Fusarium and fumonisin contamination of maize produced in Uganda
Maize (Zea mays L.) is a staple food for many Ugandans. During production and storage, the crop may be colonised by Fusarium species which may produce fumonisins that are associated with several animal and human illnesses. Contamination of maize with fumonisins may be influenced by environmental factors as well as agronomic and post-harvest handling practices. This study investigated the occurence and diversity of Fusarium species in maize grown in three agro-ecological zones of Uganda and established the pre and postharvest conditions predisposing maize to fumonisin contamination. The results indicated that maize obtained from the three agro-ecological zones was highly infected with Fusarium species with mean incidences varying between 91.03 and 99.2%. Maize from high altitude had significantly higher (p <0.05) Fusarium incidence than that obtained from mid altitude zones. Members of the Gibberella fujikuroi species complex were the most dominant Fusarium species associated with maize obtained from all the agro-ecological zones. Wide intraspecific variability was observed amongst the strains of G. fujikuroi species complex. Strains of G. moniliformis exhibited the highest level of intraspecific variability evidenced by formation of 5 distinct groups. Phylogenetic analysis did not show a direct relationship between clusters/groups of the isolates and geographical origin. All samples of freshly harvested maize obtained from farmers tested positive for fumonisins. The mean total fumonisin content ranged from 0.27 to 10 mg/kg. Maize obtained from high altitude zone had significantly higher (p < 0.05) mean total fumonisin levels (4.93 mg/kg) than maize from the mid altitude (dry) zone (4.50 mg/kg) and mid altitude (moist) zone (4.53 mg/kg). Intercropping, delayed harvesting and drying maize on bareground were positively associated with fumonisin contamination whereas crop rotation and planting treated seeds were negatively associated. Regression analysis revealed that intercropping was the major factor predisposing maize to fumonisin contamination whereas crop rotation was the major factor reducing the contamination. Majority of the G. fujikuroi strains isolated from maize produced in Uganda were high fumonisin producers. The total fumonisin production varied between 19.4 and 99.8 mg/kg. Strains identified as G. moniliformis (GU257904.1) and G. fujikuroi (EU979565.1) were higher fumonisin producers (39.9-99.8 mg/kg) than those identified as G. moniliformis (FJI54074.1) (0-24.9 mg/kg). The order of fumonisin production potential was G. moniliformis > G. fujikuroi > F. proliferatum. Seven strains (MRC 9059, MRC 9063, MRC 9054, MRC 9053, MRC 9067, MRC 9055 and MRC 9066) produced higher amounts of total fumonisins than the reference strain (MRC 826). Fusarium incidence and fumonisin levels in maize stored in traditional storage structures reduced over the six months storage period. Fusarium incidence significantly decreased (p < 0.05) from an initial average of 61.9 to 31.9% whereas fumonisin levels decreased from an average of 5.7 to 2.8 mg/kg. Maize stored in the granaries had significantly lower (p < 0.05) fumonisin levels than that stored in the mudsilos and the Tua. The total fumonisin levels in maize from all the tested storage structures after 6 months were below the limit of 4 mg/kg recommended by the Food and Drug Administration (FDA) for maize intended for human consumption. The findings of this study indicated that maize produced in Uganda is contaminated with high levels of fumonisins. This suggests that the maize consuming population in Uganda may be exposed to high levels of fumonisins. To reduce Fusarium infection and fumonisin contamination in maize, farmers are advised to adopt agronomic and postharvest practices like crop rotation, planting treated seeds and avoid intercropping maize with crops that are likely to encourage Fusarium infection and drying the crop on bareground.