Thesis (MScAgric)--Stellenbosch University, 1998. / ENGLISH ABSTRACT: Eyespot is an important disease of spring wheat (Triticum aestivum L.). Four species
of Ramulispora are associated with this disease, of which Tapesia yallundae and T.
acuformis. are common. This thesis investigates the broader subjects of genetic
variability, reproductive dynamics and fungicide resistance in Tapesia yallundae.
Each of the chapters treats specific but related topics. T. yallundae, which is the only
species thus far reported from South Africa, has been associated with yield losses of
up to 50%. To enable the implementation of more accurate and effective control
measures, understanding the dynamics of reproduction and the genetics of the
pathogen is of utmost importance. Of the many plant disease control measures such
as cultural practices, sanitation, biological control, etc., fungicide application is the
most commonly resorted to measure in eyespot control. This thesis investigates the
broader subjects of genetic variability, reproductive dynamics and fungicide
resistance of Tapesia yallzll7dae.
Fungicide application, however, is not without problems. The pathogen can
build up resistance to fungicides. The most commonly used fungicides in eyespot
control include the benzimidazole carbendazim, triazoles such as flusilazole,
tebuconazole, propiconazole, bromuconazole, flutriafol, fenbuconazole, triademinol,
and the imidazole, prochloraz. Cases of resistance to the groups listed above have
been reported. Frequent monitoring for resistance is thus crucial to prevent wastage
of fungicide and unnecessary impregnantation of the environment with potentially
ineffective chemicals. In chapter 2 of this thesis 300 isolates of T. yallundae from 15
fields were evaluated for resistance against carbendazim, flusilazole, tebuconazole,
propiconazole, bromuconazole, flutriafol and fenbuconazole. These results indicated
that to some triazoles, such as fenbuconazole, a high level of resistance was already
present in field populations.
In a sexually reproducing fungus such as T. yallundae, knowledge pertaining
to its ability to pass resistance factors to offspring is equally important. Mating
studies were, therefore, also conducted with parental strains that showed signs of
triazole resistance. Three generations were subsequently tested for resistance to five
triazoles, namely flusilazole, tebuconazole, propiconazole, bromuconazole and
flutriafol. Results of this study showed variable sensitivity in progeny, which
indicated quantitative inheritance of resistance to triazoles. Although the sexual stage has not yet been observed in the field in South Africa, this knowledge lays the
foundation for the long-term understanding of the population dynamics of the fungus.
The ability of a heterothallic ascomycete population to reproduce sexually is
dependent on the availability of its two mating types, MATI-I and MATI-2, their
distribution, and female fertility amongst other factors. In the UK. the teleomorph is
commonly observed in the field, which is in contrast to the situation in South Africa,
where it has only been induced in the laboratory. A comparative study between the
South African and the UK. populations was therefore undertaken. Isolates
representative of the two populations were mated with tester strains as both sperm
recipients and as sperm donors. This allowed the percentage of hermaphrodites to be
determined. No difference in terms of female fertility was observed between the
South African and the UK. populations, with both populations showing low effective
population numbers. These data suggested, therefore, that the teleomorph would also
occur more frequently in South Africa if the climate was more indusive to its
development.
The overall results of this study indicated that eyes pot could still be controlled
by means of fungicide application in South Africa. Although a shift in sensitivity was
observed towards fenbuconazole and flusilazole, no resistance was detected towards
carbendazim. The latter might be due to the absen<.:eof the sexual stage in the field,
coupled by the monocyclic nature of the pathogen and sensible fungicide regimes.
The absence of T. acujormis makes the disease situation less complicated in terms of
fungicide application and management. Continuous surveys will have to be
conducted, however, to monitor this situation in future. / AFRIKAANSE OPSOMMING: Hierdie studie ondersoek die genetiese variasie, reproduksie dinamika en fungisied
weerstand in Tapesia yallundae. Elke hoofstuk handel oor spesifieke maar verwante
onderwerpe. Oogvlek is 'n belangrike siekte van lentekoring (Triticum aestivum L.).
Vier spesies van Ramulispora word geassosieer met die siekte, waarvan Tapesia
yallundae en T. acuformis mees algemeen voorkom. T. yallundae, wat tans die enigste
spesie is wat in Suid-Afrika aangeteken is, het al verliese van tot 50% veroorsaak. Om
meer akkurate en effektiewe beheermaatreels te implementeer, is dit noodsaaklik om die
oorlewingsdinamika van die patogeen te verstaan. Van al die siektebeheermaatreels soos
kulturele praktyke, sanitasie, biologiese beheer ens., bly fungisiedbehandeling die mees
algemene maatreel vir die beheer van oogvlek.
