Impact of Aspergillus flavus on groundnut, Arachis hypogaea
Ntambo Mbuya Sylvain
Faculty of Agriculture and Natural Resources
Africa University
P.O Box: 1320 Mutare, Zimbabwe
Email: sntambo2009@gmail.com
Abstract Good quality seed has reasonable varietal and physical purity, a high germination percentage and is free of external and internal pathogenic organism. An experiment to study the impact of Aspergillus flavus on groundnut seeds was conducted in the laboratory using the blotter method. Groundnut seeds sample from Mutsamba were provided and four petri dishes were prepared containing each ten seeds placed on two to three sheets of wet (but not dripping with water) blotter papers. The petri dishes were incubated in the laboratory for two weeks. The results had shown that Aspergillus flavus was the seed borne fungi detected affecting the germination of seeds with 23.3%. The identification of this fungus was through use of a stereo microscope for habit characters identification and a compound microscope for conidia identification of the fungus.
Keywords: Impact of Aspergillus flavus, groundnut seeds, stereo microscope, compound microscope.
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Introduction
Groundnut is one of the most nourishing foods and it contains five important nutrients such as food energy, protein, phosphorus, thiamin and niacin. However, the crop is affected by a number of fungus diseases among which Aspergillus flavus is a part (Singh and Oswalt, 1992). Aspergillus flavus grows by producing thread like branching filaments known as hyphae. A network of hyphae known as the mycelium secretes enzymes that break down complex food sources. The resulting small molecules are absorbed by the mycelium to fuel additional fungal growth. The unaided eye cannot see individual hyphae, but dense mats of mycelium with conidia (asexual spores) often can be seen (Agrios, 1988). Aspergillus flavus causes deterioration of groundnut seeds, and infection can occur while hosts are still in the field (pre-harvest), but often show no symptoms (dormancy) until post-harvest storage and/ or transport. It produces aflatoxins that render the seed unacceptable due to high toxicity of human or animal consumption (Singh and Oswalt, 1992).
Materials and Methods
Groundnut seed sample from Mutsamba were provided in the laboratory and four petri dishes were prepared containing each teen seeds placed on two to three sheets of wet (but not dripping with water) blotter papers. A pair of forceps was used for seeds carrying from the plastic jars to the petri dishes to avoid contaminating the specimens. The petri dishes were incubated in the laboratory for two weeks using blotter paper method. Then after, the specimens were taken in the laboratory where the fungus was identified at two levels. A stereo microscope was used to identify habit characters (gross morphology of the fungus), and a compound microscope where a mass of fungus was placed on a slide with a drop of water to identify the conidia of the fungus.
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Then after observing the conidia of the fungus under a compound microscope, Common Laboratory Seed Health Testing Methods for Detecting Fungi by Mathur and Kongsdale (2000) was used for the identification of the actual shape and name of the fungus as viewed under compound microscope.
Results
The actual shape of the fungus identified from the Common Laboratory Seed Health Testing Methods for Detecting Fungi by Marthur and Kongsdale (2000) on the affected groundnut seeds that did not germinate was Aspergillus flavus with 23.3%. Table I. Results of Aspergillus flavus on groundnut, Arachis hypogaea
Dish No Fungus Aspergillus flavus
1
2
3
4
Percentages
3
-
4
-
23.3%
Discussion
Plant pathogenic fungi survive in soil, seed, weeds and they can be dispersed by insects, wind water and animals. Aspergillus flavus, is commonly found in the seed of both rotten and apparently healthy pods of groundnuts. Many strains of this fungus are capable of producing aflatoxins that render the seed unacceptable due to high toxicity of human or animal consumption (Singh and Oswalt, 1992). Aflatoxin contamination in groundnuts can occur in the stems of seedlings, pods, and seeds. The Aflatoxins produced by the Aspergillus flavus are identified as B1, B2, G1 and G2. However, aflatoxin B1 is usually predominant and is the most
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toxic of the four categories. In 1997, a European Union regulation (European commission, 1997) first set a uniform standard for aflatoxins at a maximum acceptable level of 10 parts per billion (ppb) in groundnuts subject to further processing and at 4 parts per billion in groundnuts intended for direct human consumption (European, commission, 1997). In 1998, the European regulation was then amended (European commission, 1997) by relaxing the total aflatoxin standard in groundnuts subject to further processing at 15 ppb (8 ppb for aflatoxin B1), and at 4 ppb (2 ppb for aflatoxin B1) in groundnuts intended for direct human consumption. The stringency standard varies across countries. France and Denmark, for instance set the standard for aflatoxin B1 at 1 part per billion (ppb), whereas Portugal had its standard at 25 parts per billion (ppb) (European Commission, 1997). The maximum aflatoxin level for groundnut acceptable in the United State of America is 20 parts per billion (ppb) (Singh and Oswalt, 1992). The fungus is capable of invading groundnut seeds before harvest, during postharvest drying and during storage. Aspergillus flavus first appears on groundnut cotyledons after the emergence of seedlings. Necrotic spots become covered with masses of yellow-green spore heads of the Aspergillus flavus fungi. Fungus toxins are translocated throughout the seedling in the transpiration stream. Infected plants generally become stunted with symptoms of vein clearing chlorosis on the leaflets. Such seedlings lack a secondary root system, a condition known as “aflaroot” (Joffe, 1969).
Conclusion
The experiment was of vital interest as we were able to prepare slides ourselves and identify the fungus pathogen on groundnut seeds through the compound microscope. It impacts were seen with naked eyes on the petri dish as some seeds germinated but others did not due to the effects of this fungus on the groundnut seeds.
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Acknowledgement
My great appreciation is to Dr. Manyangarirwa the senior lecturer of advanced plant pathology for this interesting and successful practical on Impact of Aspergillus flavus on groundnut, Arachis hypogaea. I would also appreciate the Jokomo Yamada Library for the resources that helped me accomplishing this present report.
References
Agrios, G. N. (1988). Plant Pathology: 3rd Ed. Academic Press. p 93.
European Commission (1997) Commission Regulation (EC) N0 194/97 of 31 January, 1997 (Setting Maximum Levels for Certain Contaminants in Foodstuffs) Official journal of the European communities L 031, 01/02/1997: 48-50
Joffe, A. Z. (1969), Effects of Aspergillus flavus on Groundnuts and on some other Plants. Journal of Phytopathology, 64: p 321–326.
Singh F and Oswalt D.L (1992). Major diseases of groundnuts, International Crops Research Institute for the Semi-Arid Tropics Patancheru, Andhra Pradesh 5, India p 12
