Agriculture Reference
In-Depth Information
inflorescences tend to appear; and (iii) growing one line from autumn-sown seeds
and the other from mother bulbs. An autumn-sown crop grown from seed will
normally bloom later than the same line grown from mother bulbs.
Seed stalk disease and seed infection
Fungal diseases, which tend to spread in conditions of high humidity when
there are prolonged periods of plant surface wetness (see Chapter 5), are
potentially very serious in seed crops. The developing seed stalk can be com-
pletely girdled and destroyed by such infections. Probably the most serious
disease in onion seed production is downy mildew,
Peronospora destructor
. In
addition to the use of fungicides, many of the cultural practices of growing
onion seed crops - for example, fairly wide row spacings and limited autumn
applications of N fertilizers to avoid excessive leaf growth - are aimed at
maintaining good ventilation within the crop, thereby making the conditions
less conducive to the spread of downy mildew (Peters, 1990). Purple blotch -
Alternaria porri
,
Stemphylium vesicarium
and various
Botrytis
, particularly
B.
squamosa
- also cause serious damage.
Pollination
Experiments in which onion umbels were bagged have established that very little
pollination was accomplished without access by insects; wind and gravity being
of little importance. From tens to hundreds of different species of insect have been
identified visiting blooming onion umbels, the number of species depending on
the locality (Currah, 1990). Honeybees are the most important, and seed
producers hire hives of bees and site them in the onion fields during flowering. In
the USA, between 12 and 37 hives/ha are used. The introduction of the beehives
is staggered in time, starting from when about 50% of umbels are showing some
open florets. After some days the bees may learn to forage on more attractive
neighbouring crops, so by staggering their introduction their initial 'naive'
behaviour, when they tend to stay on the onion flowers, is exploited. Bees forage
for nectar, not pollen, from onion flowers, and they are active at air temperatures
above 16°C. They move more frequently from umbel to umbel when the sky is
clear and cloud free, therefore increasing the chances of cross-pollination in
sunny weather.
In the USA, onion pollen appears to have decreased in its attractiveness to
honeybees over the years. The reasons for this change are not fully understood,
but may be due to high levels of potassium in the nectar of hybrid onions. Very
high sugar concentrations (> 50%) can occur in onion nectar, and this also
makes it unattractive to bees. To attract bees onions must produce abundant
nectar and, when inbreeding to produce male-sterile lines, it is essential to
ensure that the ability to produce nectar does not disappear; such lines are
useless for seed production.
Pollen fertility varies with temperature. At 14°C pollen abundance and
viability is low, but is higher at 23°C. Male sterility tends to break down at high