Fungisiedtoediening het egter ook verskeie probleme. Die patogeen kan
weerstand opbou teen die fungisied. Die mees algemene fungisiedes wat vir
oogvlekbeheer aangewend word sluit onder meer die benzimidasool karbendazim in,
triasole soos flusilasool, tebukonasool, propikonasool, bromukonasool, flutriafol,
fenbukonasool, triadimenol, en die imidasool, prochloraz. Weerstand is egter reeds teen
hierdie middels bekend. Gedurige monitering vir weerstand is dus krities om die
vermorsing van fungisied en besoedeling van die omgewing met oneffektiewe middels te
beperk. In hoofstuk 2 van hierdie manuskrip word 300 isolate van T. yallundae van 15
lande geevalueer vir weerstand teenoor karbendazim, flusilasool, tebukonasool,
propikonasool, bromukonasool, flutriafol en fenbukonasool. Resultate dui daarop dat
teen sommige van hierdie triasole, soos bv. fenbukonasool, daar reeds 'n hoe vlak van
weerstand teenwoordig was in veldpopulasies.
In 'n seksueel reproduserende fungus soos T. yalluJ1dae, is dit noodsaaklik om te
bepaal wat sy vermoe is om weerstandbiedenheid aan die nageslag oor te dra. Om die
rede is paringstudies ook op ouers wat tekens van weerstand teenoor triasole getoon het
uitgevoer. Drie generasies was gevolglik getoets vir weerstand teenoor vyf triasole,
naamlik flusilasool, tebuconasool, propikonasool, brumukonasool en flutriafol. Resultate
van die studie het 'n variasie in sensitiwiteit van die nageslag getoon, wat op 'n
kwantitatiewe oorerwing van weerstand teen £riasole dui. Alhoewel die teleomorf nog nie in lande in Suid-Afrika opgemerk is nie, Ie hierdie kennis die fondament vir die
langtermyn vertolking van die populasie dinamika van hierdie fungus.
Die vermoe van 'n heterotalliese askomiseet populasie om seksueel voort te plant
is afhanklik van die beskikbaarheid van sy twee paringstipes, MATI-I en MATl-2, hul
verpreiding, vroulike vrugbaarheid en ander faktore. Alhoewel die teleomorf algemeen
in lande in die Verenigde Koninkryk opgemerk word, is dit in kontras met die situasie in
Suid-Afrika, waar hierdie stadium nog slegs in die laboratorium gelnduseer kon word. 'n
Studie is dus onderneem om die Suid-Afrikaanse en V.K. populasies met mekaar te
vergelyk. Isolate van die twee populasies is dus gepaar met paringsisolate as beide sperm
ontvangers en sperm donors. Hierdie prosedure het dit moontlik gemaak om die
persentasie hermafrodiete te bepaal. Geen verskille in vroulike fertiliteit is tussen die
Suid-Afrikaanse en V.K. populasies bespeur nie, en beide populasies het ook 'n lae
effektiewe populasie getal getoon. Hierdie data het dus voorgestel dat die teleomorf ook
meer algemeen in Suid-Afrika sou voorkom as die klimaat meer geskik was vir teleomorf
vormmg.
Die resultate van hierdie studie het tot die slotsom gelei dat oogvlek steeds deur
fungisiedbehandeling in Suid-Afrika beheer kan word. Alhoewel daar 'n merkbare
verskuiwing in sensitiwiteit teenoor fenbukonasool en flusilasool was, was geen
weerstand teenoor karbendazim waargeneem nie. Laasgenoemde kan dalk toegeskryf
word aan die afwesigheid van die teleomorf in die veld, gekombineer met die
monosikliese natuur van die patogeen en gebruik van alternerende fungisiedes. Die
afwesigheid van T. acuformis maak die plaaslike siektetoestand minder gekompliseerd in
terme van fungisied aanwending en bestuur. Voortdurende opnames sal egter uitgevoer
moet word om hierdie situasie ook in die toekoms te monitor.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/55834 |
| Date | 12 1900 |
| Creators | Ntushelo, Khayalethu |
| Contributors | Crous, P. W., Stellenbosch University. Faculty of AgriSciences. Dept. of Plant Pathology. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | 95 pages : illustrations |
| Rights | Stellenbosch University |
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