The use of Flucarbazone-sodium to control wild oats (Avena spp.) in cultivated wheat fields of the Western Cape of South Africa

Bester, Dirk Wouter (2017-03)

Thesis (MScAgric)--Stellenbosch University, 2017.

Thesis

ENGLISH ABSTRACT: Wild oats (Avena spp.) is a prominent weed in cultivated wheat fields of the Western Cape Province of South Africa. For the sustainable production of wheat, it is crucial to apply the correct herbicide rates at the correct growth stages of both wild oats and wheat in order to achieve maximum weed control. Flucarbazone-sodium has shown to provide excellent activity against wild oats when applied as a post-emergence herbicide to wheat in field experiments conducted in Canada and the USA. Flucarbazone-sodium acts as an inhibitor of the enzyme acetolactate synthase (ALS) also known as acetohydroxyacid synthase (AHAS), an important enzyme that acts as a catalyst in the biosynthesis of the branched-chain amino acids leucine, isoleucine and valine. This product, known as Everest® 2.0, is a new sulfonylaminocarbonyltriazoline herbicide on the South African market. Flucarbazone-sodium has not yet been tested scientifically under South African conditions. The environment, soil, wheat cultivars and cultivation techniques between South Africa and North America differ immensely. In this study flucarbazone-sodium was evaluated under South African conditions in the Western Cape. The first part of the study aimed to evaluate the phytotoxic effect of flucarbazone-sodium on the locally grown wheat cultivars of the Western Cape, as herbicides have no use if they reduce yield potential, regardless of how effectively they control weeds. For this study the internationally accepted BBCH scale was used to describe the different growth stages of the crop. Flucarbazone-sodium was applied at half, one and double the recommended rates at three different plant growth stages ([12-13], [14-15] and [16-17]; BBCH scale on five cultivars (SST 88, SST 027, SST 056, SST 015 and Pannar 3408) in a phytotoxicity field trial. Yield and grain quality (protein content, hectolitre mass and thousand kernel mass) was examined at the end of the growing season. Wheat yield was not negativly affected by flucarbazone-sodium applications, however, double the recommended rate showed a significant (p < 0.05) reduction in yield and hectolitre mass for cultivar SST 056. Appart from SST 056 there is no statistical evidence to discard flucarbazone-sodium in a weed management system, as long as the application guidelines on the product label are followed. To date, no studies have been published on the effectiveness of flucarbazone-sodium in controlling wild oats biotypes in South Africa. For this study, three sites in the Swartland area (Moorreesburg and Piketberg) with severe wild oats infestations were chosen. Flucarbazone-sodium was applied at half, recommended and double the recommended rates at three different growth stages ([12-13], [14-15] and [16-17]; BBCH scale of the wheat. The results showed that flucarbazone-sodium was very effective in controlling wild oats in cultivated fields at all sites except one. At the Pools site near Piketberg in the Western Cape very low levels of wild oats control by flucarbazone-sodium was recorded. Resistance was suspected and seeds were harvested for further investigation. Glasshouse and molecular trials were conducted in order to prove that the biotype present at Pools is resistant to flucarbazone-sodium and to investigate the possible mechanism responsible for this resistance. Results showed that the sampled wild oats biotype at Pools is clearly resistant to flucarbazone-sodium. Even at eight times the recommended rate, flucarbazone-sodium could not control a single wild oats plant. Flucarbazone-sodium appeared to have some effect on the wild oats biotype sampled at Pools, since the dry mass (DM) of the wild oats was negatively v affected as the dosage rate was increased. PCR results showed the sampled wild oats biotype had homozygous resistance mutations Ala-205-Val and Trp-574, and heterozygous resistance mutations Pro-197 and Ser-653. This is the first documented case of Ala-205-Val, Trp-574 and Pro-197 mutations present in Avena spp. worldwide.

