Narcissus and Daffodil

Production of Narcissus bulbs 77
of phytotoxicity (Wallis, 1966, 1967a). In trials with carbendazim, the addition of
compounds believed to increase fungicide uptake (dimethyl sulphoxide, indolylacetic
acid and hydrochloric acid) showed no benefit to disease control (ADAS,
1976). The generation of foam in HWT tanks, which could reduce pesticidal effectiveness,
may be reduced by the addition of an anti-foam preparation, which has no
adverse effect on the crop (Tompsett, 1977). Where problems in handling bulbs
are likely due to the applied pesticides, an anti-dust ‘sticker’ may be added, as is
practised in the Netherlands (Anon., 1987).
Some pesticide labels state that bulb dip solutions should be freshly made up
each time, but this is impractical. It is usual to top up bulb dip tanks and to re-use
them for as long as practical, providing gross contamination with debris (bulb
skins, soil, etc.) can be avoided. After dipping, tanks are topped up with water and
the appropriate amounts of disinfectant, pesticide, etc., are added so that the
top-up is given at the original strength. Dutch recommendations to top-up formaldehyde
at twice the original strength have now been modified, and 0.75% commercial
formalin is now recommended (van der Weijden, 2000). For pesticides,
manufacturers’ recommendations should be consulted, although these are often
not specific. The thiabendazole fungicide ‘Storite Clear Liquid’ is an exception, as
definite top-up procedures are given. As active ingredients can be lost in use, it is
advisable to have the concentration of formaldehyde and pesticides determined by
specialist laboratories for sets of samples, at least until experience of their behaviour
is obtained, and particularly as little published guidance exists on the stability
of pesticides in bulb dips. Recent Dutch recommendations suggest using slightly
higher top-up rates for prochloraz, but using other pesticides at the original
strength (van der Weijden, 2000).
Treatments to reduce damage due to HWT
HWT damage can be reduced by warm-storing bulbs for 1 week at 30 °C before
HWT, a procedure used in south-west England to reduce damage to flowers in the
year after HWT (Wood, 1944; Slootweg, 1962; Tompsett, 1975). Warm-storage is
always recommended when treating sensitive narcissus such as Poeticus cultivars,
and is beneficial in cultivar ‘Tête-à-Tête’ (ADAS, 1988b). The exact treatment is
not critical, 3 to 8 days at 30 to 35 °C having been used (Rees and Turquand, 1967;
Turquand and Rees, 1968). Such warm-storage, however, by partly desiccating
the bulbs, induces stem nematode to migrate to the outside of the bulbs, often
near the base plate, where they form ‘nematode wool’ which can escape into the
bulb dip during HWT. Warm-storage is, therefore, used in conjunction with presoaking
bulbs (for 3 or 4 hours or, preferably overnight, at ambient temperatures
with formaldehyde) immediately before HWT, in order to hydrate the ‘wool’.
With this regime a higher HWT temperature, 46 or 47 °C, is necessary to kill stem
nematodes. It is important not to omit pre-soaking when pre-warming is used.
Warm-storage in itself may decrease crop vigor (Wallis, 1965, 1967a). Warm
storage appears to induce dormancy in the shoot initials, making them less sensitive
to damage by high temperatures, and thus extends the period over which
HWT can be safely used (Wallis, 1967c). The lower limit of the warm-storage effect
is about 18 °C. If bulbs are stored at 18 °C for 2 weeks before HWT (where storage
may otherwise have been at slightly lower ambient temperatures), HWT damage