Agriculture Reference
In-Depth Information
in a single season or over the span of a year where green, pickling, ware or seed
production crops link the seasons into a continuous cycle of onions (Maude, 1990b).
The disease is more prevalent on densely produced crops (e.g. green and pickling
onions) in the UK but it is very destructive in most onion crops in temperate parts of
the world (Lorbeer, 1992).
Primary inoculum from the sources described above and secondary inoculum
produced as conidia on the leaves of infected crop plants, spore dispersal and
survival, infection and colonisation by and the sporulation capacity of
B. squamosa
are basic requirements for the development of leaf blight in onion crops (Sutton
et al.,
1986).
(b) Epidemiology
The significance of a primary inoculum source such as sclerotia in initiating disease
outbreaks depends on the ability of such a source to produce conidia and this is
regulated by temperature and soil water potential (Clarkson
et al.,
2000). The
greatest number of conidia per sclerotium in the field were produced at temperatures
of 5 to 10
o
C and reduction in soil water potential reduced the number of conidia
produced. However, conidia were produced at water potentials as low as -2 Mpa, at
which sclerotial germination was at least 60% (Clarkson
et al.,
2000). Water
potential was more likely to be the limiting factor for conidiogenic germination
between April and September than between October and March, as temperatures
were more limiting in these winter months in the UK (Clarkson
et al.,
2000).
Disease development on onion foliage
is dependent on the positive action and
interaction of a number of epidemiological factors. For example,
B. squamosa
produces conidia at night on the necrotic tips of onion leaves when mean
temperatures during a wet period are between 8 and 22
o
C. Formation of conidia is
greatest when the duration of wetness is more than 12 h and least when there is
wetness for 5 h or less. Conidial release is favoured by decreasing or increasing RH
and by rain; once spores are detected, release occurs on a daily basis (Shoemaker
and Lorbeer, 1977; Sutton
et al.,
1978). Dispersal follows a diurnal pattern, which
peaks between 0900 to 1200h in Canada (Sutton
et al.,
1978).
When conidia land on onion leaves, temperature and leaf wetness duration are the
main variables influencing infection. Although conidia may germinate over a wide
temperature range (6 to 33
o
C), lesions develop optimally at 20
o
C and are reduced at
15 and 25
o
C; lesion numbers increase with increasing leaf wetness duration up to
48 h (Alderman and Lacy, 1983; Lorbeer, 1992). Leaf penetration is direct and
localised leaf spot lesions develop; these may expand leading to blighting when leaf
wetness is prolonged (Alderman and Lacy, 1983). The greatest production of conidia
occurs on necrotic parts of leaves and leaf tips and not on the leaf spot lesions that
represent a host hypersensitivity reaction to a weak pathogen (Lorbeer, 1992).
Routine applications of fungicides are used globally for the control of onion leaf
blight but their number can be reduced by the use of disease forecasting.
The first disease forecasting system known as BOTCAST was developed in
Canada (Sutton
et al.,
1986). Between 1988 and 1992 two forecasting models,
