Studies Of The Infection Of Cultivars Of Shallot (Allium Ascalonicum L.) In Ghana By Aspergillus Niger Van Tieghem And The Effect Of The Host On Some Aspects Of The Biology Of A. Niger

ABSTRACT The relationship between five randomly selected isolates of Aspergillus niger from naturally infected bulbs of shallot and two shallot cultivars (Allium ascalonicum L.) was investigated. The two cultivars were “Pale-brown” and “Pink”- named for the colours of their scale leaves. A. niger causes bulb rot of shallots which have important uses in food and traditional medicine in Ghana The relationship showed by the cultivars were similar in some aspects and different in many other aspects, indicating physiological differences between the cultivars. In the field, the Pale-brown plants were bigger (mean length of 3.62±0.06 cm and mean width of 2.71±0.05 cm) with greener and more lush leaves. Chemical analysis of the typically larger bulbs of Pale-brown cultivar showed higher mean concentrations of calcium (0.164%), magnesium (0.054%), nitrogen (1.456%), phosphorus (0.406%), potassium (1.142%) and sodium (0.179%). In comparison, the respective percentage values for the Pink cultivar were 0.145, 0.046, 1.192, 0.367, 0.979 and 0.175. The mean length and the mean width of the Pink bulbs were also 3.26±0.08 cm and 2.08±0.08 cm respectively. Any nutrients secreted by the scale leaves and swollen leaf bases were not able to stimulate germination of conidia of A. niger in distilled water droplets placed on them. The conidia did not also germinate in very dilute (1:40) droplets of Potato Dextrose Broth. Higher concentrations of Potato Dextrose Broth, galactose, glucose and sucrose induced appreciable levels of conidium germination. When that happened, the different types of leaves of the bulbs showed differences in their influence on the conidia. Conidium germination was better on the scale leaves than on the swollen leaf bases while percentage germination on the corresponding organs was better and germ tubes were longer on the Pale-brown cultivar than on the Pink cultivar. The mycelium of A. niger, in addition, grew better in extract of bulbs of the Pale-brown cultivar and rotted bulbs of the wound-inoculated Pale-brown cultivar faster than the Pink bulbs. Conidiophores produced in various tests were larger on Pale-brown bulbs than on Pink bulbs. Conidia formed by A. niger growing on ground bulbs of the two cultivars showed some similarities and differences when they were stored in atmospheres of different relative humidities, from zero to 100% R.H., provided and maintained with appropriate concentrations of sulphuric acid. The two sets of conidia lived longest at 60 to 100% R.H. and died quickest at zero to 50% R.H. At the best survival humidity of 100% R.H., 52.1, 51.2 and 35.8% of conidia of A. niger Isolates 1, 3 and 5 respectively formed on the Pale-brown cultivar medium were still alive on the 200th day. The corresponding values for conidia formed on Pink cultivar medium were 43.8, 42.3 and 32.3 per cent, respectively. At each percentage viability assessment, conidia formed on the Pale-brown cultivar medium showed higher survival potential. There was positive correlation between spore longevity and the length of the germtubes. The two shallot cultivars clearly stimulated fungi in the neighbourhood of their bulbs and roots over 60 days. Studies using the soil dilution plate method identified Aspergillus as the dominant genus in the phyllosphere, rhizosphere and non-rhizosphere soils. The remaining genera were Fusarium, Penicillium and Rhizopus. The Aspergilli were A. flavus, A. niger, and A. terreus. A. niger was the most abundant, constituting more than half the fungus population. Application of xxix Urea and Potassium fertilizers separately to the soil did not alter the pattern of the influence of the shallot cultivars on the fungal populations. The pattern was also the same in different farm soils under continuous cultivation with different crops. Because of the restraining effects of various sorts of the Pink cultivar on Aspergillus niger, it was considered to be the better cultivar for expanded shallot cultivation in Ghana and suitable for breeding improved stocks to reduce the threat of A. niger to the shallot industry.