Narcissus and Daffodil

96 G.R. Hanks
the boxes, exiting at the top of the stack. Small numbers of bins can be dried using
portable fans blowing into the pallet base or using ‘box tops’ fitted with fans
blowing downwards, or by placing bins over flow ducts in a bulk drier, but in
either case an arrangement of foam rubber closers or polythene film fixed around
the bins is needed to direct the air flow through the bulbs.
The forced air used to dry bulbs may be at ambient temperatures or it may be
heated. A number of recommendations state that a lift of about 3 °C at temperatures
of about 25 °C should be used (e.g., van Paridon, 1990), contrary to usual
advice to growers in the UK (e.g., ADAS, 1988a). Price (1975a,b) showed that the
incidence of rotting bulbs, and the numbers of propagules of the base rot pathogen
isolated from the base plate of healthy bulbs, increased with increasing storage
temperatures from 15 to 24 °C, then declining to 30 °C. In early studies of base rot,
Gregory (1932) and Hawker (1935) showed that bulbs should be stored below
25 °C, while Xu et al. (1987) reported that the incidence of base rot was greater with
temperatures >19 °C and Moore et al. (1979) stated that storage at 18 °C is a reasonably
acceptable and practical recommendation. ‘High temperature drying’ of
narcissus bulbs at 35 °C has been developed in the UK, and, as well as the convenience
of rapid surface drying (in two to three days), it produces cleaner bulbs as the
outer skins and soil contamination are more easily removed, and there is no
increase in base rot due to the higher temperature (Tompsett, 1977). However, the
safety of drying narcissus bulbs at 35 °C has been questioned by Linfield (1986b) on
the basis of culture experiments with the base rot pathogen on solid and liquid
media. On solid media, the fungus grew rapidly at temperatures of 20 or 25 °C, but
growth was slower outside this range and had ceased at 40 °C, confirming the findings
of McClellan (1952) that the optimum temperature for growth was 24 °C and
that there was little growth at 35 °C. In liquid media, however, Linfield (1986b)
found that growth of the pathogen was rapid over the range 15–35 °C, and had not
ceased entirely even at 45 °C, and she argued that in a freshly-lifted bulb, conditions
would be more like those of liquid culture, so that warm air drying would,
initially, favour pathogen growth: the rate of moisture removal from the tissues
would be more important than the drying temperature itself.
Bulb drying can be divided into first and second stages (Moore, 1980). Where
high temperature drying at 35 °C is used, this is only for first-stage drying, and
lower or ambient temperatures are used for second-stage drying. First-stage drying
consists of the removal of surface water, and is essential for the control of
surface moulds and other fungi. The rate of loss of surface water depends on the
rate of air movement and its temperature, and high rates of air movement are
necessary (425 m 3 /h/t for bulbs in loose bulk, and up to three-times this, for bulbs
in bulk bins to allow for leakage). With lower air flows, bulbs in the base of the
stack dry faster than those at the top, subsequently leading to variations in bulb
performance. Relative humidity should not exceed 75%. Second-stage drying
extends to the removal of internal water and ensures bulbs are thoroughly dry;
high rates of air movement are not needed (170 m 3 /h/t for loose bulbs) and the
humidity can be 80–85%. Higher ventilation and circulation rates are needed
throughout storage for disease-prone cultivars such as ‘Tête-à-Tête’. Robertson
et al. (1980) developed a computer simulation of drying times based on air flows,
temperature and bed depth, which was in reasonable agreement with experience
in practice. Bulbs may lose 20–25% of their lifted weight during drying, cleaning,