Rice is one of the leading food crops of the world, an important staple food and cash crop of Pakistan. It contributes 15 percent to the foreign exchange earnings. Its area under cultivation, production and yield per hectare also indicates its importance. This crop is very suitable, where other crops are not possible to grow or where Kharif (summer) irrigation water supply is abundant. According to Cheema et al. (1991) rice prefers 5.0-6.5 soil pH, and is moderately tolerant to exchangeable sodium percentage (ESP) in the level of 20-40%. This crop may also tolerate up to 0.40-0.60 percent of white alkali and 0.10-0.20 percent of black alkali in soil (WAPDA report, 1961). The rice is frequently grown on heavy clay soils that have an impervious, subsoil layer (hard pan) that limits 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 (Bhatti and Soomro, 1996).

The rice crop is subjected to more than forty diseases, which are one of the factors, for low yields of rice in the world (including Pakistan). 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. The fungi, bacteria, nematode and 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. Overall, Table present area, production and yield per hectare of rice crop in Pakistan. The causes of fluctuations in production may be many more, but the diseases could not be ignored, neglected and or regretted, because they also cause variable loss time by time to the crop. These all diseases are injurious in some areas, in some years and on some plant parts. All parts of plant are subject to disease and one or more diseases can occur on virtually every plant and in every field. All draw attention because of symptoms or signs and generate great concern because of their effects on the quality and/or quantity of plants, straw or grain.

According to Kamal and Moghal (1968), Hafiz (1986), Nyvall (1979), Bhatti and Soomro (1996) and Jiskani (1999), brown spot, blast, stem rot, bunt, bacterial blight, false smut, ufra and khaira (zinc deficiency) are sometimes considered important diseases at various parts of rice growing areas of Pakistan. Therefore identification, management practices and some other relevant knowledge of the most important diseases of rice are being summarized, so that the growers may protect their crop from these diseases, research workers may decide their future strategies, extension workers may also to be alert.

BLIGHT OR BROWN SPOTS (Helminthosporium oryzae)

Symptoms: This disease has been recorded all over Pakistan. Initially small dots or circular eye shaped or oval spots appear light in colour on leaves. These spots coalesce and result in linear spots

brown in colour. Later on withering and yellowing of leaves occur. Seed setting also affected and causes sterility, shrivelling and show rotting and poor germination.

Perpetuation: Diseased seeds, plant debris and soils help the fungus to survive, while air and irrigation water help to the fungus for transmitting from diseased to healthy plants.

Control: 1) Use of resistant varieties or disease free seed in healthy soils, 2) Sanitation and crop rotation, 3) Hot water seed treatment at 54 oC for 10 minutes or with seed dressing fungicides, 4) Collection and destruction of stubble and spraying with copper fungicides at right time and 5) Application of suitable foliar fungicides may help to minimise further dissemination of the disease.

BUNT OF RICE (Tilletia barclayana)

Symptoms: This disease also called black or kernel smut is generally distributed wherever rice is grown. Diseased grains are filled with black powder, which can be detected by breaking them. Only a few grains may be affected wholly or partially in an ear. If not severely infected, seeds may germinate but seedlings are stunted.

Perpetuation: The disease causing organism is soil borne.

Control: 1) Cultivation of resistant varieties, 2) Use of healthy seed, 3) Sowing early maturing varieties, 4) Avoid high rates of nitrogen fertilizer, 5) Avoid winnowing and threshing of diseased crop in field, 6) Treat the seed with suitable chemicals easily available in the market and 7) Collect and burn diseased ear heads.

Rice blast (Pyricularia oryzae)

Symptoms: Some times this disease refers as Pyrricularia blight or rotten neck, generally distributed where ever rice is grown. Small spots appear on leaves, nodes, panicles and grains and some times on leaf sheaths. The spots begin as small, water-soaked, whitish, greyish or bluish dots. These spots rapidly increase and become grey in centre. Brown to black spots also develops on inflorescence and glumes. In later stages, diseased heads appear blasted and whitish in colour. Grain development is affected and the panicles droop.

Perpetuation: The disease perpetuates through diseased plant debris lying in the field, seed and wild grasses.

Control: 1) Burn and destroy diseased plant debris and stubble, 2) Early planting, 3) Cultivation of resistant varieties, 4) Use of healthy seed, 5) Dusting the seed with any one of the organic mercurial seed dressing fungicides, 6) Spray the crop with organo-mercurials, 7) Avoid excessive depth application of irrigation water, 8) Avoid excessive plant population and 9) Control grasses and other weeds.

BAKANAE DISEASE (Fusarium moniliforme)

Symptoms: This disease is also called white stalk, generally distributed where ever rice is grown. Infected seedlings are thin, chlorotic, may die before or after transplanting. In the field, infected plants have few tillers and leaves die in short time. Live plants have empty panicles. Some infected plants may be stunted instead of elongated, while the abnormal elongation of these (infected) plants in seed bed or field is most common symptom of this disease.

Perpetuation: The disease causing fungus is seed and soil borne, through which it can survive for a long period.

Control: 1) Cultivation of resistant varieties and 2) Seed treatment.

UFRA OF RICE (Ditylenchus angustus)

Symptoms: The disease has been reported from Bangladesh, Egypt, India and South Asia. The leaves become yellow and wither, seedlings die. Brown spots appear on leaves and leaf sheaths. Stems may also bear spots. These spots become darker brown along with upper inter-nodes of the stem. Ears may not emerge or may show swellings and become twisted and distorted. Grains are not formed usually.

Perpetuation: Diseased plant debris carrying disease causing nematodes.

Control: 1) Burn the stubble, 2) Grow early maturing varieties, 3) Use healthy seed and 4) Plough the field thoroughly after harvest so as to expose the soil to sun-heat.

Bacterial blight (Xanthomonas oryzae)

Symptoms: Water soaked stripes appear along the margin of leaf blades, which later on enlarge and turn yellow. These lesions may cover the entire blade, may extend to the lower end of leaf sheath. Similar symptoms may occur on glumes of green grains.

Perpetuation: Survive in rhizophere of weed hosts, infected straw and root stubble. Disseminate by wind and water.

Control: Cultivation of resistant varieties is alone easy and safe way to prevent the crop against diseases including this disease also.

Stem Rot (sclerotium oryzae)

Symptoms: Two to three months old plants begin to wither and ultimately dry up; the sheaths soon turn somewhat dark and start rotting. Black dots (fruiting bodies of sclerotia) occur at the base of dried leaves and leaf sheaths. Stem begins to rot and become soft, plant falls down.

Perpetuation: Infested soil helps the organism for its survival.

Control: 1) Use of resistant varieties, 2) Burning of diseased rice stubble, 3) Crop rotation and 4) Antagonistic organisms.

References:

1. Bhatti, I. M. and A. H. Soomro. 1996. Agricultural inputs and field crop production in Sindh. Agricultural Research Sindh, Hyderabad.
2. Cheema, S. S., B. K. Dhaliwal and T. S. Sahota. 1991. Theory and digest of agronomy. Kalyani Pub., New Delhi.
3. Hafiz, A. 1986. Plant diseases. PARC, Islamabad.
4. Jiskani, M. M. 1999. A brief outline “THE FUNGI” (Cultivation of mushrooms).
5. Kamal, M and S. M. Moghal. 1968. Studies on plant diseases of South West Pakistan. Agricultural Research Institute, Tandojam.
6. Nyvall, R. F. 1979. Field crop diseases handbook. AVI Pub. Co. Inc. Westport.
7. WAPDA Report, 1961.

December, 2004