September 11, 2006	Monday	Sha'aban 17, 1427

ONION is an important bulb vegetable crop which can help in earning foreign currency through export. But in times of shortage it is also imported.

According to the ‘Agricultural Statistics of Pakistan 2004-2005’, onion was cultivated on 55.4 thousand hectares and produced 633.1 thousand tons during 1987-88, which increased up to 127.8 thousand hectares and 1,764.9 thousand tons during 2004-05 with an average 11.4 to 15 tons yield per hectare.

Seasonal fluctuations between the yield of Sindh and Balochistan have several constraints which include low quality seed, imbalanced fertilizers, uneven irrigations, and attacks of insect-pests and diseases.

Globally, the vegetable is attacked by 66 diseases including 10 bacterial, 38 fungal, six nematode, three viral, one phytoplasmal, one parasitic, and seven miscellaneous diseases and disorders. While in Pakistan, several soil-borne diseases become widespread by limiting its production.

The diseases commonly known as downy mildew (Peronospora destructor), purple blotch (Alternaria porri), grey mold (Botrytis sp.) and basal/pink rot (Fusarium sp.) etc., are the most destructive ones which damage crop and reduce bulb yields sometimes up to 100 per cent. Several chemical, cultural and biological methods are used to control onion diseases but most growers are unaware of it. Many aspects need special attention by the researchers while the extension workers require updating.

Downy mildew is a serious disease. Many allium species including onion and garlic etc., serve as a mean of its alternate hosts. The disease rapidly destroys susceptible tissues and causes fungus which produces oospores that can be soil borne for several years. Growers are advised not to plant infested bulbs.

Frequent sprays with fungicide are effective. Well drained field, lower plant densities and wind help the disease spread. In 2002, the Integrated Disease Management Model (IDMM) for control of onion downy mildew was developed. This model reduced the area under disease progress curve (AUDPC) and increased substantially the bulb size and yield.

Botrytis blight, bulb and neck rot is the next important disease and can damage the crop in field and as well as during storage. The infections originate in the field but symptoms appear after harvest. It can be controlled through long rotations with non-host crops.

Irrigation within 10 to 14 days of lifting onions is avoided, tops should be dried approximately one week before, roots under bulbs should be cut before harvest, mature crop should be harvested, and during dry weather damaging of necks or bulbs at harvest should be avoided.

Black mold appears as black discoloration at the neck. Shallow sunken lesions also occur on outer scales and clusters of black spores below the outer dry scales. The entire surface of the bulb may turn black and all scales may be affected in advanced stages. The bulb may become dry and shrivel, but more often soft-rot bacteria follow the infection. The spores of this fungus are very common in the air and soil. Avoiding crop injury reduces black mold incidence. Bulbs should be protected from moisture during harvesting and shipping. Black mold development can be suppressed by maintaining transit and storage temperatures below 12.77OC.

Blue mold appears at harvesting or in storage. A blue-green powdery mold develops on the surface of lesion. In advanced stages of the disease the bulb may become tough and rubbery or a soft-rot may develop due to action by bacteria that may invade tissue affected by blue mold. The fungus commonly grows on dead material and invades onion bulbs through injuries, bruises and uncured neck tissue. Avoid wounding the bulbs at harvest and storing below 5.55OC temperatures is recommended.

Onion smut plants are usually stunted, and often die. Surviving plants have poorly developed bulbs or produce bulbs that deteriorate rapidly in storage. Plant symptoms turn from dark brown to black with slightly thickened streaks or blisters on the cotyledonary leaf and other leaves or scales. Blisters in localized areas of leaves often rupture to expose black powdery masses of smut spores. Seed-bed solarisation reduces smut pathogen and improves seedling stand and other characters including root and shoot lengths.

Stemphylium leaf blight produces small, light yellow to brown and water-soaked lesions on leaves and leaf sheaths. As lesions expand, they coalesce, causing extensive blighting of the leaves. The centre of lesions turn brown to tan, then dark olive brown and finally black as fungus sporulates. Sometimes fruiting bodies may appear in infected tissue as small, black, pinhead-like raised bodies. Infection is usually limited to leaves, and does not extend down to the scales of the bulb. The fungus normally invades dead and dying onion tissues. Severe damage can occur on healthy leaves during warm weather when leaves are wet for more than 24 hours. Long-term rotation with unrelated crops may reduce losses. Also, good field drainage and reduced plant density may lessen disease severity.

Purple blotch occurs as water soaked areas followed by white necrotic spots on foliage. Girdling lesions on stem and black concentric rings on purplish lesions also appear. Fungus requires rain or persistent dew for reproduction and penetration of plant tissue. It can survive in crop debris. Rotation with non-host crops, maintaining good air movement around foliage is suggested. The growers can also use proper fungicides.

Fusarium basal rot affected plants wilt; decay of bulbs progress up from basal plate, white mycelium on and surrounding basal plate are typical characteristics. Infection is increased by insect injury to roots, stem plate or bulb and the pathogen is soil borne. Minimum of four years crop rotation is proposed.

Translucent scale disease is cussed by frequent high temperatures and high humidity during the last 6-8 weeks of growth. The scales are translucent and resemble freezing injury; become greyish and water soaked and brownish from inside. Affected scales shrink, giving bulb slightly irregular surface. Symptoms continue to progress in storage, delays of 2-4 weeks between curing and cold storage can increase the incidence. The problem needs research for solution.

Root knot nematode feeds on roots, results stunting and reduced stand and produces characteristic galls on roots. Growing non host crops for several years is helpful, but is not usually feasible because nematodes possess wide host range. However, soil amendments, oilseed cakes, plant extracts and recommended nematicides help in decreasing nematode population.