Black mould (Aspergillus niger) commonly occurs on onions stored at temperatures above about 25°C, with an optimum at about 30°C, especially at > 80% RH (Hayden et al., 1994). This disease is the main cause of the rejection of onion bulbs (van Konijnenburg and Ardizzi, 1997) from hot production areas, such as Texas, Egypt, India and Sudan (Hayden and Maude, 1997). Regulation of the storage environment effectively controls black mould in the UK. However, in Sudan, for example, where suitable conditions for the growth of the pathogen occur in the field and in uncontrolled storage environments, it is difficult to limit the inoculum entering the store. The fungus is apparent on bulbs within a few days of storage. The symptoms are the abundant black conidia produced on and sometimes under the outer skins of onion bulbs in store (Hayden and Maude, 1997). The source of the inoculum is infected seed and soil. Seed treatments are partially effective. Reducing mechanical damage and wounding and using short-term high-temperature drying, followed by storage at < 80% RH, are the most effective treatments.
Preharvest treatment of bulbs with sulphur dioxide at 1% (v/v) for 72 h or heat treatment at 50°C for 3 h can reduce losses due to A. niger in store (Thamizharasi and Narasimham, 1992); sulphur dust treatment is an effective long-term storage control method (Chavan et al., 1992; Padule et al., 1996).
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