Agriculture Reference
In-Depth Information
identified a small-spored 16-chromosome form (type A1) and a larger-spored
32-chromosome form (type A11) of the fungus. These
B. aclada
subgroups (A1 and
A11) were genetically different from
B. byssoidea
(the cause of mycelial neck rot)
and significantly different from each other (Nielsen
et al.,
2001). From the use of
universally primed (UP) polymerase chain reaction (UP-PCR) DNA fingerprints, it
is suggested that subgroup A11 of
B. aclada
is a possible hybrid species between
B. byssoidea
and subgroup A1 of
B. aclada
(Nielsen and Yohalem, 2001). Both the
smallest-spored group (
B. aclada
) and the largest-spored group (
B. byssoidea
) each
have 16 chromosomes, while the intermediate group (
B. allii
) has 32 (Yohalem
et al.,
2003). This research shows that the three groups are genetically distinct and
that isolates of
B. aclada
and
B. byssoidea
were the possible ancestors of the
polyploid
B. allii
(Yohalem
et al.,
2003). Yohalem
et al.,
(2003) suggest that both
B. aclada
and
B
.
allii
are valid names and they propose that
B. aclada
is reserved for
the small-spored sub-group (A1) and
B. allii
for the larger-spored sub-group (A11)
of
B. aclada
.
In this chapter the name
B. aclada
(
sensu
Fresenius) has been used for both
subgroups (A1 and A11) replacing
B. allii
(
sensu
Munn).
Botrytis aclada
is the
asexual form (anamorph) of an ascomycete fungus. There is no known teliomorph
(sexual stage). The fungus is a necrotroph.
The disease only becomes evident when onions have been in store for eight or
more weeks. Typically the upper parts of affected bulbs are soft when pressed and
removal of the brown wrapper scales (leaves) reveals a black mass of sclerotia
enclosing the neck tissues. Individual sclerotia are up to 5 mm in diameter and a
grey mould sometimes extends below them on bulbs. The fungus invades
downwards in the food scales of bulbs and these have a brown 'cooked' appearance.
The pathogen became important in commercial onion bulb crops in the 1970s in
the UK through the use of imported diseased seed (Maude and Presly, 1977 a,b;
Maude, 1983). In a recent survey,
B. aclada
has been detected in many seed lots
harvested from bulb onion seed crops grown in the semi-arid Columbia basin of
Washington, US (du Toit
et al.,
2004). Preliminary molecular analysis of isolates of
B. aclada
collected during this survey may indicate that both
B. aclada
and
B. allii
(
sensu
Yohalem
et al.,
2003) were present.
(b) Epidemiology
The epidemiology of most foliar pathogens of onions can be deduced by relating the
symptoms produced at various stages of crop development to the environmental
conditions that prevailed immediately prior to symptom appearance. In this respect,
onion neck rot is different: the disease is virtually symptomless in the growing crop
and to record its spread, which occurs by the aerial dissemination of conidia, it is
necessary to incubate onion foliage to demonstrate the presence of conidiophores
and conidia of the fungus on those tissues. This reveals that there is a direct linear
relationship between incidence of seedborne infection, the number of infected plants
in onion crops and the number of neck rot infected bulbs in store (Maude and Presly,
1977a,b; Maude, 1983). During dry growing seasons, there is a virtual one-to-one
