Nutritional Deficiencies/Disorders of Onion

Nitrogen

Leaves become yellowish green erect and upright curled, wilted and dwarf. At maturity tissue above bulbs become soft.

Correction measure: Foliar spray of Urea 1% or DAP 2% twice at weekly intervals.

Phosphorus

Slow growth, maturity blazed. Leaf colour becomes light green and bulbs have few dried outer peals. Tip burn in older leaves.

Correction measure: Soil application of recommended dose of phosphorous should be applied at the time of sowing or planting. Foliar spray of DAP 2% twice at fortnightly intervals.

Potassium

Since potassium is very mobile within the plant, symptoms only develop on young leaves in the case of extreme deficiency. Tip burn symptoms, leaves become dark green and erect. Bolting promoted. Older leaves become yellow and necrotic.

Correction measure: Foliar application of K₂SO₄ @1% twice at weekly interval.

Sulfur

The leaves show a general overall chlorosis. The yellowing is much more uniform over the entire plant including young leaves.

Correction measure: Foliar spray of K₂SO₄ or CaSO₄ @ 1% twice at fortnightly interval.

Manganese

Leaves show tip burn, light coloured and curling. Growth restricted. Bulbing delayed with thick necks.

Correction measure: Foliar spray of MnSO₄ @ 0.3% twice at fortnightly interval.

Zinc

Growth restricted. The leaves show interveinal necrosis. In the early stages of zinc deficiency the younger leaves become yellow and pitting develops in the interveinal upper surfaces of the mature leaves. As the deficiency progress these symptoms develop into an intense interveinal necrosis but the main veins remain green.

Correction measure: Foliar spray of ZnSO₄ @ 0.5% twice at for nightly interval.

Iron

Complete yellowing of young leaves. The most common symptom for iron deficiency starts out as an interveinal chlorosis of the youngest leaves, evolves into an overall chlorosis, and ends as a totally bleached leaf. Because iron has a low mobility, iron deficiency symptoms appear fi rst on the youngest leaves. Iron deficiency is strongly associated with calcareous soils, anaerobic conditions, and it is often induced by an excess of heavy metals.

Correction measure: Foliar spray of FeSO₄ @ 0.5%.

Physiological and nutritional disorders

Sprouting of bulbs in the field is noticed sometimes towards the start of maturity stage of bulbs particularly when there are winter rains or excessive soil moisture and nitrogen supply. This disorder is, however, not of permanent nature and varies from variety-to-variety. Early-planting also causes sprouting. Splitting is also noticed sometimes in some varieties, which is due to delayed harvesting or irrigation after long spell of drought. For past few years rubberification problem is noticed in Rajkot area and also Nilgiri hills of Tamil Nadu. It is increasing day-by day. The effects of insecticides, fungicides, micronutrients and growth regulator were studied. It was observed that rubberification was totally controlled by application of micronutrients i.e. zinc sulphate and ammonium molybdate. It was also controlled by neem cake insecticides and growth regulator like gibberellic acid. The aerial bulbil formation is also quite common in onion where lower temperature prevails for more period or there are more temperature variation. It, however, does not affect much on yield or quality of bulbs and varietal variation is noticed. The rubberification and premature sprouting of bulbs are main physiological disorders in onion. The reasons for these disorders are summarized below for hill grown onion.

Rubberification and premature sprouting of bulbs are noticed mostly in fields which are located in low-lying areas of watershed where there is heavy deposition of nutrients along with silt during heavy rains. These problems are severe in onion fields which are more frequently irrigated than the normal requirements of onion bulbs. With the application of higher levels of nitrogen, there is an increased level of pre-mature sprouting of bulbs which results in splitting and rubberification of bulbs. Rubberification incidence also increases when higher level of nitrogen is applied in the form of urea. Due to increased levels of nitrogen, in addition to the increased production of rubberized, there is an increased level of thrips incidence in leaves during the later stage of crop. The thrips lacerate leaves and cause severe damage to the crop. Through lacerated wounds by thrips, there is a severe incidence of blast disease, caused by Boltrytis allii, when crops is 70-90 days old, which further deteriorates the quality of onion bulbs. Short duration type of onion is more susceptible than long duration type. Premature sprouting of bulbs is more in the crop (April-August) when there is high moisture level in soil at maturity of crop due to heavy rains. Delayed harvest during rainy season has increased premature sprouting and splitting of bulbs. Wider spacing of cloves at the time of planting increases uptake of nitrogen and water by the individual plants which increases premature sprouting and rubberification of onion bulbs

IPM for Onion

To know the IPM practices for Onion, click here.

Source: NIPHM and Directorate of Plant Protection, Quarantine & Storage