Agriculture Reference
In-Depth Information
the field. The variability of these results reflect earlier reports in which DADS
or onion oil applied to infected fields lowered subsequent white rot incidence
by up to 75%, but in other trials they were not effective (Entwistle, 1990).
COMPOSTS AND CROP RESIDUES
Considerable volumes of onion waste are
generated from packhouses and food-processing factories. The knowledge that
white rot sclerotia are stimulated to germinate by flavour volatiles prompted
research into using such processing waste as a cheap soil amendment, which
contained such volatiles and could be used to destroy sclerotia.
Experiments established that sclerotia in onion waste are destroyed by 3
days at temperatures of 48°C and above, so this was the minimum treatment
needed to avoid spreading disease from infected waste (Coventry
et al.
, 2002).
Pot experiments have shown that sclerotial destruction does occur in soil
treated with such composts, but a high ratio of compost to soil is needed for
much effect. For example, compost produced in 7 days at 54°C caused more
than 60% destruction of sclerotia in a 50:50 soil:compost mix kept at 13°C for
6 months. Although the composted waste contained flavour volatiles, there
was evidence that the destruction of sclerotia by compost was due to other
factors in addition. Further experiments showed that composts from brassica
or carrot waste had a similar effect, although onion waste was the most
effective (Coventry
et al.
, 2005).
Fungicides
Fungicidal treatments for soil-borne diseases are difficult, because roots and
stem bases are inaccessible to chemicals. It is usually only at seed sowing, set or
bulb planting or seedling transplanting that fungicides can be placed where
they are going to be most effective. The broad-spectrum dithiocarbamate
protectant fungicide, thiram, combined with systemic, curative fungicides like
thiobendazole or carboxin, is routinely applied as film-coats to allium seeds
(Taylor
et al.
, 2001). This treatment reduces losses to the damping-off diseases.
Fungicidal dips can be applied to garlic cloves, onion sets and transplants and
can give protection against
Fusarium
basal rot (Cramer, 2000).
Selective systemic fungicides have proved effective for white rot control.
Currently, the systemic fungicide tebuconazole is very effective (Melero-Vara
et
al
., 2000; Wood
et al
., 2002; Clarkson
et al
., 2006). A seed-coating containing
5 g of tebuconazole/kg of seed reduced the incidence of white rot on bulb
onions harvested from a white rot-infected field from 47% for untreated
controls to 9% for the seed treatment (Clarkson
et al
., 2006).
Similarly, trials on garlic showed that dipping cloves in a tebucanazole
solution before planting reduced infection with white rot, and this treatment
plus four sprays of the fungicide solution aimed at the stem base increased bulb
yield by 33% and reduced the number of plant deaths from 17 to 1.5% (Melero-
Va r a
et al
., 2000). This treatment was as good as solarization in overcoming
white rot. As with many fungicides, there is some phytotoxic effect of
