Narcissus and Daffodil

76 G.R. Hanks
caution. Higher rates of formalin can also damage the base plates of bulbs, producing
corky areas, especially when HWT is being carried out early or soon after
lifting (Briggs, 1988), and in dwarf cultivars and others which form early, fine
roots and which are susceptible to formaldehyde damage (Vreeburg, 1984b; van
der Weijden, 1989). However, formaldehyde continues to be the material of
choice for most growers because of its effectiveness, cheapness and availability.
The use of formaldehyde is now coming under scrutiny due to health issues (Zell,
1984), and it may be unavailable in some localities. Linfield (1991) evaluated
alternative disinfectants for killing the chlamydospores of the base rot fungus
under HWT conditions: commercial preparations of glutaraldehyde, hydrogen
peroxide–peracetic acid and thiabendazole were highly effective and non-phytotoxic.
A disinfectant based on peroxyacetic acid (peracetic acid) was effective in killing
stem nematodes (both free-swimming and in the ‘wool’ stage) and chlamydospores
of the base rot pathogen at HWT temperatures (Hanks and Linfield, 1997, 1999).
In this study, peroxyacetic acid killed the chlamydospores of the base rot fungus
within 1 hour, whereas total kill was not achieved with formaldehyde even after 4
hours. Used in HWT, the peroxyacetic acid-based disinfectant was not phytotoxic
to bulbs. Glutaraldehyde was shown to be effective against stem nematodes at HWT
temperatures (M.J. Lole, personal communication). Alternatives such as bleach
and chlorine dioxide are being evaluated by Chastagner (1999).
Fungicides and insecticides may be added to HWT tanks along with formaldehyde.
In the UK, thiabendazole is the material of choice for controlling the base rot
fungus and other pathogens such as Penicillium. Thiabendazole, although used in
agriculture and horticulture as a fungicide, was originally introduced to farming as
an anthelminthic, and it appears to have useful activity against stem nematode in
HWT (Hanks and Linfield, 1999). Because thiabendazole is more soluble at very
acidic pH values, and, like many pesticides, is broken down by alkaline hydrolysis,
for bulb dips it is used in an acidic formulation (‘Storite Clear Liquid’ in the UK).
To further reduce the pH value of the dip an acidifier, sodium hydrogen sulphate
(sodium bisulphate), is sometimes added to HWT tanks by growers on the basis of
anecdotal evidence, and this is now being investigated experimentally (Hanks,
1999). For stocks severely affected by base rot, thiabendazole is used in HWT even
when it has been applied post-lifting, this double treatment providing the greatest
initial reduction in the number of diseased bulbs (Hanks, 1996b). Prochloraz-based
fungicides are also used, while a number of other fungicides have been found to
be effective, including thiram, benomyl and carbendazim (ADAS, 1974, 1976;
de Rooy, 1975). Fungicides used in the Netherlands include zineb/maneb, captan,
benomyl, carbendazim and Topsin M, sometimes used in a mixture, especially for
dwarf cultivars (Anon., 1987). Recent recommendations are for a combination of
formaldehyde, captan, prochloraz and carbendazim (van der Weijden, 2000). Lower
concentrations are used for ex-forced bulbs, because of the extra uptake by the
drier bulbs.
The insecticide chlorpyrifos may be added to HWT tanks to prevent subsequent
infection of bulbs by the larvae of the large narcissus fly, although this gives
protection only in the first year (Tones and Tompsett, 1990; Hanks and Linfield,
1997). Cold dips or application at planting were less effective.
It is usual to add a non-ionic wetter to HWT tanks to enhance the effects of
pesticides, and other types of wetters should be avoided because of the possibility