Contribution of push-pull cropping system to management of ear rots and mycotoxin contamination in Maize in Western Kenya

Abstract

Push-pull is a cereal cropping system that has recently been reported to reduce incidences of ear rots and mycotoxins in maize. However, the effectiveness and mechanism involved is not yet understood. In the current study, the (i) socio-economic and agronomic factors associated with the occurrence of ear rots and contamination of maize with mycotoxins in different cropping systems in western Kenya, (ii) impact of insect management under push-pull cropping system in managing aflatoxin and fumonisins, (iii) role of soil health improvement under push-pull on the population of mycotoxin-producing fungi and (iv) effect of desmodium roots exudates on mycotoxin producing fungi of maize were determined. A household survey covering 116 farmers who practiced push-pull and 139 farmers practicing other cropping systems was conducted in five counties of western Kenya. At least 10 maize ears were sampled per farm during harvest and analyzed for ear rot fungal pathogens, aflatoxin and fumonisins. Sixty push-pull farms, each with a neighboring control farm were examined for damage due to stem borer, fall armyworm, ear rots, ear rot fungal pathogens, aflatoxin and fumonisin levels. Soil was sampled and analyzed for mycotoxigenic fungi and nutrient content at planting, flowering and at harvest. Dried desmodium roots exudates were extracted with methanol and dichloromethane and tested in vitro for growth inhibition of toxigenic isolates of A. flavus and F. verticillioides. All the respondents were small holder farmers with over 50% being female. Twenty six percent of the respondents had knowledge on aflatoxin while over 50% had knowledge of maize ear rots. Most farming practices were similar between cropping systems but significantly (P < 0.05) lower population of F. verticillioides and A. flavus were isolated from the maize samples from push-pull farms. All push-pull samples were contaminated with aflatoxin below 10 μg/kg (Kenyan regulatory threshold) while 4.3% of the samples from non-push-pull had levels above 10 μg/kg. Five percent and 9.4% of the maize from push-pull and non-push-pull farms, respectively, had fumonisin above 1000 μg/kg European Commission regulatory threshold. Knowledge on aflatoxin was 7.5 times higher among elderly aged 45 to 60 years while knowledge of ear rots increased 6 times with level of education (P < 0.05) and non-push-pull respondents were 34% more knowledgeable. Fumonisin and aflatoxin contamination in maize increased 3.9 times and by 28%, respectively, with application of diammonium phosphate (DAP) fertilizer during planting (P < 0.05). Aflatoxin levels also significantly increased 2 times with stemborer infestation of maize. Stemborer and fall armyworm damage on foliage and ears of maize were significantly (P < 0.05) reduced by slightly over 50% under push-pull cropping system. Fusarium ear rot was the most common ear rot with mean incidence of 5 and 10% under push-pull and non-push-pull, respectively (P < 0.05). Populations of F. verticillioides and A. flavus were significantly low under push-pull. Aflatoxin levels were not significantly different between cropping systems, but fumonisin were significantly lower by 39% under push-pull cropping system. There was positive and significant correlation among insect damage, ear rot, ear rot fungi and mycotoxin levels in maize. Populations of fungi and nutrients in soils were not significantly different between the cropping systems and did not have significant correlation between them. Methanolic extracts of desmodium roots showed significant reduction in radial growth of toxigenic A. flavus by 11-17% and F. verticillioides by 53-61% through reduced spore germination and germ tube elongation. The results showed that planting maize under push-pull cropping system indirectly reduced mycotoxin contamination through reduced insect damage. Reduced growth of toxigenic fungi in soils under push-pull by chemicals produced into the rhizosphere by desmodium roots was suggested as a potential mechanism of reducing mycotoxin contamination