Graduation date: 1995

Thesis (MScAgric)--University of Stellenbosch, 2005. / ENGLISH ABSTRACT: Ripgut brome (Bromus diandrus Roth.) is a weed that causes great problems in the most wheat and grain producing areas and also in livestock practices. Until recently (1995) there were no registered chemicals for the management of ripgut brome in wheat, in South Africa. After the registration of sulfosulfuron and iodosulfuron + mesosulfuron for the management of ripgut brome in wheat, these two herbicides were widely used and in the case of wheat monocultures, it was used repeatedly. During the last few years, reports of ripgut brome that were suspected to be resistant to these chemicals, increased. With the development of herbicide resistance it is of great importance to investigate methods to confirm resistance and also to control it. The goal of this study was firstly, to confirm resistance in ripgut brome and secondly to compare growth and development of resistant ripgut brome popualtions to that of susceptible ripgut brome populations. The dormancy of Bromus seed was also investigated as were effective methods to break seed dormancy. Lastly, quicker methods to confirm resistance were investigated. A short summary of the experiments follows. In the first experiment the degree of resistance of three different ripgut brome populations were determined, by using the pot spray method. One population each of B. pectinatus and B. rigidus were also included in the study. The seed of the Bromus populations were germinated after which it was planted in plastic pots and were placed in the glasshouse until the three to four leaf stage. Subsequently the plants were treated with the following four herbicides: sulfosulfuron, iodosulfuron + mesosulfuron, imazamox and haloxyfop-R methyl ester, at seven concentrations namely, the recommended dosage, one quarter, one half, twice, four times and eight times the recommended dosage. After six weeks the percentage survival and the dry mass of the plants were determined. Results showed that the three ripgut brome populations had different degrees of resistance to sulfosulfuron and iodosulfuron + mesosulfuron, varying from no resistance to moderate resistance to strong resistance. There was no resistance to imazamox and haloxyfop-R methyl ester. The B. rigidus population exhibited strong resistance or tolerance (natural resistance) to the two sulfonylureum herbicides. In the second experiment the seed of the same Bromus populations were germinated and planted in plastic pots that were filled with three litres of river sand to determine the growth, development and seed production of the plants. The number of leaves for each plant as well as the plant height were measured weekly, until the plants became reproductive. The two resistant populations grew much faster than the susceptible population and they also produced taller plants. The susceptible population produced more leaves, but seed production was delayed considerably. This probably relates more to the plant’s adaptation to their enviroment, than to adaptation due to resistance. The susceptible population was collected from a natural environment, while the others were collected from wheat fields. In the third experiment the seed dormancy of the Bromus populations was investigated. The effect of different treatments on the dormancy of the seed was also investigated. The treatments that were applied were gibberrellic acid, fumigation with ammonia gas and an ammonia treatment combined with a cold treatment. Seed dormancy in all populations was short-lived and the cold treatment was an effective way of stimulating fresh seed to germinate. The last experiment was performed to develop a quicker method for the evaluation of resistance in Bromus spp. In this experiment the petridish method was investigated. Only sulfosulfuron and iodosulfuron + mesosulfuron were used, because resistance to them was proven earlier. Different concentrations of the herbicides were applied to the dishes with the seed and were exposed to a cold treatment before being placed in a germination chamber. The seed in al the treatments germinated and it was decided to let the seed grow for two weeks in the petri dishes to observe whether the herbicides may have a detrimental effect on the growth of the small seedlings. After two weeks there were no differences between treatments and the experiment was terminated. The study showed that resistance is present in some of the Bromus populations and that there are biological differences between populations with different degrees of resistance. However, the fact that the susceptible population comes from a completely different environment than the other populations, complicate matters and further studies are required to obtain a clearer picture. / AFRIKAANSE OPSOMMING: Predikantsluis (Bromus diandrus Roth.) is ‘n onkruid wat in die meeste koringen garsproduserende gebiede, asook in sommige vee praktyke, probleme veroorsaak. Tot redelik onlangs (ongeveer 1995) was daar in Suid-Afrika geen middels geregistreer wat predikantsluis in koring kon beheer nie. Nadat sulfosulfuron en iodosulfuron + mesosulfuron