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Onion diseases and IPDM
By M. Mithal Jiskani
Assistant Professor (Plant Pathology)
Faculty of Crop Protection, Sindh Agriculture University Tandojam


Onion is an important bulb vegetable crop, commercially grown in many countries of the world including Pakistan. Onion is being used as a raw and given protection to human beings from sun stroke, normally is consumed green as well as in mature stages almost by every one (poor as well as rich people), by different mean. The crop is of importance in such a mean too that increased production earns foreign currency through export and in case of shortages onion is imported by spending many million rupees.

According to ‘Agricultural Statistics of Pakistan 2004-2005’, onion was cultivated on 55.4 thousand hectares and produces 633.1 thousand tons during 1987-88, which increased up to 127.8 thousand hectares and 1764.9 thousand tons during 2004-05, respectively. The similar source also reported 11.4 to 15.0 tones as average yield per hectare (Table), which shows fluctuation in per hectare yield from 9.2 (Sindh) to 20.1 (Balochistan). This fluctuation may be due to several constraints that affect onion yield adversely in our country, which may includes the use of low quality seed, imbalanced fertilizers, and uneven irrigations; and above all, attack of various insect-pests and diseases.



In the world, onion is attacked by 66 diseases including 10 bacterial, 38 fungal, 06 nematode, 03 viral and 01 phytoplasmal disease, 01 parasitic plant and 07 miscellaneous diseases and disorders (Mohan and Schwartz, 2005). In Pakistan, several serious diseases, such as soil-borne diseases, become widespread and serious enough to limit 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 diseases, damage the crop and reduced bulb yield sometimes up to 100%. Several chemical, cultural and biological methods are used to control onion diseases, but most of the growers are unaware to that, similarly many disease aspects needs special attention by the researchers to work more and extension workers needs updating themselves to guide the concerned. Therefore, causes, identification and control of most important diseases is discussed for the due attention of onion growers, researchers and extension workers.

Downy Mildew caused by Peronospora destructor is one of the most serious diseases. Many Allium species including onion and garlic etc serve as a mean of its alternate hosts. The disease is characterized as elongated leaf patches that are slightly paler green or tan and collapse soon. When wet with dew, grayish violet and furry growth visible on lesion surface. Under favorable conditions, this disease rapidly destroys susceptible tissues. The disease causing fungus produces oospores that can be soil borne for several years. Pathogen also over-winters in onion bulbs and as oospores in debris from diseased foliage. Spores can survive for up to 3 days, are produced at night and dispersed during day. The spores need 9-16 days between infection and sporulation. Sporangia can be carried long distances on air current and are favored by cool temperatures (43-80OF) and wet weather.

Growers are advised to do not plant infested bulbs. Rotate with non host crops for at least 3-4 years. Prompt tillage after harvest to hasten breakdown of onion tissues. Frequent sprays with fungicide are effective. Well drained field, lower plant densities and wind help the disease spread, hence avoid thick sowing and frequent (heavy) irrigation to avoid prolonged leaf wetness.

Ahmed and Khan (2002) developed an Integrated Disease Management Model (IDMM) for control of onion downy mildew, which included the testing of NPK fertilizer 120:90:60 kg ha*1, plant population 0.5 million plants ha*1, 8 irrigations/season, fungicide Ridomil @ 250 g 100 L*1 plus Antracol/Dithane M-45 @ 200/300 g 100 L*1 and herbicide Ronstar @ 5 ml L*1. This model reduced the Area Under Disease Progress Curve (AUDPC) and increased substantially bulb size (cm) and bulb yield (t ha*1) by 3.7-4.1 and 20.3-28.2%, respectively. The multi location testing of IDMM proved its superiority over FOPs in minimizing the disease attack and improving the crop yield.

Botrytis blight, bulb and neck rot caused by Botrytis allii, B. squamosa, B. cinerea, is the next important disease, can damage crop in field and bulbs in field as well as during storage. The infections originate in the field but symptoms usually appear after harvest. The disease first appears as a soft, brown rot at the neck. When conditions are humid, gray fungal growth develops on the bulb and between the scales. Black irregularly shaped fungal resting structures (sclerotia) produced on the outer neck tissue or the rotten outer scales at advanced stages of the disease. The neck tissues collapse shows internal watery decay but the necks are damaged before they are well dried. Sclerotia of the fungus frequently found on the neck of onion. The disease develops best under cool and humid conditions (50° to 75°F). In the absence of a host, the fungus survives in the soil and in rotted bulbs as sclerotia for long periods. The fungus can also be seed borne or taken into field on infected bulbs planted for seed production. Airborne spores can penetrate succulent host tissue without a wound while moisture is present. Necks that are not cured are extremely susceptible to infection. The fungus is unable to penetrate well-dried neck tissue.

