Onion crop Diseases and their management

Onion is attacked by a number of diseases, some of them are as under:

1. Pink root: The root turns pink, shrinks and dies. The causal organism is a fungus which lives in soil. The plant pro- duce new roots but do not complete with the growing season so yield is lowered. It is more serious in hot, dry weather and at the time of bulb formation.

Control:  Adopt crop rotation and apply calcium cyanamide at the rate of 2500 litres per hectare.

2. Onion smut: It is caused by Urocystis cepulae. The fungus lives in the soil year after year. It is inactive at 30°C or above. The fungus attacks on young seedling plants. Dark, slightly thickened areas on small leaves of younger seedlings occur. Then other leaves are attacked. They are swollen and try to bend downwards. Then small black pustules appear.

Control:  

(I) Treat the seed with Thiram 3 g per kg of seed,

(2) Grow disease resistant varieties like Nebuka, etc. and  

(3) Apply fungicide in furrows along the lines of seed sown in nursery.

3. Neck rot : It is caused by the fungus Botrytis alii. It is found in all regions where bulbs are stored. The fungus also attacks injured leaves. The lesions appear as sunken, dried areas near the node or the bulb or involve whole bulb. The fungus over-winters in the diseased onions in storage.

Control:  i. Harvest the crop when the tops are completely dry. ii. Store the onion at about O°C and with humidity at about 65 % with good dry air circulation. iii. Remove the diseased plants and bum them.

4. Downy mildew: The disease spreads rapidly during cool and wet weather. Some varieties of onion have the fungus during winter also in storage. Dew collects on the plant at night by which the spores germinate and enter the onion leaf.

Control: i. Grow resistant varieties. ii. Spray Zineb, about 6-10 application at an interval of 6 to 8 days. iii. Remove diseased plants and burn them.

5. Stemphylium blight: Small, yellow to pale orange spots appear in the middle of flower stalk as well as leaf on one side but other side remain quite green. These spots then become spindle shaped, diffused lesions then coalesce. The whole flower stalk is attacked in a short period affecting seed development and the leaves of the bulb crop dry prematurely.

Control: (i) From first week of February, spray the crop with Dithane M-45 -2-5 g per litre of water, repeat the process at fortnightly interval, total 4-5 sprays are sufficient. (ii) Remove the diseased plants and burn them. (iii) Adopt the crop rotation.