Agriculture Reference
In-Depth Information
form so that sharp corners are avoided. Large bins need to
be handled by fork-lift trucks, and require special physical
planning of the store. For ventilation of bin stores, a plenum
chamber, formed like a narrow room, runs down one side of
the main store, and 'letter box' slots for ventilation are pro-
vided, which can be lined up with the bins at different lev-
els. With this system the air passes up through the onions in
each bin (Schouten 1987). Normally such systems are for
cool climates or are used with refrigeration. However, in
warm climates which are not humid for prolonged periods,
it may be possible to combine bin or bulk storage with
intermittent forced ventilation using ambient air of the
appropriate temperature and RH at certain times of day: the
development and management of such systems is discussed
in detail by Brice
et al
. (1997). Farmers in Zambia and
Zimbabwe independently developed heated air onion stores
in the 1980s, often making use of tobacco drying barns
equipped with heaters (Currah & Proctor 1990).
Where onions are still largely hand harvested, sacks may
be filled in the field and stacked on pallets in ventilated
stores. In this case an open area should be left in the centre
of each pallet so that each sack is aerated from both sides.
Failure to do this invites trouble, as the onions in the centre
of each palleted mass will tend to get damp and may sprout
and eventually rot. Other storage arrangements include
using shelves to store bulk onions, using small crates
arranged on shelving, or stackable trays with permeable
sides and bases, or storing onions in bunches or in strings.
In all storage set-ups, however, it should be emphasised
that air needs to circulate freely around the onions and that
if it is impeded, trouble will follow. The same advice can
be given about shallots and garlic. Traditional stringing
methods are still in use in some places, such as Korea for
'northern' garlic (J.K. Lee, personal communication, 2003).
prefer white cultivars. Red onion skins contain phenols,
protocatechuic acid and catechol, which are not found in
the outer scales of brown onions (Walker
et al
. 1929; Link
& Walker, 1933). Both phenolic compounds are toxic to
the fungus
Colletotrichum circinans
responsible for the
disease smudge (Link & Walker 1933). However, not all
fungi are affected by these phenols including
B. allii
which
is thought to suppress or degrade the onion bulb's natural
defences known as tsibulins which are cyclopentane
phytoalexins that accumulate in bulb scales at the site of
infection (van Baarlen
et al.
2004). In general terms, onions
with high dry matter and high pungency are more resistant
to pathogens, and softer, juicy onions are more susceptible
and therefore less suitable for long-term storage. But by
breeding for good skin quality and higher pungency, even
onions with quite low dry matter content can be obtained
which can withstand long storage. Traditionally, for exam-
ple, Egyptian and Spanish storage onions fit this category.
The use of high-temperature (>25°C) drying to discour-
age neck rot has already been mentioned. Several bacterial
and fungal diseases can be hard to control if the infected
onions are not detected before they are stored. Mechani-
cally or biologically damaged onions are a menace in store
since they give off more water vapour than normal through
their raised respiration rate, and their damaged flesh can
develop disease infections. Many of the diseases which
onions can suffer in storage were illustrated by Snowdon
(1991), who also depicted some of the other conditions
which can lead to onion downgrades, such as skin staining
by water during transport.
Some of the more serious storage pathogens of onions
and the conditions which favour them are shown in
Table 17.8.
The development of fungi on irradiated onions in store
was studied by Benkeblia and Selselet-Attou (1997) in
Algeria. The highest ionising treatment, at 0.31 kGy, was
most effective in reducing fungal loads on onions. Fungal
loads fell at first during storage on the control samples
but after a time in store (more rapidly at ambient than at
4°C) they gradually increased again until they reached
the original levels after several weeks of storage. After
the most severe ionising treatments plus cool storage, the
original microflora levels were not regained. However,
onion rotting in store could not be correlated the numbers
of onion fungi present, and it is suggested that other less
specialised organisms are also involved.
In the Korean Republic, novel methods of controlling
fungi on onions by biological means have shown promise
(Lee
et al
. 2001). The most prevalent diseases were basal
rot, black mould and neck rot. A number of antagonistic
ONION PEST AND DISEASE PROBLEMS
IN STORAGE AND DURING TRANSPORT
Pathogens
A range of fungal and bacterial pathogens feed on and
damage onion bulbs. Usually these are organisms which
are common in the field or may even be seed-transmitted,
as was found with
Botrytis allii
and certain bacterial
pathogens. Each pathogen species has its own temperature
requirements for development, while all are encouraged to
grow by high humidity (>80% RH) during storage. They
include low temperature pathogens such as neck rot, and
those needing high temperatures (most of the bacteria,
black mould or
Aspergillus niger
, and some other fungi).
Some fungi preferentially attack coloured cultivars; others
