Management of aspergillus flavus link infections ingroundnut using bacterial endophytes as biological control agents.
Loading...
Date
2023
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
Microorganisms deteriorate groundnut seeds pre- and post-harvest. One of these microorganisms
includes a fungus known as Aspergillus flavus Link. This pathogen reduces seed germination rate, see
total oil content, carbohydrates, and proteins. Aspergillus flavus is a mycotoxigenic fungus that
produces aflatoxins as secondary metabolites. As a result, seeds infected or colonized by A. flavus are
considered not biologically unsafe for consumption and processing. Current management strategies
have only provided temporary relief and increased susceptibility to the pathogen. Therefore, the use of
environmentally friendly strategies is important. This study aimed to evaluate the efficacy of potential
endophytic bacterial strains isolated from seeds, roots, leaves and stems of beans (Phaseolus vulgaris
L.), peas (Pisum sativum L.), and groundnuts (Arachis hypogaea L.) in controlling A. flavus during
storage and greenhouse trials.
During the in vitro studies, 106 endophytic bacterial isolates were isolated and screened against A.
flavus using the disc diffusion method. The best 10 isolates were selected for secondary screening
against A. flavus in the dual culture assay. Only 13 (12%) of the isolates inhibited the growth of A.
flavus in vitro after nine days of incubation. The best isolates from the screening trials include Isolate
ALA (55.2%), followed by Isolates KI (40.9%) and KG (40.2%) inhibitions, respectively. Isolate ALA
was obtained from Phaseolus vulgaris L. (dry bean) leaves, while Isolates KI and KG were isolated
from groundnut stems and leaves. From the secondary screening, the best five isolates were sent to
Inqaba Biotech Industries (Pty) Ltd for molecular characterization and identification at the species
level. Three of the isolates KI, KG, and BB that were gram-positive were identified as either Bacillus
siamensis, Bacillus velezensis or Bacillus amyloliquefaciens. Isolate BB also had similarities to B.
subtilis. Isolate GNLA and ALA were identified as Pantoea dispersa and Pseudomonas fluorescens,
respectively. Three of these strains (Bacillus siamensis/ Bacillus velezensis/Bacillus subtilis/Bacillus
amyloliquefaciens strains BB, KG, and KI) were then used in in vivo experiments to assess their
efficacy in controlling A. flavus on two groundnut cultivars (Akwa and Sellie-Plus) during a storage
trial. In a short-term trial (7-day period), culture filtrate of B. amyloliquefaciens/B. siamensis/ B.
velezensis strain KG showed potential as a biological control agent against A. flavus. An average
percentage infection of 38.2% on Cultivar Akwa and 33.0% and Cultivar Sellie-Plus were observed.
In contrast, the rest of the treatments (bacterial cells and crude lipopeptide extract) provided the least
control. In the medium-storage trials (21-day period), the best endophytic bacterial treatment was the
acidic cultured filtrate of B. amyloliquefaciens B. siamensis/ B. velezensis strain KG, which provided
constant control for 21 days. The interaction of these isolates with A. flavus was studied using Scanning
Electron Microscopy (SEM).
In the greenhouse trials, three treatments were evaluated. These were (i) B. amyloliquefaciens/B.
siamensis/ B. velezensis strain KG was used as a seed treatment, (ii) P. fluorescens strain ALA as a
foliar spray treatment, and (iii) the combination of both seed treatment and foliar spray to control A.
flavus during the greenhouse trial using the two groundnut cultivar Akwa and Sellie-Plus. Both
cultivars performed somewhat differently for each treatment. The highest number of seeds and seed
weight was recorded for the seed treatment for Sellie-Plus. In contrast, the combination of seed and
foliar spray treatment recorded the highest number of seeds and seed weight for Cultivar Akwa.
However, the combination of seed and foliar spray treatment showed potential as the best treatment
against A. flavus with an average percentage disease incidence of 54.8% and 46.8% for Akwa and
Sellie-Plus respectively.
In conclusion, the endophytic bacterial strains demonstrated potential in controlling A. flavus infection
in groundnut seeds. Adjusting the pH of cultured filtrate provided a constant and enhanced control
against the pathogen. Furthermore, the combination of seed and foliar spray treatment using the two
bacterial strains showed potential as a biological control agent against A. flavus.
Description
Masters Degree. University of KwaZulu-Natal, Pietermaritzburg.