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In addition, the mycofloral balance of Aspergillus spp. on a single sample of onion
seeds can alter depending on the recovery temperature that is used. Thus, A. niger
was recovered from naturally infected seeds over an incubation temperature range of
20 to 35 o C but at 40 and 45 o C, it was replaced by A. fumigatus from the same seed
sample - presumably because this fungus has a higher growth optimum (see Fig.
19.1) and was more competitive at those temperatures (Hayden and Maude, 1994).
(b) Transmission and transmission rate
Transmission and transmission rate are factors of epidemiological importance in the
initiation of spread of disease. Consequential transmission of many seedborne
organisms, and of seedborne fungal pathogens of onions in particular, requires that
the incidence of infected seeds (i.e. the inoculum threshold) is sufficient to transfer
infection to a proportion of the emerging seedlings causing disease outbreaks of
economic proportions. Inoculum thresholds have been determined for many
seedborne organisms (Maude, 1996). They are of critical importance where infected
seeds are the main source of crop disease.
The epidemiological consequences of sowing onions with a threshold level of
1% B. aclada -infected seeds are such that, under wet summer conditions, there is
considerable spread of disease in the growing crop, resulting in up to 10% of bulbs
with neck rot in store (Fig. 19.2). This is the maximum incidence of diseased bulbs
that can be handled in commercial grading lines and seed samples with 1% or more
infected seeds should be discarded or should be treated before sowing.
Figure 19.2. Relationship between Botrytis allii - infected onion seeds and post-harvest neck rot
of bulb onions:
, dry summer (1973);
, wet summer (1974) (redrawn from Maude, 1983).
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