Agriculture Reference
In-Depth Information
The optimum harvest date has been investigated in several studies (see
Brewster, 1990b). All experiments show that bulb yield continues to increase
until well after foliage fall-down, probably while the leaves remain at all green;
this is normally for at least 2 weeks after fall-down. However, most studies show
that the time to sprouting is maximum when plants are harvested at 50-80%
fall-down. Earlier or later harvest results in earlier sprouting in store. Also skin
quality, as shown by staining and splitting, deteriorates if bulbs are harvested
at a later stage, particularly if the weather is wet. Therefore, the date of harvest
is a compromise between maximum yield and maximum storage life and skin
quality. Skin quality has assumed great importance in Europe recently, and
therefore bulbs are normally harvested at the 50-80% foliar fall-down stage.
In the warm, dry climate of California, experiments have shown maximum
storage life and least rotting in store when bulbs were harvested when the
foliage was fully senesced, well after fall-down (Currah and Proctor, 1990).
Therefore, in dry climates the optimum stage for harvest may be later than in
wetter, cooler regions.
Onion seed production
The physiology of flowering finds its most important application in seed
production. This has been reviewed by Peters (1990), pollination biology in seed
production by Currah (1990) and seed production in tropical regions by Currah
and Proctor (1990). Seed yields of about 500 kg/ha are common in many
regions, but in the USA 800-1000 kg/ha are typical. In trials in California, seed
yields from bulbs stored at optimum pre-planting temperatures varied almost
fourfold from year to year, with a maximum yield of 1554 kg/ha (Jones and
Emsweller, 1939). Yields of more than 3000 kg/ha have been obtained in
experiments using high plant densities and irrigation in Washington State, USA
(Vincent, 1960). In contrast, the best trial yields of seeds from a multiplier cv. in
the Philippines was 111 kg/ha (Currah and Proctor, 1990). Thus, there are
clearly enormous differences in average seed yields depending on genotype,
locality, season and method of seed production.
Locations for seed production
Regions with reliably warm, dry summers are best for onion seed production.
Low atmospheric humidity reduces the risk of diseases like downy mildew and
purple blotch (see Chapter 5). Clear, bright, warm weather during flowering is
favourable to a high activity from pollinating insects. In the USA, parts of Idaho
and Oregon are climatically well suited for producing seed of the 'long-day' US
varieties. 'Short-day' types are produced in the south-western USA, particularly
the Imperial Valley of California (Jones and Mann, 1963). In northern Europe,
