Narcissus and Daffodil

Production of Narcissus bulbs 73
carried out by Hewitt (1914), but the treatments used (1–6 hours at 48.9 °C)
resulted in the death of the bulbs. The technique was further developed by Ramsbottom
(1918, 1919), Staniland (1933) and Staniland and Barber (1937), resulting
in a standard recommendation of 3 hours at 43.5 °C. The early studies have been
fully described by several authors (Slootweg, 1962; Turquand, 1966; Tompsett,
1982; Lane, 1984; Gratwick and Southey, 1986). Treatments were later increased
to 3 hours at 44.5 °C or 4 hours at 43.5 °C, to improve control (e.g., van Slogteren,
1931; Chitwood and Blanton, 1941; Woodville and Morgan, 1961). Lees (1963)
gave the critical treatments for crop damage. Growth was not impaired by extending
a 3 hour treatment to 46.1 °C, nor by extending a 43.5 °C treatment to 6½
hours, but a 3 hour treatment at 47.2 °C impaired vigor and at 54.4 °C a 45 minute
treatment was lethal. For N. tazetta var. chinensis, Lin et al. (1987) reported that
bulbs grew normally after HWT for 45 minutes at 50 °C or 25 minutes at 55 °C,
although complete control of the target nematodes (in this case Aphelenchoides spp.)
was not achieved. The chemicals applied do not penetrate beneath the bulb scales
at ambient temperatures unless a vacuum treatment is applied (Newton et al.,
1933). ‘Vapour heat treatment’ was also investigated, but the regime necessary to
control stem nematodes – 8 hours at 47.7 °C – had adverse effects on the crop
(Chitwood and Blanton, 1941). Other methods of treating stem nematode (such as
nematicidal dips or field applications) are either ineffective, or suitable chemicals
are not now available (Hesling, 1971; Damadzadeh and Hague, 1979; Windrich,
1986).
With two-year-down growing, effective HWT becomes even more important,
and this is reflected in the UK recommendation to treat all planting stocks for a
full 3-hour period at a temperature of 44.4 °C. The treatment period is usually
taken as starting when the tank temperature regains 44.4 °C after the cooling effect
of loading the bulbs. In the Netherlands, using one-year-down growing and particularly
where soil sterilisation is used to control nematodes, it is usual to apply
a milder treatment (2 hours at 43 °C), adequate to control mites and narcissus fly
larvae. Where a nematode problem is suspected, or when using two-year-down
growing, a longer, hotter regime (4 hours at 47 °C) is recommended, and a 4 hour
treatment at 48 °C is used for ex-forced bulbs (Vreeburg et al., 1999). At the time
of writing, anecdotal evidence is that growers in the UK and the Netherlands are
treating bulbs at up to 49 °C. Warm-storage and pre-soaking are necessary prior to
HWT in these cases (see below).
A variety of designs of HWT tanks is used, the usual type being a front-loaded
design where bulk bins of bulbs are loaded and unloaded using a fork-lift truck
whilst the dip solution is temporarily pumped to a holding (or ‘slave’) tank (Figure
4.3). Tank capacity varies depending on the scale of the operation, but may be
from 0.5 to 10 t of bulbs. Since the temperature of HWT is critical for killing nematodes
without causing unnecessary damage to the bulbs, good temperature control
and tank circulation are key factors in HWT tank design. Many other factors
are also important, such as rapid heating, good insulation, a suitable water:bulb
ratio, temperature monitoring and recording, operator safety and convenience of
use. Aspects of the design and use of HWT tanks have been described by Gratwick
and Southey (1986). Although HWT may be used to control pests and diseases in a
variety of other plant material, flower bulbs (in particular narcissus) are the main
subjects for HWT, and an HWT facility is likely to be the major dedicated outlay