Agriculture Reference
In-Depth Information
for store operatives, unlike low-oxygen stores. Ethylene, being a gas, reaches all
bulbs in a store uniformly. Once removed from store the ethylene diffuses out of
the bulbs within 3 days leaving no residue, unlike maleic hydrazide (MH). As a
result, although shelf-life after removal from store is better than for untreated
bulbs, it not as good as for MH-treated bulbs. Since there is increasing demand
from supermarkets and consumers for produce to be free of residues from
applied chemicals, this technique is being rapidly adopted for sprouting control
in onion stores.
However, for onions destined for long periods of transport after removal
from store, MH is likely to remain useful since the ripening- and senescence-
inducing effects of ethylene on other fruits and vegetables make it unlikely that
it will be acceptable for use in sprout suppression in onions on ships carrying
produce exports. As yet there are scant scientific data reported on ethylene and
onion storage. The optimum concentrations of ethylene have not been reported,
although 10
l/l was very effective in inhibiting sprout leaf growth in dormant
bulbs stored at 3
0
C (Bufler, in press, b). Effects on bulb quality have also yet to be
established, although first reports indicate that ethylene prevents the increase in
pungency with time in storage but does not prevent the decrease in fructan
content and increase in fructose (Bufler, in press, b).
Translucent and leathery scale disorders of onion bulbs
During the late 1970s and the 1980s there were widespread reports from
northern Europe of bulbs with one or more scales in a broken down and watery
state. This was not associated with a causal pathogen, and the increased
prevalence of the disorder coincided with harvesting methods changing to
involve mechanical handling, artificial curing and bulk storage. New cultivars
with strong, thick skins suited to mechanical harvesting and bulk storage may
also have been a factor.
The problem was particularly serious in Norway, and research there
established that the symptoms could be induced by submerging bulbs in water
for 1-2 days, or by sealing the necks of bulbs with vaseline, both treatments
that restrict gas exchange and which lead to internal carbon dioxide levels
above 13% (Hoftun, 1993). High internal carbon dioxide is considered the
primary cause of watery scale, and this has been termed the 'choke theory' for
watery scale.
Further studies showed in detail how such symptoms can arise (Solberg,
1997). Solberg divided watery scale disorders into leathery scales (LS) - which
appear as thick, dark, moist scales between the dry outer skin and the inner
fleshy scales - and translucent scales (TS), which are water-soaked but firm,
fleshy scales occurring initially in the middle or upper parts of outer fleshy
scales. The symptoms become more severe after long storage, a tart smell and
grey or bright green colours can develop and symptoms can spread to the
