Agriculture Reference
In-Depth Information
pink root control in Idaho, USA, and a 3 or more-year gap is recommended for
Fusarium
basal rot control in Colorado, Tasmania and the Philippines. The
pathogens responsible for pink root,
Fusarium
basal rot and southern blight all
have a wide host range, and non-susceptible crops should be rotated with
onions. However, some soil-borne pathogens have extremely long-lived resting
bodies - in particular, the sclerotia of white rot, so that control by rotation is
not an option.
The development of disease is dependent on temperature (see Table 5.6),
and it is sometimes possible to avoid root disease by growing allium crops at the
times of year when soil temperatures do not favour disease. For example, in
Israel early onion cultivars produce mature bulbs before soil temperatures rise
above 25°C and favour pink root, southern blight or
Fusarium
basal rot. In
Louisiana, USA, shallots planted in August, rather than the usual September-
October, can be harvested in November, before soil temperatures decrease to
the range at which white rot becomes a problem. In the UK, overwintered salad
onions can be harvested in April, before soil temperatures rise sufficiently for
white rot outbreaks.
With the damping-off diseases, infection risk is minimized by shortening
the time that plants are in the vulnerable seedling stage. This can be achieved
by sowing when temperatures favour rapid emergence and growth, and by
providing good seedbed conditions.
Resistant cultivars
There is a long history of breeding onions for resistance to pink root in the south-
eastern USA, and cultivars with a fair degree of tolerance have long been grown
there. Resistance appears to involve both the capacity of roots to resist infection
and the capacity to produce new roots after infection (Entwistle, 1990). True
resistance needs to be distinguished from situations where early maturity allows
a cultivar to avoid temperatures conducive to the disease. The situation is similar
for resistance of onions to
Fusarium
basal rot (FBR): resistance to the disease is not
absolute, but losses are greatly reduced. Numerous techniques of screening for
resistance have been developed, including seedling tests. Resistance to FBR was
found to be highly correlated with resistance to pink root in one population
(Cramer, 2000). There remain some regions where photothermally adapted
cultivars resistant to pink root and FBR still need to be developed. Despite
considerable research efforts worldwide, reliable resistance to white rot has not
yet been identified (Hovius
et al.
, 2005).
Destruction of propagules
There is often a correlation between the population density of pathogen
propagules in soil, which is termed the inoculum density, and the severity of
disease. There are a number of measures which reduce inoculum density -
crop rotation discussed above being one - and others are now considered.
