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

Abstract

English: Sorghum is the fifth most grown cereal worldwide, and is a staple food in 30 countries that sustains 500 million people in the semi-arid tropics. Sorghum grain mold (SGM) is the one of the most important pre-harvest biotic constraints in sorghum production. Over 40 genera of pathogenic fungi occur on sorghum grain and cause SGM. Fusarium graminearum, a causal agent of SGM, is responsible for the majority of economically and medically important mycotoxins associated with the disease. This, however, is not an individual pathogen, but a complex of species or a combination of related Fusarium species and is referred to as the Fusarium graminearum species complex (FgSC). The FgSC formed the focus of this study. Grain from nine sorghum cultivars, harvested over three seasons at Cedara and two seasons at Alma, was evaluated for grain mold severity, mycotoxin contamination and the stability of the grain mold response over changing environmental conditions. Lower visual threshed grain disease ratings and total fungal biomasses, determined using ergosterol analysis, were observed in grains with an elevated tannin content. However, no correlation between threshed grain disease ratings and ergosterol content was observed indicating that the former criterion is not a reliable measure of grain colonisation by grain mold fungi. Quantitative PCR analysis indicated a FgSC DNA content in grain over a range of 5.52 ng.μl-1 in PAN8625 to 55.43 ng.μl-1 in PAN8806 with significant differences between cultivars. Only three of the 162 grain samples had deoxynivalenol (DON) concentrations that exceeded 10 μg.kg-¹ and DON was therefore excluded from further analysis. However, nivalenol (NIV) and zearalenone (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 to changing environments with most cultivars yielding an IPCA1 score <1. Robust regression was applied to quantify the relationship between NIV and ZEA accumulation in grain relative to the FgSC DNA concentration and indicated that host genotype influences mycotoxin production despite similar colonisation levels. Results indicate the need for the inclusion of environmental variation in the screening and selection for resistance to SGM in sorghum genotypes, to ensure quality grain and human and animal health. The development of an epidemiological model which quantifies the risk of grain molds and mycotoxins in sorghum production areas could enable producers to ensure that timely management decisions are made to reduce FgSC infection and mycotoxin contamination. Sorghum grain collected over two seasons from 18 South African sorghum production areas were analysed for FgSC colonisation and DON, NIV and ZEA contamination. FgSC colonisation and concomitant mycotoxin accumulation coincided with weather conditions during early-post flowering, 82-95 days after planting (d.a.p.) and soft dough stage, 92-115 d.a.p., which are the critical periods for grain colonisation and mycotoxin accumulation. FgSC development and colonisation were significantly, positively correlated with maximum relative humidity 82-95 d.a.p. and significantly inversely correlated with maximum temperature and evapotranspiration 82-95 d.a.p. DON, NIV and ZEA accumulation were significantly positively related to FgSC DNA concentration. DON had borderline significant positive relationship with maximum temperature 101-115 d.a.p., however NIV and ZEA had significant inverse relationship with minimum temperature 91-104 and 100- 113 d.a.p., respectively. Preliminary models based on stepdown multiple regression analysis were developed. Future studies could include localities with more available and accurate weather data to further calibrate and validate the models developed. A range of commercial sorghum grain samples were collected from a sorghum processing company as well as two finished products were taken from storage. In addition a total of 180 sorghum grain samples consisting of four cultivars from three localities in South Africa (Greytown, Standerton and Potchefstroom) were collected during one production season and decorticated using a tangential abrasive dehulling device (TADD) for five time periods. Ergosterol concentrations were highest in sorghum bran and lowest in 22% dehulled grain, indicating that a high proportion of fungal contamination lies in the outer grain layers. In contrast, FgSC DNA concentrations were detected at lowest levels in sorghum bran and at highest levels in 22% dehulled grain. The assumption was therefore, made that FgSC infections were deep-seated within the grain endosperm. Furthermore, controlled decortication by TADD only resulted in a significant reduction in FgSC DNA content after four minutes i.e. 35% decortication. The assumption is further supported by NIV concentrations in both commercial and controlled samples which were relatively high in 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. The assumption can be made that ZEA and DON are associated with superficial FgSC infections and accumulate in the outer layers of the grain while NIV is associated with pathogenesis in the deeper endosperm layers. Cultivars differed in hardness and variation in hardness of grains was associated with prevailing weather conditions at each locality, as well as endosperm texture with lower colonisation and mycotoxin levels in the harder grain. The highest FgSC DNA concentrations and DON, NIV and ZEA accumulation was recorded in grain from Greytown where weather conditions during the critical grain development stages promoted infection and the contamination of grains by mycotoxins. Understanding the effects of decortication on FgSC DNA concentrations and the accumulation of DON, NIV and ZEA could assist commercial processors to make the best management decisions for the removal of these harmful mycotoxins.