vir predikantsluisbeheer in koring geregistreer is, is die twee middels op groot skaal, en in die geval van koring monokultuurstelsels, aanhoudend toegedien. Gedurende die afgelope paar jaar is berigte ontvang dat beheer van predikantsluis met die middels nie meer so doeltreffend is nie, moontlik as gevolg van onkruiddoderweerstand wat ontwikkel het. Met die ontstaan van onkruiddoderweerstand is dit belangrik om praktyke en maniere te vind om weerstand vinniger te bevestig en doeltreffend te bestuur. Die doel van hierdie studie was eerstens om weerstand in predikantsluis te bevestig en tweedens om die groei en ontwikkeling van plante afkomstig van vermoedelike weerstandbiedende predikantsluis populasies te vergelyk met plante uit ‘n vatbare populasie. Die saadproduksie en dormansie van die saad is ook ondersoek asook effektiewe metodes om dormansie te breek. Laastens is ondersoek ingestel na ‘n vinniger manier (petribakkie metode) om weerstand te bevestig. Hieronder volg ‘n oorsig oor die vier eksperimente wat uitgevoer is. In die eerste proef is die mate van weerstand van drie verskillende predikantsluis populasies bepaal, deur van die gewone potspuit metode gebruik te maak. Daar is ook een populasie elk van Bromus pectinatus en vermoedelik Bromus rigidus ingesluit in die studie. Die sade van die verskillende populasies is toegelaat om te ontkiem en daarna is dit in plastiese potjies geplant en in ‘n glashuis geplaas totdat die drie tot vier blaarstadium bereik is. Die plante is daarna gespuit met die volgende vier middels: haloksifop-R-metielester (Gallant Super), imasamoks (Cysure), iodosulfuron + mesosulfuron (Cossack) en sulfosulfuron (Monitor), teen sewe konsentrasies elk, nl. teen die aanbevole dosis, asook teen een kwart van, een helfte van, twee keer, vier keer en agt keer die aanbevole dosis. Na ses weke is die persentasie oorlewendes en die droëmassa van die plante bepaal. Resultate het getoon dat die drie predikantsluis populasies verskillende grade van weerstand teen die twee sulfonielureums (sulfosulfuron en iodosulfuron + mesosulfuron) toon, dit wil sê van geen tot matig tot sterk weerstandbiedend. Daar is egter geen weerstand teen haloksifop-R-metielester (Gallant Super) en imasamoks (Cysure) waargeneem nie. Die B. rigidus populasie het sterk weerstand of toleransie (natuurlike weerstand) teen die sulfonielureum middels getoon. In die tweede proef is saad van dieselfde Bromus populasies ontkiem en oorgeplant in plastiese potte gevul met 3 liter riviersand om die groei en ontwikkeling en saadproduksie van die plante te evalueer. Die aantal blare per plant en hoogte van die plante is weekliks bepaal totdat die plante reproduktief geraak het. Hierna is die metings gestaak om te voorkom dat die saadproduksie van die plante benadeel word. Die resultate het getoon dat die twee weerstandbiedende predikantsluis populasies vinniger groei as die vatbare populasie en ook langer plante vorm, terwyl die vatbare populasie vinniger en meer blare vorm, maar langer neem om saad te vorm. Hierdie waarnemings hou egter waarskynlik meer verband met die oorsprong van die populasies as met die graad van weerstandbiedendheid. Die vatbare populasie is versamel in natuurlike veld vêr van enige landerye terwyl die ander populasies almal uit graanlande afkomstig is. In die derde proef is saaddormansie van die Bromus populasies ondersoek. Daar is ook ondersoek ingestel na verskillende behandelings om dormansie te breek. Die behandelings wat toegepas is, is ‘n gibberelienesuur behandeling teen verskillende konsentrasies, beroking met ammoniak vir verskillende tye en ‘n ammoniak behandeling tesame met ‘n koue behandeling. Die resultate het getoon dat saaddormansie van die Bromus populasies van korte duur is, maar dat kouebehandeling effektief is om ontkieming van vars saad te stimuleer. Die vierde proef is uitgevoer om vas te stel of daar vinniger evaluasiemetodes is vir die evaluasie van weerstand in Bromus spp., deur van die petribakkie metode gebruik te maak. In hierdie proef is slegs die middels iodosulfuron + mesosulfuron (Cossack) en sulfosulfuron (Monitor) gebruik, omdat daar ‘n mate van weerstand teen hulle waargeneem is in die eerste proef. Die middels is teen verskillende konsentrasies in petribakkies gevoeg, tesame met die sade en toe blootgestel aan ‘n kouebehandeling voordat dit in ‘n ontkiemingskabinet geplaas is vir ontkieming. Die sade in al die behandelings het ontkiem en daar is besluit om die saailinge uit die ontkiemingskabinet te haal en vir twee weke te laat groei sodat daar bepaal kon word of die middels ‘n effek op die groei van die plantjies het. Na twee weke kon geen verskil in die groei van die plantjies waargeneem word nie en die proef is beëindig. Die studie het getoon dat daar wel weerstand in sommige van die Bromus populasies voorkom, en dat biologiese verskille voorkom tussen predikantsluis populasies met verskillende grade van weerstand. Die feit dat die vatbare populasie uit ‘n heeltemaal verskillende omgewing kom as die ander populasies, maak definitiewe afleidings moeilik. Daar sal opvolgstudies uitgevoer moet word om van die onduidelikhede op te klaar.