introduced into an area it is wise if there is time to try them out for a year or so in a
small way, to compare them with the local types.
Growth period: 9–150 days depending on whether seed or sets are planted, the
tend to mature faster when they are stressed, such as when they are planted too close
together.
Temperature: varieties vary greatly in their temperature requirement, but low
temperatures encourage bolting, if the bulbs are large enough. Cool, wet weather is
best for early growth, with warm, dry weather for harvest.
Rainfall: requirement depends on variety. Often irrigated. If the soil is liable to
become waterlogged, onions should be planted on raised beds.
Pests: maggots of the Onion fly can stop the plants developing altogether. Seed-
sown crops are worst affected, setts rarely suffer badly. Eggs are laid in the bulbs of
young onions, the yellow maggots eat the bulb and the leaves go yellow and die. Hot
weather and dry soil favours spread of this pest. To prevent the flies from laying
eggs, either sprinkle soot or wood ash along the row, or spray water mixed with
paraffin (1 fluid ounce/1 gallon water). As soon as this pest is identified, infected
plants should be uprooted and burned, to avoid leaving maggots in the soil.
- Thrips—the leaves develop silvery blotches.
- Eelworms—very tiny worms, they make the tops of the plants wilt. Control by
burning/destroying all infested plants and strict rotation of 6 years without allowing
either other Allium species or Chickweed, an alternative host, to grow in or near the
infected area.
Diseases: Downy Mildew causes leaves to develop a greyish or violet coloured
Bordeaux powder.
- Purple Blotch
become purple with a yellow halo; Pink Rot & White Rot—roots are attacked, then
the leaves go yellow and die. Soil-borne; Yellow Dwarf Virus—worldwide. Yellow
streaks on leaves. Spread by contact, or by sets or aphids, but not by seed. - Storage Diseases:
Black Mould (Aspergillus) and Neck Rot (Botrytis) are both common. Neck Rot
attacks onions in store. A grey mould forms on the skins, and later on the centres
turn brown. Prevent by thoroughly drying the onions before storing and store in a
cool airy place.
YIELD
In the tropics 7–10 MT/ha of freshly harvested onions is about the average, while
40 MT/ha.
33 inches in diameter) was grown in Fife, UK in 1997. The variety name is Kelsae
Sweet Giant.
— leaves develop white sunken spots, which grow larger and236 TONY WINCH
season/temperature when planted, rainfall, day length and the variety chosen. Plants
streaks on the leaves, especially in humid conditions. Can be controlled with
sophisticated farming systems in ideal growing conditions can produce more than
At the time of going to press, the world’s largest onion (7.03 kg, 15 lb 5 oz and