PLANT PROTECTION


Growing healthy and vigorous crop is the ultimate aim of all farmers because a poor and diseased crop simply deprives funds available with one without any output. Onion crop is victim of several enemies each of which requires specific management practices. Major diseases and insect pests and their control measures are given below :

DISEASES

Damping off

Caused mainly by Fusarium oxysporum fungus, this is very common in almost all onion-growing pockets. Pythium sp. has also been reported to cause damping off disease in some pockets. The disease is more prevalent in Northern and Eastern parts of the country during kharif season, causing 60-75% damage. Two types of symptoms are observed.

Pre-emergence damping off : The fungus kills the radicle and plumule of seed before emergence from soil.

Post-emergence damping off : The pathogen attacks the collar region of seedlings on the surface of soil. The collar portion rots and ultimately the seedlings collapse and die.

For managing the disease, healthy seeds should be selected for sowing. The seed should be treated with thiram @ 2g/kg before sowing. Continuous raising of nursery in a particular plot should be avoided. Top soil of nursery should be treated with thiram @ 5 g/m² area and nursery should be drenched with the same chemical @ 2 g/litre of water at fortnightly interval. Soil solarization using 250-gauge polythene for 10 days before sowing and application of Trichoderma viride in soil @ 1,250 g/ha is also effective to control damping off to a considerable extent.

Purple Blotch

Purple blotch is caused by Alternaria porri . The disease occurs under favourable conditions of temperature 28-30ºC and 80-90% relative humidity. It is more common in kharif season in Maharashtra. Small, sunken, whitish flecks with purple coloured centres are common symptoms occurring on leaves and flower stalks. Further, large purple area develops forming dead patches. The intensity of disease varies from season-to-season. It causes losses of 25% in rabi, 50% in kharif in Maharashtra, while in Northern and Eastern parts 25-90% damage in bulbs and seed crop occurs when the disease appears along with stemphylium blight.

For managing the disease effectively, healthy seed should be used. Crop rotation of 2-3 years with non related crops should be followed. Further, mancozeb @ 0.25% or chlorothalonil @ 0.2% or rovral @ 0.25% should be sprayed at fortnightly interval commencing from one month after transplanting. The sticker triton/sandovit should be mixed in spray solution.

Purple blotch

Stemphylium blight

This disease is very common in Northern and Eastern India on onion leaves as well as flower stalks. Disease intensity varies from 20 to 90% in seed crop and 5 to 50% in bulb crop. Infections occur on radial leaves of transplanted seedlings at 3-4 leaf stage during late-March and early-April. Small yellow to pale-orange spots or streaks develop on middle of leaves/flower stalks on one side. The casual organism is Stemphylium vesicarium. Disease can be managed effectively if mancozeb @ 0.25% along with monocrotophos @ 0.18% with sticker triton is sprayed at fortnightly interval starting before appearance of symptoms.
Stemphylium blight

Colletotrichum blight

The disease has been reported from Maharashtra. It is caused by Colletotrichum gloeosporides. Its symptoms initiate as pale-yellow water-soaked spots which increase in length covering whole leaf producing numerous black coloured slightly raised structures in central portions. These structures appear in concentric rings. Affected leaves shrivel, droop down and finally wither. Spraying of mancozeb @ 0.25% or carbendazim @ 0.1% gives good control.
Colletotrichum blight

Twister disease

Caused by Golmerella cingulata, it has made its occurrence recently in India during kharif season. Curling, twisting of leaves, chlorosis and abnormal elongation of neck portion are major symptoms. Bulbs development is affected. Crop hygiene, use of quality seeds, seed treatment with thiram @ 2 g/kg before sowing and spraying with ziram based fungicides like cuman L @ 0.3%, dithane Z-78 @ 0.25% or soil treatment with benlate @ 2 g/m² and spraying @ 0.1% benlate are its control measures.
Twister disease
Downy mildew

Caused by Perenospora destructor,it is reported from Northern hilly track and plains particularly humid locations. On the surface of leaves or flower stalks violet growth of fungus is noticed which later becomes pale-green yellow and finally leaves or seed stalks collapse. For managing the disease effectively, onion bulbs meant for seed crop should be exposed to sun for 12 days or heated for 4 hours at 41ºC to destroy the fungus. Spraying of 0.2% zineb also gives good control.

Onion smut

Onion smut is caused by Urosystis cepulae. In India, it is reported in Karnataka on small onions. The fungus survives in soil in area where temperature remains below 30ºC. Since the fungus remains in soil, disease appears on cotyledons of young plants soon after it emerges. Smut appears as elongated dark, slightly thickened areas at the base of seedlings. The black lesions appear near the base of scales on planting. The affected leaves bend downwards abnormally. On older plants numerous raised blisters occur near the base of leaves. The lesions on plant at all stages often expose a black powdery mass of spores. The fungus becomes inactive at soil temperature of 26.7ºC and above.

The disease can be controlled by treating the seeds with captan or thiram @ 2.5 g /kg of seed before sowing. Seed-bed soil treatment with methyl bromide at 1 kg/25 m2 or alternative application of formaldehyde solution @ 8 ml/litre of water /10m2 is effective in controlling the disease.