AFRIKAANSE OPSOMMING: Wildehawer (Avena spp.) is ‘n prominente onkruid in koring produksie areas van die Wes-Kaap van Suid-Afrika. Vir die volhoubare produksie, van koring is dit belangrik dat die korrekte onkruiddoder-konsentrasie toegedien word tydens die korrekte groeistadium van beide koring en wildehawer vir optimale onkruidbeheer. In Kanada en Amerika het flukarbason-natrium uitstekende beheer getoon teen wildehawer wanneer dit as ‘n na-opkoms middel toegedien is. Flukarbason-natrium dien as ‘n inhibeerder van die ensiem asetolaktaat sintase (ALS) ook bekend as asetohidroxysuur sintase (AHAS). Hierdie belangrike ensiem dien as ‘n katalis vir die biosintese van aminosure leusien, isoleusien en valien. Die produk genaamd Everest® 2.0 is ‘n nuwe sulfonielaminokarbonieltriasolinoon-onkruiddoder op die Suid-Afrikaanse mark. Flukarbason-natrium is nog nooit wetenskaplik getoets onder Suid-Afrikaanse toestande nie. Daar is geweldige groot verskille tussen die omgewing, grondtipes, koring kultivars en bestuurspraktyke van Suid-Afrika en Noord-Amerika. In hierdie studie is flukarbason-natrium getoets onder die Suid-Afrikaanse toestande van die Wes-Kaap. Die eerste gedeelte van hierdie studie was gefokus op om vas te stel of flukarbason-natrium enige fitotoksiese effek sal hê op die koringkultivars wat in die Wes-Kaap geplant word, want daar is geen nut vir ‘n onkruiddoder wat die opbrengspotensiaal van ‘n gewas benadeel nie, ongeag hoe effektief dit onkruid beheer. Vir hierdie studie is die internasionaal aanvaarde BBCH skaal gebruik om die verskillende groeistadiums van beide koring en wilde hawer te beskryf. Flukarbason-natrium is teen half, aanbevole en dubbel die aanbevole dosis op drie verskillende groeistadiums ([12-13], [14-15] and [16-17]; BBCH skaal) van vyf koringkultivars (SST 88, SST 027, SST 056, SST 015 en Pannar 3408) toegedien. Opbrengs en graankwaliteit (proteïeninhoud, hektolitermassa en duisendkorrelmassa) is aan die einde van die groeiseisoen gemeet. Die resultate het gewys dat koringopbrengs nie negatief beïnvloed is deur flukarbason-natrium nie. Dubbel die aanbevole dosis van flukarbason-natrium het egter ‘n beduidende (p < 0.05) afname in opbrengs en hektolitermassa veroorsaak op die SST 056 kultivar. Al is daar ligte tendense wat dui op ‘n afname in koringobrengs en -kwaliteit, is dit nie beduidend genoeg om produsente te ontmoedig om die produk deel te maak van ‘n geïntegreerde onkruidbestuursplan nie. Dit is egter van uiterse belang dat die riglyne op die etiket gevolg word. Tot op hede is daar nog geen studies gepubliseer wat die beheer van wildehawer biotipes in Suid-Afrika met flukarbason-natrium bestudeer nie. Vir hierdie studie is daar drie persele in die Swartland (Moorreesburg en Piketberg) geïdentifiseer waar hoë wildehawer populasies teenwoordig was. Flukarbason-natrium is teen half, aanbevole en dubbel die aanbevole dosis toegedien op drie verskillende wildehawer groeistadiums ([12-13], [14-15] en [16-17]; BBCH skaal). Die resultate het getoon dat flukarbason-natrium wildehawer uiters effektief beheer op ‘n vroeë groeistadium by alle persele, behalwe vir die perseel by Pools naby Piketberg in die Wes-Kaap. ‘n Uiterste lae vlak van beheer deur flukarbason-natrium is waargeneem en moontlike weerstand is vermoed. Wildehawersaad wat by die Pools perseel geoes was is gebuik in glashuis en molukulêre studies om weerstand te bevestig. Die moontlike meganisme wat verantwoordelik is vir die weerstand was ook bestudeer. Resultate het getoon dat die biotipe wat by die Pools perseel teenwoordig is wel weerstandbiedend is teen flukarbason-natrium. Selfs teen agt keer die aanbevole dosis was daar nie ‘n enkele plantsterfte aangeteken nie. Flukarbason-natrium het wel ‘n effek gehad op die wildehawer biotipe teenwoordig by Pools, aangesien die droëmassa afgeneem het soos die konsentrasie van flukarbason-natrium verhoog het. PCR resultate het ook aangedui dat die wildehawerpopulasie by Pools homosigotiese weerstand mutasies Ala-205-Val en Trp-574 bevat, asook heterosigotiese weerstand mutasies Pro-197 en Ser-653. Dit is die eerste gedokumenteerde geval wêreldwyd waar Ala-205-Val, Trp-574 and Pro-197 mutasies in Avena spp. waargeneem is.

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