Crop Protection

Because of this, weed control is essential to reduce competition for moisture and nutrients. Their erect growth habit means that even when they are established they do not compete well with weeds. Pre and post emergence herbicide programs are commonly employed. 

A range of nematodes attack the stem and roots, restricting moisture and nutrient uptake. Stem and bulb eelworm (Ditylenchus dipsaci) is particularly damaging, attacking stems and causing shoot twisting, distortion and young plant death. Seed needs to be nematode free, and soil sterilization and good rotational practice is needed to minimize nematode population build up. 

Onion thrips are also a damaging pest worldwide, particularly in warmer regions. They congregate between the young leaf and the plant neck, extensively damaging leaves and slowing growth.

Onion fly or maggot, shallot aphid and leek moth, are other major pests all of which can be controlled using insecticides. Onions are also very susceptible to foliar diseases and bulb rots. Bacterial diseases significantly reduce bulb quality, e.g. 

Pseudomonas alliicola, which causes slippery skin within the bulb and P. cepacia - slimy or ‘sour’ outer scales. 

Neck rot (Botrytis allii), mildew (Peronospora destructor) rust (Puccinia porri) and leaf rot (Botrytis squamosa) are among the key diseases that need fungicidal control.

Bulbs need to be carefully handled to minimize bruising and cuts that can allow disease to enter. Controlled atmosphere storage is increasingly practiced to maintain quality in store over longer periods and extend the marketing period. 

Maintaining CO2 levels at 5% inhibits fungal sporulation and O2 at 3% reduces sprouting. Temperatures should be around 1°C (34°F) to help reduce respiration and relative humidity kept at 65-70% in order to inhibit fungal spread.

Storage 

Onions should be mature and dry when harvested. Unless crops are to be cured in the field, harvesting of dry onions usually starts when 60-80% of the tops have fallen over. Formation of chlorophyll - green scales - in the outer skin can occur when crops are left to cure for too long or when bulb shoulders are exposed to sunlight in the field. 

Bulbs need to be carefully handled to minimize bruising and cuts that can allow disease to enter. Controlled atmosphere storage is increasingly practiced to maintain quality in store over longer periods and extend the marketing period. Maintaining CO2 levels at 5% inhibits fungal sporulation and O2 at 3% reduces sprouting. 

Temperatures should be around 34°F to help reduce respiration and relative humidity kept at 65-70% in order to inhibit fungal spread.