Afrikaans: Sorghum is wêreldwyd die vyfde mees geplante graan en in 30 lande vir 500 miljoen mense in die semi-aride gebiede. Tydens produksie is sorghum graanskimmel (SGS) een van die belangrikste voor-oes biotiese stremmings faktore en meer as 40 genera patogeniese swamme word op graan aangelief. Fusarium graminearum, ‘n veroorsakende organisme van SGS is verantwoordelik vir die belangrikste ekonomise en medise mikotoksiene wat met die siekte geassosieer word. Dit is nie ʼn enkel patogeen nie, maar ʼn kompleks van individuele spesies of ʼn kombinasie van verwante Fusarium spesies en word na verwys as die Fusarium graminearum spesies kompleks (FgSK). Die FgSK was die basis van hierdie studie. Graan van nege sorghum kultivars, wat oor drie seisoene op Cedara en twee seisoene op Alma geoes is, is geëvalueer vir graan skimmel strafheid, mikotoksien produksie en die stabiliteit van die graan skimmel reaksie oor veranderende omgewingstoestande. Minder visuele sikte waarnemings en totale fungus biomassa, soos bepaal met ergosterol analise, is in grane met ʼn hoër tannien inhoud waargeneem. Geen korrelasie is tussen visuele siekte lesings en ergosterol inhoud waargeneem nie wat daarop dui dat eersgenoemde nie ʼn betroubare maatstaf is vir die bepaling vir graan kolonisering nie. Kwantatiewe PCR analise het betekenisvolle verskille tussen kultivars aangedui, waar die FgSK DNA inhoud van graan vanaf 5.52 ng.μl-1 in PAN8625 tot 55.43 ng.μl-1 in PAN8806 varieer het. Slegs drie van 162 graan monsters het meer as 10 μg.kg-¹, deoksinivalenol (DON) bevat dus is geen verdere DON analises gedoen nie. Nivalenol (NIV) en zearalenone (ZEA) was deurgaans teenwoordig met die uitsondering van vier by NIV en twee by ZEA. “Additive Main Effect and Multiplicative Interaction (AMMI)” ontledings van FgSK DNA, NIV en ZEA konsentrasies het relatief stabiele waarneem op kultivars teen veranderende omgewingstoestande aangedui met “IPCA1” tellings <1 in meeste van die kultivars. Onsensitiewe regressie is toegepas om die verhouding tussen NIV en ZEA produksie in graan teenoor die FgSK DNA konsentrasie te kwantifiseer wat ‘n aanduiding is dat gasheer genotipe mikotoksien produksie beïnvloed ongeag eweredige kolonisasie vlakke. Resultate dui aan dat dit belangrik is om omgewingstoestande as ʼn veranderlike in te sluit tydens weerstandseleksie van SGS om sodoende kwaliteit graan te produsêer en menslike- en dieregesondheid te verseker. Die ontwikkeling van ʼn epidemiologiese model om die risiko van graan skimmel en mikotoksiene in die sorghum produksie areas te kwantifiseer, kan produsente in staat stel om bestuurspraktyke aan te pas om FgSK infeksie en mikotoksien produksie te verminder. Sorghum graan is oor twee seisoene in 18 produksie gebiede versamel en vir analises FgSK kolonisering, DON, NIV en ZEA produksie gedoen. Resulte van FgSK kolonisering en gepaardgaande mikotoksien produksie ooreenstem met die twee kritiese tye (vroë blom, 82-95 dae na plant (d.n.p) en sagte deeg stadium, 92-115 d.n.p.) en mikotoksien