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Black Mould of Onions, Garlic and Shallot
The fungus may appear as a black, sooty mould on the outer dry scales of the mature/harvested bulb if relative humidity is 80% or above. If the dry outer scales are removed, this sooty mould may be observed growing along the 'veins' of the fleshy bulb tissue. The neck of infected bulbs may be discoloured, and a cross-section of such bulbs reveals ingress of the fungus into the fleshy bulb tissue. This and any other damaged areas show water soaking. An initial dry rot may develop into a wet rot, especially if the primary fungal infection is aggravated by bacterial infection. Alternatively, the infection may remain dry. Shrivelled and mummified bulbs may result.
Aspergillus Rot of Yam
The flesh develops firm, fawn or brown lesions sometimes tinged with purple. Secondary infection by soft-rot bacteria may occur.
Collar or Crown Rot of Groundnut
A. niger is often a component of the mycoflora of groundnuts and may invade the hypocotyl and radicle of germinating seedlings at temperatures greater than 30°C, causing collar or crown rot. Initially, the hypocotyl becomes watersoaked then light brown, after which the black sporulating fungus may become apparent.
Aspergillus Ear Rot of Maize
The fungus may develop on the ears of maize, the sooty mould eventually covering the whole ear. Subsequent seed germination is reduced and seeds are discoloured and shrivelled.
Storage Rot of Soyabean
Usually a mixture of species including A. niger which can cause discoloration, caking, mustiness and heating.
Aspergillus Rot of Harvested Fruits
The fungus may invade harvested fruits, especially after wounding or damage and under warm, humid conditions. Considerable soft rotting of fruit such as tomatoes and mangoes may occur due to the production of degrading enzymes, including cellulases, pectin lyase and polygalacturonase, by A. niger. In some fruits, e.g. lemon, the toxins produced after fungal invasion are considered a health hazard. Watery rot of bananas is caused by a mixture of fungi (including A. niger) and bacteria which produce cellulolytic enzymes. A. niger wet rot of guava is associated with high temperatures (23-33°C). The disease may cause calyx end rot of grapes and dates, fig smut (Doster et al., 1996), bunch mould of grapes and rot of pomegranates and peaches. In melon, rots develop as initially soft lesions which become water soaked, yellow at first and later covered with sooty black spores.
Aspergillus Soft Rot of Cocoyam
Two fungi, one of which was A. niger, were consistently associated with soft rot of corms of cocoyams (Colocasia esculenta) in Nigeria.
Stem and Rhizome Rots of Sansevieria spp. and Dracaena sanderana
Affected tissue of the stem ends and rhizomes becomes watersoaked and then sunken. Tissue underlying the epidermis rapidly disintegrates. Lesions with sporulating hyphae may form near leaf bases and stem ends.
Resistant Crop Cultivars
Onion varieties with red scales tend to be more resistant than those with white scales. This was thought to be due to the fact that the onion varieties with pigmented outer scales contain catechol and protocatechuic acid, which are phenolic compounds and which diffuse out from the dead red scales and prevent spore germination (Walker and Murphy, 1934; Walker and Stahmann, 1955). However, it should also be noted that A. niger can metabolize these substances and it is likely that other constituent compounds are involved in resistance to black mould. Ozer et al. (1999a) found differential accumulation of antifungal compounds during infection of germinating onion seeds. Whereas onion varieties vary in their degree of susceptibility to A. niger, and red onions are often found to be less susceptible, the link between skin colour and disease resistance is not straightforward. Varietal resistance to collar rot has been found in groundnut germplasm (Bhatia and Gangopadhyay, 1996; Dasgupta and Raj, 1997).
Chemical Control
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any
specific chemical control recommendations. For further information, we recommend you visit the following resources:
A. niger is most commonly observed on stored dry-bulb onions, especially when these have been produced in areas with hot, dry climates or grown under temperate conditions, but dried using warm air. Black mould has been reported as a major cause of rot of stored yellow onions in Texas, USA between 1974 and 1976, where it caused losses of 22-76% in onions stored for 3 months at 26.7°C and 70% RH (Miller and Dillon, 1979). In Japan, losses of 33% in 1979 and 61% in 1980 were recorded for onions stored over the summer (Tanaka and Nonaka, 1981). Black mould has been reported on 10% of the total dry onion shipments inspected in the New York market during 1972-84 (Ceponis et al., 1986). The disease has also been reported as the major component of the mycoflora of stored bulbs in the Sudan, where it has been reported that up to 80% of stored bulbs of varieties Wad Ramli (red) and Dongla (white) may be affected with the fungus (Musa et al., 1973).
