ONION DISEASES - The Epidemiology of Plant Diseases - page 494

relationship between percentage infected seeds and percentage infected bulbs in
store, but in wet humid seasons the storage disease levels are proportionately greater
(Fig. 19.2) (Maude, 1983).
Stewart and Franicevic (1994) obtained similar seed/bulb relationships with
higher seed infections resulting in greater levels of bulb rot in store in New Zealand.
Tylkowska and Dorna (2001) in Poland reported a stronger seed/bulb neck rot
relationship from infected versus contaminated seed. There was no apparent
relationship between plant infection and B. aclada infection of the harvested seed
from infected bulb onion seed crops in Washington, USA (du Toit et al., 2004).
The latency of this disease in onion crops increases the difficulty of establishing
the critical epidemiological parameters. The most consistent correlation over a
period of seven years in the UK was the relationship between the log % neck rot and
RH in the month of June; relative humidities over 85% then were correlated ( r =
0.97; P < 0.05) with high levels of neck rot in stored onions six to seven months
later (Maude et al., 1985).
Thus, rainfall as evidenced by high RH appears to be important in determining
levels of infection in field crops and consequently of neck rot in stored onion bulbs.
The effects if any, of the crop microflora and mycoflora on Botrytis aclada on/in
onion leaf tissues may have a further influence on the epidemiology of the disease.
Laboratory experiments on dead onion leaf pieces held under controlled
moist/wet conditions (Köhl et al., 1995; Köhl et al., 1997; Nielsen et al., 2000)
demonstrated that there were a number of mycofloral antagonists that might
adversely affect the sporulation of B. aclada . Their modes of action include:
antifungal effects causing hyphal plasmolysis, antibiosis, and exclusion by rapid
colonisation of necrotic leaf tissues (Köhl et al., 1997). Parasitism was not observed
(Köhl et al., 1997). Their role in the onion crop is not known but there may be a
potential use for some of them as biocontrol fungi.
19.4.2 Botrytis squamosa (cause of leaf rot, leaf blight, blast)
(a) Pathogen and disease
Botrytis squamosa is the asexual state of the ascomycete Sclerotinia squamosa
(teliomorph) and both asexual spores (conidia) and sexual spores (ascospores) may
be important sources of infection for this necrotroph.
This fungus differs from B. aclada in that it produces symptoms appearing as
greyish white leaf lesions (spots) which are circular to elliptical in shape, 1 to 10
mm long by 1 to 4 mm wide (Hancock and Lorbeer, 1963). Severe infection leads to
dieback of onion leaf-tips and sporulation occurs only on this dead tissue (Lorbeer,
1992). The incidence of leaf spots and their density on the leaf surface has been used
to monitor the progress of disease outbreaks.
Sources of the pathogen in the United States are represented by conidia from
onion debris and cull piles in seed production fields, and ascospores from apothecia
produced from sclerotia, or conidia from sclerotia present on the surface of soil
which had previously borne onions (Ellerbrock and Lorbeer, 1977; Lorbeer, 1992;
Clarkson et al., 2000). Inoculum bridging also occurs in successionally sown onions