Onion smut

Black Mould

Caused by Aspergillus niger, it is characterized by black powdery mass of spores that are borne on exterior of the scales and can be rubbed off easily. Black spore masses are also seen on inner scale. For effective control, bulbs should be thoroughly cured after harvesting and crop should be saved from rains.

Black mould

Basal rot

Basal rot caused by Fusarium oxysporum, is a widespread disease. Yellowing of leaves and stunted growth are its first symptoms. Later on drying of leaves from tip to downwards is noticed. In early stage of infection, roots of plants become pink in colour and rotting takes place later. In advanced stage, bulbs starts decaying from lower ends and ultimately whole plants die. Drenching with carbendazim @ 0.1% is good for checking the growth of fungus. Five-year rotation with other crop is also recommended as its pathogen survives in soil. Dipping of seedling in 0.1% carbendazim also reduces basal rot in bulb crop significantly.

Basal rot

Onion smudge

Caused by Colletotrichcum circinans, it is not very common in India. Sometimes it occurs on white bulb varieties, reducing market value of bulbs due to blemishing dots on outer scales. It is characterized by small-dark green to black dots which appear on outer scales.

The disease can be managed by thorough curing of bulbs after harvesting and storing them in well-ventilated rooms.

Onion smudge

Bacterial brown rot

The disease is caused by bacteria , Pseudomonas aeruginosa. It is very serious disease of stored onions in tropical countries especially in India. The rot begins at the neck of bulbs, later it gives off smell through the neck when squeezed. Proper curing and rapid drying of bulbs after harvesting are essential for managing the disease. Bulbs showing disease symptoms should be discarded before storage. Spraying of streptocycline @ 0.02% before harvesting if rains occur at maturity, is suggested to reduce this disease.
Bacterial brown rot

Onion yellow dwarf

This is a viral disease caused by onion yellow dwarf virus. Its diagnostic symptoms are severe stunting of plants, dwarfing and twisting of flower stalks. The affected leaves and stems change their normal green colour to various degrees of yellow and leaves tend to flatten and crinkle and as a result bend over. The virus is transmitted mechanically as well as by insect vectors. Diseased plants should be removed and burnt. Spary malathion (0.1%) or metasytox (0.1%) . Healthy bulbs should be used for seed production.

 

INSECTS

Onion Thrips

Thrips (Thrips tabaci) is most injurious insect pests of onion. These are common all over the country where onion crop is grown. Onions infected with thrips develop spotted appearance on leaves which turn pale-white blotches due to drainage of sap. Sometimes 50-60% losses in bulb crop are observed due to thrips attack. Seed production including viability of seeds are hampered due to attack of thrips. The insect is very minute and yellow to dark brown in colour. The life-cycle of insect is 8-10 days. It is found in the axil of green leaves where it sucks juice of early-emerging leaves. The adults hibernate in soil, on grass and other plants in onion fields. The thrips also over-winter in bulbs and act as source of infestation in the following year. They multiply in a large number during March-April on seed and bulb onions in Northern parts of India, whereas at Nasik incidence is observed even in January-February. The infested plants show stunted growth with twisted leaves. If the infestation comes in early stage of growth, bulb formation stops completely and plants die slowly. Bulbs in store are also attacked by thrips. Malathion (0.1%) or metasystox (0.1%) and cypermethrin (0.01%) or deltamethrin (20 ml/ha) should be sprayed along with triton or sandovit to control the insect. Soil application of phorate or carbofuran granules @ 1 kg ai/ha is also recommended.


Onion thrips
 

Head-border

Head-borer (Heliothis armigera) is a serious pest of onion seed crop in Northern India. Its larva cuts the pedicel of flowers and feeds on stalks. Many flower stalks are damaged by single larva. The bulb crop is also damaged by larva by cutting and eating aerial portions of plant. The fully-grown larva is greenish with dark brown gray lines along with the side of body and measures about 35-45 mm in length. Larva pupates inside scape. It also pupates in soil. Spraying of endosulfan or zolone (2-3 ml/litres of water) along with triton/sandovit sticker is recommended to control it. Soil application of chloropyriphos (5 ml/litre) can also control this pest.

Onion maggot

The adults of onion maggot (Hylemia antiqua) appears like house fly. The flies lay their eggs on old leaves or on soil and larva enters into soil and damage disc portion of onion bulb. Infested plants turn yellowish brown and finally dry up. The affected bulbs rot in storage as infestation leads to secondary infection by pathogenic organisms. The crop rotation should be followed and thimet should be applied in the soil before transplanting.

Cut worms

The larvae of cut worms (Agrotis ipslon) are seen in nursery-beds and newly-transplanted onion fields. The tender plants are damaged at ground level during the night. The moth of insect has dull brownish forewings with numerous wavy lines and spots, and brown coloured hind wings. The larva is dark brown with a red head. Soil application of carbofuran (1 kg ai/ha) at the times of planting is recommended. Cloropyriphos (5 ml/litres of water) also controls this pest.

Mites

Dry weather is conducive to infestation of mites (Rhizoghyphus sp.). It sucks the sap turning the plants pale with sticky appearance. It also infects stored onions, inducing sprouting. The mite is reddish-green with 8 legs and measures about 4 mm. Application of sulhpur dust (22 kg/ha) is recommended. Spraying of kelthane (0.1%) also gives good control to mites.