Fusarium graminearum mycotoxins associated with grain mould of maize and sorghum in South Africa

[摘要] English: Maize and sorghum are important crops in South Africa, meeting the dietary needs of millions of people as well as the needs of the feed industry. Fungi within the Fusarium graminearum species complex (FGSC) have recently become the subject of importance locally. Maize kernels and sorghum grain were collected from the National Cultivar Trial over three and two seasons respectively. A total of 558 samples comprising of six maize and seven sorghum cultivars were collected from 34 and 22 localities, respectively. Although different conventional detection, isolation and identification methods were utilized, very low numbers of FGSC isolates could be obtained from the grain. Molecular identification was based on the translation elongation factor-1α (TEF1-α) and the ammonia ligase (URA) genes. F. boothii and F. graminearum s.s. were identified from maize kernels while F. acaciamearnsii, F. cortaderiae and F. meridionale where associated with sorghum. Results indicated host-specificity between members of the FGSC. Total genomic DNA was extracted from all milled kernel and grain samples and analysed for the presence or absence of FGSC DNA using TaqMan real-time PCR. F. graminearum s.l. DNA was detected and quantified in over 47% of the samples. These findings contradict previous reports that stated that this group of fungi is of minor importance. Reasons may be improved detection techniques as well as changes in agronomic practices, such as rotation of maize with wheat in South Africa, or to other shifts in environmental factors. DON and ZEA concentrations were quantified using CD-ELISA. In the absence of an ELISA test for NIV, a selected number of samples were used in LC-MS/MS based detection and DON, NIV and ZEA were quantified. Concentrations varied considerably and no correlation between the two techniques was found. Based on CD-ELISA, maize and sorghum contamination with DON and ZEA differed significantly between seasons, localities and in some instances cultivars. An analysis of variance across all three seasons showed significant crop by locality interactions. In maize, high levels of DON were detected in the 2007/08 season, in particular at Delmas, while during the same season ZEA contamination of maize was significantly higher at Bothaville. In sorghum, DON and ZEA contamination was highest during the 2008/09 season, although both toxins were detected in the preceding season. In both the 2007/08 and 2008/09 seasons, Cedara was the most conducive locality for grain contamination with both toxins. Trichothecene chemotyping was conducted using both simplex and multiplex PCR. All the F. boothii and F. graminearum s.s. isolates from maize were DON/15-acetyldeoxynivalenol (15-ADON) producers and F. acacia-mearnsii, F. cortaderiae and F. meridionale isolates from sorghum grain produced NIV. DON can act as a virulence factor in plant disease and is usually associated with greater pathogenicity on plants than NIV producers. On the other hand, NIV is believed to be more toxigenic to both humans and animals. Isolates of the 15- ADON chemotype are reportedly less toxic that those of the 3-ADON chemotype. The 3- ADON chemotype was not recorded in this study. However, more FGSC isolates need to be collected from South African cereal grains and evaluated for their mycotoxin potential to establish safety guidelines for end users of these products. The results of this study showed that levels of field infection of maize and sorghum grain by the FGSC are of significance. Future studies need to quantify the relationships between members of the FGSC and their tricthothecene and ZEA production in South African maize and sorghum production systems including the role of cultivar choice, weather, rotation, tillage and other practices with the aim of establishing intervention technologies.

[发布机构] University of the Free State