Botrytis blight, bulb and neck rot can also be controlled through long rotations with non-host crops. Wide row spacing and avoiding excessive and late applications of nitrogen disfavor disease development. Following production practices that promote crop storability e.g. do not irrigate within 10 to 14 days of lifting onions, allow tops to dry approximately 1 week before topping, cut roots under bulbs before harvest, harvest only when the crop is mature, and during dry weather, avoid damaging necks or bulbs at harvest, cutting necks 1” above bulbs is preferable to ripping off tops are also useful. To provide good ventilation for curing onions before storage and proper storage conditions: just above freezing and prevent moisture condensation on bulbs is also helpful to decrease disease.

Black mold caused by Aspergillus niger produces its typical characteristic as black mold, growth of the disease causing fungus, 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. Optimum temperatures for disease development ranged from 82 - 95 OF and moisture also favor disease development.

Avoiding crop injury reduce 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 55 OF.

Blue mold caused by Penicillium sp. appears at harvesting or in storage. The first symptoms appear as water soaked areas on the outer surface of the scales. A blue-green powdery mold will develop on the surface of the 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. Optimum conditions for disease development include high relative humidity and temperatures of 70O– 77OF.

Only avoid wounding the bulbs at harvest and storing below 42OF temperatures is recommended.

Onion smut caused by Urocystis cepulae, develop typical smut symptoms on leaves and bulbs. Affected plants are usually stunted, and often die. Surviving plants often have poorly developed bulbs or produce bulbs that deteriorate rapidly in storage. Plant symptoms consist of dark brown to black, 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 solarization significantly reduced smut pathogen and improved seedling stand and other characters including root and shoot lengths, number of leaves, fresh end dry weighs.

Stemphylium Leaf Blight of Onion caused by Stemphylium vesicarium (teleomorph: Pleospora allii), initially produces small, light yellow to brown, and water-soaked lesions on the leaves and leaf sheaths. As the lesions expand, they coalesce, causing extensive blighting of the leaves. Typical lesions are found in higher numbers on the side of leaves facing the prevailing wind. The centers of lesions turn brown to tan, then dark olive brown and finally black as the fungus sporulates. Sometimes fruiting bodies called perithecia 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 caused by Alternaria porri, occurs as water soaked areas followed by white necrotic spots on foliage. Girdling lesions on the stem and black concentric rings on purplish lesions also appear. Fungus requires rain or persistent dew for reproduction and penetration of plant tissue, 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, caused by Fusarium oxysporum f. sp. Cepae, results yellow and necrotic leaf tip. Affected plants may wilt; decay of bulbs progressing up from basal plate, white mycelium on and surrounding basal plate are also typical characteristics of the problem. Infection increased by insect injury to roots, stem plate or bulb and the pathogen is soil borne

Minimum of 4 years crop rotation is proposed.

Translucent Scale disease is cussed by frequent high temperatures (>90° F) and high relative humidity during the last 6-8 weeks of growth. The scales are translucent and resemble freezing injury, become grayish and water soaked, are brownish 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 incidence.

The problem needs research for solution.

Root knot nematode (Meloidogyne hapla, M. incognita, M. javanica, and M. chitwoodi) feeds on the roots, results stunting and reduced stand and produces characteristic galls on roots. Galls induced by M. hapla are generally small and difficult to see, whereas galls produced by the other root knot species are larger.

Stubby root nematode (Paratrichodorus sp.) also feed on roots and plants become stunted.

Lesion nematode (Pratylenchus penetrans) suppresses the growth and yield.

Growing non host crops for several years is helpful, but is not usually feasible because nematodes posses wide host range. However, soil amendments, oilseed cakes, plant extracts and recommended nematicides help to decrease nematode population.


Courtesy: The writter is parmanent contributer of Pakissan.com
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