Agriculture Reference
In-Depth Information
including alternative host plants, survival in soil and in crop debris, modes of
pathogen spread and how this is affected by weather. Disease management
utilizes such information to avoid or minimize initial bacterial disease inoculum
and to apply bactericides starting only when weather conditions make the
spread of disease likely, thereby minimizing the risk of bactericide-tolerant
strains of pathogen evolving.
Integrated disease management programmes to achieve these objectives
must include:
1.
Ensuring seed and sets are pathogen free. This is particularly important in
the era of global seed companies trading seeds between countries and
continents.
2.
Maintaining good crop hygiene by removing or burying diseased debris,
volunteer alliums and weeds on which the pathogens survive as epiphytes.
3.
Utilizing crop rotations of non-host crops with a minimum of 1 year
between allium crops. For example, small grain crops are recommended in
rotations to reduce
X. axonopodis
pv.
allii
.
4.
Separating vulnerable crops both in time and space so that there is not a
continuous 'infection cycle' from one allium crop to another.
5.
Avoiding excessive nitrogen fertilizer, which can result in luxuriant foliage
that is easily damaged by wind and cultivation traffic and which tends to
maintain a wet microclimate in the crop.
6.
Minimizing damage to plants from cultivations and by having good pest
control.
Conditions around harvest time can be critical in determining the level of
bulb rot in stored onions. A series of studies near Pukekoe in New Zealand has
shown that dry neck and leaf tissue is less prone to infection. The bacteria can
invade the neck and enter the bulb by infecting the green leaves or through
moist cut necks. The aim of harvesting systems should be to dry the onion
necks and foliage as quickly as possible, to cut the foliage from the necks when
they are as dry as possible and to ensure the necks dry quickly after cutting.
Forced ventilation with dry air can help to achieve this. It is important to avoid
wounding the tops of bulbs by cutting necks too low. Cutting without bulb
damage is easier the drier the necks (Wright and Triggs, 2005). Bacterial bulb
rots in store tend to be more serious following a wet harvesting season.
Resistant cultivars would be useful in disease control, but there are few
reports of resistance to bacterial disease. Wright and Grant (1998) surveyed a
wide range of allium species for resistance to leaf rot by strains of
Ps. viridiflava
and
Ps. marginalis
isolated from rotten onion bulbs. There were wide differences
in susceptibility but most vegetable species included susceptible types, although
there were highly resistant garlic and chive lines, as well as some resistant wild
alliums. Different authors have reported differences in the susceptibility of other
allium vegetables to
Ps. syringae
pv.
porri
virulent on leeks (Noble
et al.
, 2006).
This may be because of differences in techniques or it may reflect genuine