akkumulasie. Weerstoestande is belangrik deurdat FgSK kolonisering van wat betekenisvol gekorreleer is met maksimum relatiewe humiditeit, 82-95 d.n.p. en betekenisvol negatief gekorreleerd is met maksimum temperatuur en verdamping, 82-95 d.n.p. Produksie van DON, NIV en ZEA is betekenisvol gekorreleer met FgSK DNA konsentrasie bepalings. DON het ʼn betekenisvolle positiewe verwantskap aangedui met maksimum temperatuur, 101- 115 d.n.p., terwyl NIV en ZEA ‘n betekenisvolle negatiewe verwantskap met minimum temperatuur 91-104 en 100-113 d.n.p. Voorlopige modelle gebaseer op stapsgewyse regressie analise is onwikkel. Verdere studies, wat lokaliteite insluit waar betroubare weerdata oor tyd beskikbaar is word benodig om die modelle verder te verfyn en te bekragtig. Verder is ʼn aantal kommersieel goeste sorghum graan monsters in ʼn sorghum verwerkings maatskappy versamel en twee stoor produkte is ingesluit vir ontledings. ʼn Addisioneel is 180 sorghum graan monsters, wat vier kultivars vanaf drie lokaliteite (Greytown, Standerton en Potchefstroom) insluit versamel gedurende een produksieseisoen en met ʼn “tangential abrasive dehulling device (TADD)" vir vyf tyd periodese ontdop. Sorghum semels het die hoogste ergosterol konsentrasies aangedui en 22% bewerkte graan die laagste, wat daarop dui dat die meeste fungus kontaminasie op die buitenste graanlaë van die saad voorkom. Die teenoorgestelde is aangedui deur FgSK DNA konsentrasies waar die laagste vlakke in sorghum semels en die hoogste konsentrasies by 22% TADD bewerkte graan waargeneem is. Die aanname dat FgSK binne die endosperm voorkom, was gemaak. Beheerde bewerking van graan met TADD het eers na vier minute d.w.s. op 35% ʼn betekenisvolle afname in FgSK voorkoms aangedui. Die aanname word verder ondersteun deur NIV produksie in albei kommersiële en beheerde monsters wat relatief hoë konsentrasies in sorghum semels en 4% TADD bewerkte graan en die afname na 0.00 μg.kg-1 in beide 22% en in die ses minute TADD bewerkte graan. In vergelyking met NIV, was DON en ZEA deur kort TADD bewerkings periodes vanaf die graan verwyder. Die aanname kan dus gemaak word dat ZEA en DON produksie met oppervlakkige infeksies van FgSK gepaardgaan, terwyl NIV met patogenese in die dieper endosperm lae gepaardgaan. Kultivars verskil in hardheid en herdie variasie word geassosier met weerstoestande van die onderskeie lokaliteite sowel as endosperm tekstuur. Laer koloniserings en mikotoksien vlakke is bepaal in die harder graan, die hoogste FgSK konsentrasies en DON, NIV en ZEA produksie is in graan vanaf Greytown waargeneem. Dit word toegeskyfaan gunstige weerstoestande tydens die kritiese graan ontwikkelings stadiums wat infeksie en ontwikkeling van die patogeen kompleks en dus kontaminasie deur mikotoksiene bevoordeel het. Kommersiële verwerkers kan die beste bestuurs besuite maak waneer hulle die invloed van TADD bewerkings effek op FgSK DNA konsentrasies en die produksie van DON, NIV en ZEA verstaan om sodoende die skadelike miktoksiene vanaf graan te verwyder.