop of Pakistan. It contributes 15 per cent to the foreign exchange earnings.

The rice is frequently grown on heavy clay soils that have an impervious, subsoil layer (hard pan) that limits the drainage, because it requires a constant and plentiful supply of water. The rice production can be used to reclaim saline soils, because flooding leaches salts from the soils.

In those areas, where other crops are not possible to grow or where Kharif irrigation water supply is abundant, this crop is very suitable. Rice plants prefer 5.0-6.5 soil pH, and are moderately tolerant to exchangeable sodium percentage (ESP) in the level of 20-40 per cent. This crop may also tolerate up to 0.40-0.60 per cent of the white alkali and 0.10-0.20 per cent of the black alkali in the soil.

The rice crop is subjected to more than 40 diseases, which are one of the reasons for the low yields of rice in the world, including Pakistan.

The fungi, bacteria, nematode and the virus cause different infectious diseases. Non-infectious diseases may be caused because of low or high temperature, decrease or increase in different nutritional elements essential for the crop. The causes of fluctuation in the production may be more, but the diseases could not be ignored, neglected or regretted, because they also cause variable loss of time to the crop.

The diseases may appear at any stage of the growth and development of plant, attacking the seed sown, root system, foliage, stalk, leaf sheath, inflorescence and even the developing grain. All these diseases are injurious in some areas, in different years and on various parts of the plant. All parts of plant are subject to disease and one or more diseases can occur on virtually every plant and in every field. These all draw attention because of the symptoms or signs and generate great concern because of their effects on the quality or the quantity of plants, straw and grains.

The brown spot, blast, stem rot, bunt, bacterial blight, false smut, ufra and khaira (zinc deficiency) are sometimes considered important diseases at various parts of the rice growing areas of world including Pakistan. The brown spot or blight of rice is a much more widespread and a common disease in almost all rice growing areas of the world. Therefore cause, symptoms (identification), perpetuation (survival from season to season), transmission and control (disease management practices) of this most important disease of rice is being summarized, so that the growers may protect their crop, the research workers may decide their future strategies and extension workers may also become alert.

CAUSE:

A FUNGUS, HELMINTHOSPORIUM ORYZAE, CAUSES THE DISEASE.

SYMPTOMS:

The disease has been recorded all over the country. Initially small dots or circular eye-shaped or round to oval spots appear in light colour on the leaves. These spots increase in size, coalesce with each other and result in linear spots brown in colour on diseased leaves. Later on the withering and the yellowing of leaves occur. Similar symptoms appear on glumes and kernels. The seed setting may also be affected, cause sterility, shrivelling and shows rotting and poor germination. Mostly emerged seedlings become blighted, if the disease is severe.

PERPETUATION:

The diseased seeds, plant debris and soils of infected crop help the fungus to survive. In our conditions, the disease causing fungus mostly survive and multiply on diseased crop debris lying in the field as well as on the certain grasses and weed plants, from which the organism can pass to regular rice crop under favourable conditions. Air and irrigation water helps the fungus in transmitting from the diseased to healthy plants.

CONTROL:

The following disease management practices are proved to be the best for control of brown spot or blight of rice:

1) The use of resistant varieties or disease-free seed in healthy soils.
2) Sanitation and crop rotation.
3) Hot water seed treatment at 54C for 10 minutes or with seed dressing fungicides.
4) Collection and destruction of stubble and spraying with copper fungicides at right time.
5) Application of suitable foliar fungicides help to minimize further dissemination of the disease.
6) Improving drainage system is reported beneficial.
7) Fulfilment of nutritional requirements of plants, mostly in poor soils, increase disease resistant.