Modelling the colonisation of sorghum grain by the Fusarium graminearum species complex and concomitant mycotoxin production

[摘要] English: Sorghum is the fifth most grown cereal worldwide, and is a staple food in 30countries that sustains 500 million people in the semi-arid tropics. Sorghum grainmold (SGM) is the one of the most important pre-harvest biotic constraints insorghum production. Over 40 genera of pathogenic fungi occur on sorghum grainand cause SGM. Fusarium graminearum, a causal agent of SGM, is responsible forthe majority of economically and medically important mycotoxins associated with thedisease. This, however, is not an individual pathogen, but a complex of species or acombination of related Fusarium species and is referred to as the Fusariumgraminearum species complex (FgSC). The FgSC formed the focus of this study.Grain from nine sorghum cultivars, harvested over three seasons at Cedara and twoseasons at Alma, was evaluated for grain mold severity, mycotoxin contaminationand the stability of the grain mold response over changing environmental conditions.Lower visual threshed grain disease ratings and total fungal biomasses, determinedusing ergosterol analysis, were observed in grains with an elevated tannin content.However, no correlation between threshed grain disease ratings and ergosterolcontent was observed indicating that the former criterion is not a reliable measure ofgrain colonisation by grain mold fungi. Quantitative PCR analysis indicated a FgSCDNA content in grain over a range of 5.52 ng.μl-1 in PAN8625 to 55.43 ng.μl-1 inPAN8806 with significant differences between cultivars. Only three of the 162 grainsamples had deoxynivalenol (DON) concentrations that exceeded 10 μg.kg-¹ andDON was therefore excluded from further analysis. However, nivalenol (NIV) andzearalenone (ZEA) were present in all but four and two samples, respectively.Additive Main Effect and Multiplicative Interaction (AMMI) analysis of FgSC DNA,NIV and ZEA concentrations indicated a relatively stable response in cultivars tochanging environments with most cultivars yielding an IPCA1 score <1. Robustregression was applied to quantify the relationship between NIV and ZEAaccumulation in grain relative to the FgSC DNA concentration and indicated that hostgenotype influences mycotoxin production despite similar colonisation levels. Resultsindicate the need for the inclusion of environmental variation in the screening and selection for resistance to SGM in sorghum genotypes, to ensure quality grain andhuman and animal health.The development of an epidemiological model which quantifies the risk of grainmolds and mycotoxins in sorghum production areas could enable producers toensure that timely management decisions are made to reduce FgSC infection andmycotoxin contamination. Sorghum grain collected over two seasons from 18 SouthAfrican sorghum production areas were analysed for FgSC colonisation and DON,NIV and ZEA contamination. FgSC colonisation and concomitant mycotoxinaccumulation coincided with weather conditions during early-post flowering, 82-95days after planting (d.a.p.) and soft dough stage, 92-115 d.a.p., which are the criticalperiods for grain colonisation and mycotoxin accumulation. FgSC development andcolonisation were significantly, positively correlated with maximum relative humidity82-95 d.a.p. and significantly inversely correlated with maximum temperature andevapotranspiration 82-95 d.a.p. DON, NIV and ZEA accumulation were significantlypositively related to FgSC DNA concentration. DON had borderline significantpositive relationship with maximum temperature 101-115 d.a.p., however NIV andZEA had significant inverse relationship with minimum temperature 91-104 and 100-113 d.a.p., respectively. Preliminary models based on stepdown multiple regressionanalysis were developed. Future studies could include localities with more availableand accurate weather data to further calibrate and validate the models developed.A range of commercial sorghum grain samples were collected from a sorghumprocessing company as well as two finished products were taken from storage. Inaddition a total of 180 sorghum grain samples consisting of four cultivars from threelocalities in South Africa (Greytown, Standerton and Potchefstroom) were collectedduring one production season and decorticated using a tangential abrasive dehullingdevice (TADD) for five time periods. Ergosterol concentrations were highest insorghum bran and lowest in 22% dehulled grain, indicating that a high proportion offungal contamination lies in the outer grain layers. In contrast, FgSC DNAconcentrations were detected at lowest levels in sorghum bran and at highest levelsin 22% dehulled grain. The assumption was therefore, made that FgSC infectionswere deep-seated within the grain endosperm. Furthermore, controlled decorticationby TADD only resulted in a significant reduction in FgSC DNA content after four minutes i.e. 35% decortication. The assumption is further supported by NIVconcentrations in both commercial and controlled samples which were relatively highin sorghum bran and 4% dehulled grain, but reduced to 0.00 μg.kg-1 in both the 22%dehulled grain and grain from the six minutes decortication. In contrast with NIV,ZEA and DON, were removed from grain by short periods of decortication. Theassumption can be made that ZEA and DON are associated with superficial FgSCinfections and accumulate in the outer layers of the grain while NIV is associatedwith pathogenesis in the deeper endosperm layers. Cultivars differed in hardnessand variation in hardness of grains was associated with prevailing weatherconditions at each locality, as well as endosperm texture with lower colonisation andmycotoxin levels in the harder grain. The highest FgSC DNA concentrations andDON, NIV and ZEA accumulation was recorded in grain from Greytown whereweather conditions during the critical grain development stages promoted infectionand the contamination of grains by mycotoxins. Understanding the effects ofdecortication on FgSC DNA concentrations and the accumulation of DON, NIV andZEA could assist commercial processors to make the best management decisionsfor the removal of these harmful mycotoxins.

[发布机构] University of the Free State