Narcissus and Daffodil

Production of Narcissus bulbs 75
not been reported; late HWT is probably more damaging, because of the potential
loss of leaf area and roots. In cases where a problem with nematodes is suspected
or known in a stock, bulbs should be lifted early (June), cleaned, graded and given
HWT promptly. Earlier recommendations (e.g., Hastings and Newton, 1934) and
some Dutch recommendations state that HWT in this case should be done within
three to four weeks of lifting, but current recommendations in the UK are to treat
sooner than this (J.B. Briggs, personal communication). The increase in stem nematode
numbers in bulbs during storage was shown by Winfield and Hesling (1966).
Even when applied correctly, HWT can reduce crop vigor, but this disadvantage
is outweighed by the control of stem nematode and, in any case, the loss of vigour
is insignificant in two-year growing. If HWT is carried out too early or too late, for
too long a time or at too high a temperature, or following the storage of bulbs at
low temperatures, a variety of damage occurs. This varies from mild damage to the
flowers (unimportant where crops are being grown for processing), through damage
to the leaves (from mottling of the leaf tips to severe distortion or stunting
resulting in bulb yield loss) or roots (resulting in severe yield loss), to the death of
the bulbs. Excessive HWT temperatures may result in the abnormal production of
additional bulblets (Edwards, 1965; H.Y. Alkema, personal communication).
Hot-water treatment – the use of chemicals
While stem nematodes in the bulbs are killed by the high temperatures of HWT
alone, chemicals are added to HWT tanks to control pests and diseases better. The
basic material added is the disinfectant formaldehyde, used as ‘commercial formalin’
(containing 38–40% formaldehyde) (Hawker, 1944). For California, Qiu et al.
(1993) reported that the standard HWT regime was 4 hours at 44 °C, but showed
that such a treatment was sufficient to control stem nematode without added
formaldehyde; using formaldehyde, 150 minutes was sufficient and caused no
crop damage. However, the time for the centres of bulbs to attain the target temperature
after placing in hot water should be added to these basic times: this was
calculated as equal (in minutes) to –15 + 3.4x, where x is bulb circumference in
cm (Qiu et al., 1993). Higher temperatures could be used for shorter times: without
formaldehyde, stem nematode was controlled by 60 or 15 minute treatments
at 46 or 48 °C, respectively, without crop damage; with formaldehyde, control was
achieved by 90, 45 and 30 minute treatments at 46, 48 and 50 °C, respectively, but
this caused crop damage. HWT for 4 hours at 44 °C reduced the number of fungal
colonies (Penicillium sp., Fusarium oxysporum f.sp. narcissi and Mucor plumbeus)
recovered from bulbs, but the effects of formaldehyde, glutaraldehyde and
sodium hypochlorite in controlling these fungi were variable.
Despite these possibilities for dispensing with a disinfectant, it is generally
considered essential to add formaldehyde, which is effective at HWT temperatures
in killing free-swimming stem nematodes (nematodes that escape from the bulbs
into the tank dip and which are more resilient to high temperatures), as well as the
spores of the base rot fungus. Formaldehyde is usually added as 5 litres commercial
formalin per 1000 litres (0.2% a.i.). Increasing the concentration of formaldehyde
gives little extra benefit in fungicidal activity, but can result in crop toxicity,
with fewer and deformed flowers (Price and Briggs, 1976; Linfield, 1991), so any
recommendations to use higher rates (e.g., Higgins, 1999) should be treated with