Abstract:
The study evaluated the effects of soil organic matter amendments on temporal dynamics of
populations of A. flavus and antagonistic microbes, and on aflatoxin contamination in
groundnut under field conditions in Malawi. The study also investigated the implications of
rainfall on populations of the microbes. Treatments included commercial organic fertilizer
applied at the rates of 0.2, 0.4 and 0.6 t ha-1; and farmyard manure at rates of 2.5, 5.0 and 7.5
t ha-1, while an untreated plot was used as a control. The short-duration groundnut variety
JL24 was used as the experimental material. The experiment was laid out in a randomized
complete block design with three replications at Chitedze Agricultural Research Station in
Malawi during the 2018/2019 main cropping season. Soil samples were collected at monthly
intervals from date of planting of groundnut and were analyzed using plate count method on
selective media for colony forming units of A. flavus, Trichoderma spp. and P. fluorescens to
monitor their populations per treatment over time. Aflatoxin B1 content in groundnut samples
from all plots was detected using an ELISA protocol. Dried and shelled groundnut samples
were weighed for estimation of yield. There were no significant effects of treatments on
population. However, populations of A. flavus and Trichoderma spp. decreased with an
increase in population of P. fluorescens over time, suggesting an antagonistic effect of P.
fluorescens against both A. flavus, and Trichoderma spp. There was a positive correlation
between rainfall and populations of A. flavus, and Trichoderma spp. The highest application
rates of both COF and FYM gave lower median AFB1 concentration (<10 ppb) than the lower
rates which gave AFB1 concentrations above 10 ppb. The control treatment gave a lower
median AFB1 concentration (12.87 ppb) than the two lower rates of COF (> 30 ppb) and FYM
(≤ 15.36 ppb). Overall, FYM treatments gave lower median AFB1 concentrations compared to
plots treated with COF. There were no significant effects of treatments on yield. The highest
rates of COF and FYM can be recommended to reduce aflatoxin contamination. However,
further study is recommended where toxigenicity and antagonistic potential of A. flavus and
its antagonists respectively, as well as pre-treatment soil aflatoxin concentration will be
assessed.
