Mycobiota profile, phenology, and potential toxicogenic and pathogenic species associated with stored groundnuts (Arachis hypogaea L.) from the Volta Region, Ghana

Abstract

This study updates the mycobiota resident in groundnut seeds, their phenology during storage with the view to ascertain their occurrence, potential toxigenic species, and pathologically important species in the stored samples. The moisture content of the seeds ranged from 5.7% to 6.5% within the stipulated safe moisture content of 8% for extension of shelf life. Culturing the seeds on mycological media (Sabouraud's Dextrose Agar SDA; Oxytetracycline Glucose Yeast Extract OGYE, Potato Dextrose Agar, PDA) caused a de novo growth of the quiescent spores at 28-30°C for 7-14 days. Fungal population counts on the three media ranged from 2.01 to 2.16 log₁₀ CFU/g samples to a final 6-month count of 1.67-2.60 log₁₀ CFU/g. Eighteen different fungal species belonging to ten genera were encountered on the media, namely Aspergillus, Cladosporium, Curvularia, Fusarium, Penicillium, Trichoderma, Rhizopus, Rhodotorula, Sporendonema, and Paecilomyces. Aspergillus spp. (A. niger, A. flavus, A. fumigatus, and A. terreus) were the most frequently isolated, followed by Fusarium species (F. oxysporum, F. solani, and F. verticillioides), Trichoderma (T. harzianum and T. viride), Rhizopus spp (R. oligosporus and R. stolonifer), and Penicillium verrucosum. The species which were seed borne (A. niger, A. flavus, A. terreus, A. fumigatus, F. solani, F. verticillioides, T. viride, C. herbarum, and Curvularia lunata) were isolated on both surface sterilized and non-surface sterilized seeds. The phenology of the encountered fungal species generally followed five patterns. The most frequently isolated Aspergillus niger, A. flavus, and A. fumigatus predominated throughout the 6 months sampling period, while A. ustus and A. terreus appeared sporadically and disappeared. The early colonizers (R. oligosporus, R. stolonifer, and Paecilomyces) could not be isolated after 2-3 months owing presumably to stronger antibiosis competition from the Aspergillus species. The most predominant Aspergillus species initially constituted 36%-48% of the total population but declined to 10%-36% in 6 months. Mycobiota encountered with mycotoxigenic potential and human health importance were A. niger, A. flavus, A. fumigatus, F. verticillioides, and Penicillium verrucosum. Other species of pathological importance to plants were Curvularia lunata and Fusarium oxysporum. The practical implications of these findings are discussed.

Keywords: Fusarium and Curvularia species; Groundnut; Penicillium; mycobiota phenology; mycotoxicogenic and pathogenic Aspergillus.

FIGURE 1

Comparative population of resident fungi (log₁₀ CFU/g sample) in Arachis hypogaea L stored for up to 6 months and incubated on PDA, SDA, or OGYE after each sampling interval at 26–32°C. Abbreviations: PDA, Potato Dextrose Agar; SDA, Sabouraud Dextrose Agar; OGYE, Oxytetracycline Glucose Yeast Extract Agar

FIGURE 2

(a) Changes in mean total fungal counts/plate with attendant changes in moisture content of seeds during storage for 6 months at 30°C. (b) Relationship between fungal colonies and moisture content

FIGURE 3

Phenology of resident fungi in groundnut (Arachis hypogaea) stored in the laboratory for up to 6 months (SSO—Surface Sterilized; 0—initial, non‐surface sterilized)

PLATE 1

Fungi appearing on non‐surface sterilized groundnut seeds (Kumawu red variety)

PLATE 2

Fungi appearing on surface‐sterilized groundnut seeds (Kumawu red variety)