Agriculture Reference
In-Depth Information
Such transmission relationships fit Van der Plank's equation (1963), which states:
the amount of initial inoculum
(x
o
)
, the average infection rate
(r)
and the time over
which infection occurs
(t)
determines the amount of disease that develops
(x)
and
therefore
x = x
o
e
rt
. In the example of
B. aclada
-infected onion seeds, the minimum
tolerance level would be based on reducing the initial inoculum (i.e. infected seeds)
(x
o
)
so that the disease threshold
(x
d
)
is not exceeded, even under conditions
favouring the average infection rate
(r)
.
The rate of transmission of seedborne onion pathogens is modified by a number
of factors, including fungal type (i.e. biotroph, necrotroph), severity of seed infection,
soil conditions (e.g. temperature and moisture) and soil microflora.
For example, the biotroph
Peronospora destructor
(cause of downy mildew of
onion) is commonly present in onion seed production fields, where it may invade
developing onion seeds. In common with many other downy mildews, its survival
on onion seeds is relatively short-lived and so there are opposing views on whether
transmission of the disease can occur from a seed source.
Other pathogens (mainly necrotrophic) are probably sufficiently persistent on or
in seeds to transmit and cause outbreaks of disease. Of these,
Botrytis aclada
(
B. allii
- cause of neck rot) is the main temperate seedborne pathogen of onions.
Aspergillus niger
(black mould) (Hayden and Maude, 1992) and
Alternaria porri
(cause of purple blotch) (Wu, 1979) are hot climate equivalents (Table 2). The
importance of infected seeds in the transmission of either of the latter fungi is
difficult to quantify, because field sources of infection exist (Pandotra, 1965;
Hayden
et al.,
1994a,b).
19.4 CASE HISTORIES: FOLIAR DISEASES
Many temperate (
Botrytis aclada, Botrytis squamosa, Peronospora destructor
, etc.)
and hot climate (
Alternaria porri, Glomerella cingulata, Aspergillus niger,
Cercospora duddiae
, etc.) pathogens, from a variety of sources, infect the foliage of
growing onion crops causing major outbreaks of disease. A number of epidemiological
factors constrain or facilitate leaf disease development, some of which are reviewed
here in relation to specific pathogens.
19.4.1
Botrytis aclada
(cause of onion neck rot)
(a) Pathogen and disease
Neck rot is the main storage disease of bulb onions in temperate parts of the world
(see Maude, 1990a). Munn identified the pathogen as
Botrytis allii
n.sp. causing
small sclerotial neck rot disease of onions in Michigan and New York State, USA
(Munn, 1917). This identification was confirmed by Walker (1925, 1926).
Hennebert (1963, 1973) following examination of herbarium material and the use of
adjusted spore measurements concluded that the fungus named
B.
aclada
by
Fresenius in 1850 (Fresenius, 1850) was the same as
B. allii
.
New research (light microscope - Shirane
et al.
(1989); DNA fingerprinting -
Nielsen
et al.,
2001; Nielsen and Yohalem, 2001; Yohalem
et al.,
2003) has
