STUDIES ON THE BIOLOGY OF BLACK MOLD (ASPERGILLUS-NIGER) ON TEMPERATEAND TROPICAL ONIONS .1. A COMPARISON OF SOURCES OF THE DISEASE IN TEMPERATE AND TROPICAL FIELD CROPS

Aspergillus niger colonies were present on 60-90% of the leaves sample d from onion crops grown under hot, tropical conditions (Shambat, Suda n), but only on 21% of leaves in crops grown under temperate condition s (Wellesbourne, UK). The seed stocks used in the Sudan were naturally contaminated with A. niger, whereas the fungus was absent from seed u sed in the UK. Inoculation of seed with A. niger resulted in an increa se in the incidence of the fungus on onion plants (mean 83 % of plants with affected leaves) in the field and on bulbs in store in crops gro wn in the UK. In the UK, A, niger is infrequent in the soil and air, b ut it is a major component of the soil and air mycoflora in the Sudan. Most of the bulbs grown in Sudan were contaminated with A. niger at h arvest because of the high incidence of the fungus in the field.