Fungal and mycotoxin contamination of wheat from Nakuru and Nyandarua districts of Kenya and pathogenicity of fusarium species on wheat
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Mycotoxin poisoning resulting from contaminated cereal grains has become rampant in Kenya. Understanding the fungal causes and management is important both to the farmers and the consumers of the cereal grains and their products. A survey was carried out during the 2004 wheat growing season in Nakuru and Nyandarua districts. A total of 9 agroecological zones were selected, within which 10 farms in each agroecological zone were randomly selected. A total of 89 farmers were interviewed on wheat agronomic practices and they gave 94 samples for mycological and toxicological analysis. The wheat kernels were plated on Potato Dextrose Agar and Spezieller Nahrastofarmer Agar media that were amended with inorganic salts and the fungi that grew were identified based on colony and other morphological characteristics to genus level and some to species level. Fourteen Fusarium species were tested for pathogenicity on variety 'Mbuni' wheat ears in two greenhouse growing cycles where Fusarium head blight disease severity was assessed as the percentage of the spikelets bleached and the area under disease progress curve. Re-isolation of the Fusarium species was done. Contamination of wheat with mycotoxins zearalenone, T-2, deoxynivalenol and aflatoxin Bl was determined by competitive ELISA method. Eight wheat varieties grown in Kenya were also tested for susceptibility to Fusarium head blight. Pathogenic strains of F. graminearum were used to inoculate varieties under greenhouse conditions. The head blight was assessed as percentage of spikelets bleached and area under the disease progress curve and kernel weight reduction compared to un-inoculated control. Results of the survey showed that, a total of 18 wheat varieties were grown in the area and most farmers (77.9%) were found to use their own seeds for planting which were neither clean nor certified. Wet conditions during flowering, ripening and harvesting of wheat, that favoured diseasedevelopment prevailed in both districts. Farmers were found not taking any precautionary measuresto avoid the build up of the disease. Prevalence of Fusarium head blight was found to be 80.9%. Kernel contamination with fungi was high; Alternaria (100.0%), Epicoccum (100.0%), Fusarium (85.0%), Aspergillus (72.0%), Penicillium (18.0%) and other fungi (2.0%). Fourteen Fusarium species were isolated, with F. poae, F. graminearum, F. avenaceum, F. chlamydosporum, F. moniliforme, F. equiseti and F. oxysporum being the most prevalent. F. graminearum, F. avenaceum and F. moniliforme were found to be highly pathogenic, with a high proportion of ears bleached. The area under disease progress curve was negatively correlated (P :s 0.05) to 10 ear weight (r = -0.511) while disease severity was positively correlated (P:S 0.01) (r = 0.973) to area under disease progress curve. Grain yield measured as 10 ear weight and 100 seed weight differed significantly (P :s 0.01) depending on Fusarium species used to inoculate wheat ears. The re-isolation rate of the Fusarium species ranged from 30.0-90.0%. Most samples (93.0%) were found to be contaminated with at least a toxin, with the most toxic T-2 toxin having the highest prevalence. All the toxins were found to be at low levels with the mean content being: 128.2Jlg/kg for deoxynivalenol, 5.5Jlg/kg for zearalenone, 25.9Jlg/kg for T-2 and l.Sug/kg for aflatoxin B1. The wheat varieties were found to be susceptible to head blight but they differed in the level of disease severity and kernel weight reduction. Most susceptible varieties ('Mbuni' and 'Chiriku') had a high mean disease severity rating (59.3%, and 55.5%, respectively) and high area under disease progress curve (994.0, and 932.0, respectively). The least susceptible variety ('Njoro Bw2') had a mean disease rating of 13.0% and area under disease progress curve of 188.0. Mean reduction in kernel weight varied in the varieties and ranged from 13.5% to 74.0% and was not correlated to mean disease rating and area under disease progress curve. The results indicated that Fusarium head blight and mycotoxin contamination in wheat kernels is prevalent in the wheat growing areas of Nakuru and Nyandarua districts and there is need to sensitize wheat growers on the disease management. Most Kenyan wheat varieties are susceptible to Fusarium head blight.