Monitoring fusarium, gibberella and diplodia ear rots and associated mycotoxins in maize grown under different cropping systems

Mabuza, Londiwe (2017-03)

Thesis (MSc)--Stellenbosch University, 2017.

Thesis

ENGLISH SUMMARY: Maize ear rots represent a significant problem in most maize production areas resulting in reduced yield and quality due to visible fungal infection and mycotoxin contamination of maize grain. Mycotoxigenic fungi affecting cereal grains are particularly important for humans and animals as they pose food safety and security concerns. Increased maize productivity relies on integrated management strategies which include limiting soil erosion and water runoff. Therefore, agricultural practices that involve no-till and the retention of previous crop residues and/or cover crops are steadily increasing in maize production areas in South Africa. The relationship between no-till, the presence of crop residue in the field and maize ear rot disease severity and mycotoxin contamination is not well understood. The increase in the use of cropping systems that support the retention of crop residues in the field could have substantial impacts on maize production and food safety in South Africa. Adequate understanding of the role of agricultural practices in disease outbreaks can assist in enhancing management of maize ear rot pathogens. In this study, the influence of different cropping systems on F. verticillioides and F. graminearum accumulation, Diplodia ear rot (DER) incidence as well as mycotoxin contamination in maize grain was determined. Cropping systems did not significantly affect F. verticillioides accumulation, zearalenone and nivalenol contamination in all the years of evaluation. Fusarium graminearum accumulation, DER incidence and deoxynivalenol contamination were, however, significantly affected in certain years when disease development was favoured. A survey to establish the effect of no-till and conventional tillage practices on Fusarium ear rot, Gibberella ear rot and DER in maize grain and resultant mycotoxin contamination in maize grain was also conducted in commercial farms in South Africa. Additionally, the survival of F. graminearum and F. verticillioides as well fumonisin contamination in crop residue samples collected from conservation and conventional tillage commercial farms in South Africa was also investigated. Tillage practices did not have an effect of fungal accumulation, disease incidence and mycotoxin contamination in maize grain. The results from this study indicate that under local conditions, conservational agricultural practices can be used without the potential risk of enhanced disease accumulation and mycotoxin contamination. Fusarium graminearum and F. verticillioides accumulation and traces of fumonisins were quantified from all analysed crop residues and did not differ between tillage practices. The recovery of these ear rot-causing fungi from crop residues is an indication of its potential to act as inoculum reservoirs for these fungi. Although the levels of fungal target DNA quantified from the crop residues was low, the fungi may reproduce, survive and infect subsequent hosts.

AFRIKAANSE OPSOMMING: Mielie-kopvrotte verteenwoordig 'n groot probleem in die meeste mielie-produserende gebiede en lei tot verminderde opbrengs en kwaliteit as gevolg van sigbare swam-infeksie en mikotoksienbesmetting van mieliegraan. Mikotoksigeniese swamme wat kopvrot veroorsaak, is veral belangrik vir mense en diere omdat dit die veiligheid en voedselsekuriteit beïnvloed. Verhoogde produktiwiteit van mielies is moontlik met geïntegreerde bestuurstrategieë wat die beperking van grond-erosie en afloopwater insluit. Landboupraktyke wat geenbewerking en die behoud van vorige oesreste en/of dekgewasse behels, is stadig besig om in mielie-produserende gebiede in Suid-Afrika toe te neem. Die verhouding tussen grondbewerking, die teenwoordigheid van oesreste op die land, en mielie-kopvrotsiektes en mikotoksienbesmetting, word nie goed begryp nie. Die toename in die gebruik van verbouingsstelsels wat die behoud van oesreste op die land ondersteun, kan ʼn aansienlike impak op mielieproduksie en voedselveiligheid in Suid-Afrika hê. Voldoende begrip vir die rol van landboupraktyke in die uitbreek van siektes kan help met verbeterde bestuur van mielie-kopvrotpatogene. In hierdie studie is die invloed van verskillende verbouingsstelsels op F. verticillioides en F. graminearum opeenhoping, Diplodia kopvrot-voorkoms, asook mikotoksienbesmetting in mieliegraan vasgestel. Verbouingsstelsels het nie F. verticillioides opeenhoping en zearalenone en nivalenol besoedeling in al die jare van evaluering betekenisvol geaffekteer nie. Fusarium graminearum opeenhoping, DER voorkoms en deoxynivalenol besoedeling is egter betekenisvol beïnvloed in sekere jare wanneer siekte-ontwikkeling bevoordeel is. 'n Opname om die effek van geenbewerking en konvensionele bewerkingspraktyke op Fusarium kopvrot, Gibberella kopvrot en DER in mieliegraan, en die gevolglike mikotoksienbesmetting in mieliegraan vas te stel, is in kommersiële plase in Suid-Afrika uitgevoer. Daarbenewens is die voortbestaan van F. graminearum en F. verticillioides, sowel as fumonisien besoedeling in oesreste monsters wat vanaf bewaring en konvensionele bewerking kommersiële plase in Suid-Afrika versamel is, ook ondersoek. Bewerkingspraktyke het nie 'n effek op swam-opeenhoping, siekte-voorkoms en mikotoksienbesmetting in mieliegraan gehad nie. Die resultate van hierdie studie dui daarop dat onder plaaslike toestande, bewaringslandboupraktyke gebruik kan word sonder die potensiële risiko van verhoogde siekte-opeenhoping en mikotoksienbesmetting. Fusarium graminearum en F. verticillioides opeenhoping en spore van fumonisien is vanaf alle ontlede oesreste gekwantifiseer en het nie tussen bewerkingspraktyke verskil nie. Die terugkry van hierdie kopvrot-veroorsakende swamme vanaf oesreste is 'n aanduiding van hul potensiaal om as inokulumbron vir hierdie swamme op te tree. Hoewel die vlakke van swamteiken DNA, gekwantifiseer vanaf die oesreste, laag was, kan die swamme oorleef, vermeeder en volgende gashere infekteer.

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