Thesis (MSc)--University of Stellenbosch, 2001. / ENGLISH ABSTRACT: Phytophthora cactorum (Lebert & Cohn) Schrot. is the primary cause of crown, collar and
root rot diseases of apple (Malus domestica Borkh.) trees worldwide. This pathogen is most
destructive in commercial apple orchards under waterlogged soil conditions and has recently
been identified as causing serious disease in some South African apple orchards. Crown,
collar and root diseases are difficult to control because of their unpredictability and
catastrophic nature. The use of resistant cultivars and rootstocks is economical and
environmentally considerate. Therefore the need to develop screening techniques that will
enable the selection of desirable disease resistant traits as part of an apple-breeding program in
South Africa was identified. The work undertaken in this study was aimed at optimizing
different techniques to test resistance.
Using two direct inoculation techniques (excised stem and intact stem) the
aggressiveness of lO isolates of P. cactorum on apple rootstocks was determined. The
susceptibilities of five apple rootstocks were also compared. Results have shown isolate by
rootstock interaction which means isolate aggressiveness was influenced by rootstocks tested.
The selectivity of isolates suggests that there may be several strains of the pathogen.
Population studies of the pathogen might contribute valuable information that could lead to
better interpretation of results. Rootstock susceptibility was monitored in vitro throughout the
season by inoculating at monthly intervals for 26-months. It was observed that during winter,
rootstock susceptibility was low compared to high susceptibility during summer. These
results have revealed new information regarding changes in the relative resistance of the
different rootstocks over the growing season, e.g. the susceptibility pattern of rootstock
MMl06 occurred 1 to -2 months later than that of other rootstocks. This finding has
important implications on the way in which resistance test results are interpreted, and
emphasizes the importance of not relying on point sampling. Furthermore, useful information
has been acquired regarding the epidemiology of the disease with regard to "windows of
susceptibility". The phenomenon of a phase shift in susceptibility of different rootstocks needs to be tested on a broader scale to assess whether it has any practical application on
resistance testing.
Although different inoculation techniques are applied in breeding programs, up to now
there is no consensus on which technique works best for seedling selections. Since large
numbers of individuals must be tested to improve the chances of detecting resistant genotypes,
mass inoculations of young seedlings is a rapid way of identifying resistant individuals. Two
different screening methods were tested during this study. Using the sand-bran technique,
seedlings were transplanted onto inoculated soil and the root mass was used as a measure of
resistance. In a second method zoospore inoculum was applied to seedlings growing in a
sand:bark mixture at different concentrations and the seedlings were subjected either to water
drenching or not. In both trials the aggressiveness of isolates differed significantly from each
other and only higher inoculum concentrations were effective in causing disease. The age of
seedlings used in tests emerged as an important factor. Seedlings under five-months-old
should not be used. Drenching inoculated seedlings enhanced disease development but the
production of sufficiently high numbers of zoospores was a laborious task. Thus, it is
recommended that the sand-bran inoculum technique be tested with the drenching treatment
for mass selection.
In conclusion this study confirms the importance of both choice of isolate and choice of
inoculation intervals in determining susceptibility of rootstocks to infection. In spite of the
fact that stem inoculation bioassays have limited resemblance to natural disease situations,
these bioassays are useful for obtaining an indication as to whether genotypes have a degree of
resistance and merit further testing. For this reason refinement of the stem inoculation
bioassay is worthwhile pursuing. With regard to seedling trials, both the sand-bran and the
zoospore technique appear promising but refinement of these techniques is necessary in order
to present a more practical way of testing large volumes of seedlings. / AFRIKAANSE OPSOMMING: Evaluering van inokulasietegnieke om weerstand teen Phytophthora cactorum in appels
te evalueer:
Phytophthora cactorum (Lebert & Cohn) Schrot. is die primêre oorsaak van kroon-, kraag en
wortelvrot van appelbome (Malus domestica Borkh.). Dit is die mees verwoestende patogeen
in kommersiële appelboorde waar daar versuipte toestande grond voorkom. P. cactorum is
onlangs identifiseer as die patogeen wat ernstige kroon- en kraag-verotting in Suid Afrikaanse
appelboorde veroorsaak. Kroon-, kraag- en wortelvrot is moeilik om te beheer as gevolg van
die onvoorspelbaarheid en rampspoedige aard van die siekte. Die gebruik van kultivars en
onderstamme wat weerstandbiedend is teen siektes en plae is omgewingsvriendelik en is
ekonomies van belang, dus het die behoefte ontstaan om inokulasietegnieke te ontwikkelom
weerstandige saailinge te identifiseer en te selekteer as deel van 'n appelteelprogram in Suid
Afrika. Die doelwit van hierdie studie is om verskillende inokulasietegnieke te toets en te
verfyn om weerstand in appelsaailinge te identifiseer.
Deur gebruik te maak van twee inokulasietegnieke (die afgesnyde loot- en intakte loot
tegniek), is die relatiewe aggressiwiteit van 10 isolate van P. cactorum en die vatbaarheid van
vyf appelonderstamme ondersoek. Resultate het aangetoon dat die aggressiwiteit van die
isolate gevarieer het na aanleiding van die onderstam wat getoets is. Die selektiwiteit van die
isolate is 'n aanduiding dat daar moontlik verskeie rasse van die patogeen voorkom.
Toekomstige studies op die populasiestruktuur van P. cactorum sal 'n belangrike bydrae
maak tot die interpretasie van resultate oor weerstand en weerstandsteling.
Die vatbaarheid van onderstamme was ook in in vitro proewe ondersoek deur
maandelikse inokulasies toe te pas oor 'n tydperk van 26 maande. Dit is opgemerk dat die
onderstamvatbaarheid gedurende die winter laag was in vergelyking met die somer. Nie al die
onderstamme het dieselfe gereageer gedurende verskillende toetstye nie. Hierdie resultate
toon aan dat die relatiewe weerstand van verskillende onderstamme oor die groeiseisoen
verskil, byvoorbeeld die vatbare reaksie van die onderstam 'l\.1MI06' het een tot twee maande
later voorgekom in vergelyking met ander onderstamme wat getoets is. Hierdie bevinding het belangrike implikasies op die interpretasie van weerstandstoetsing en beklemtoon die
moontlike tekortkominge in enkelproefwaarnemings. Bruikbare inligting ten opsigte van die
epidemiologie van die siekte is versamel wat beskryf kan word in terme van vensters van
vatbaarheid wat verskil van onderstam tot onderstam. Verdere ondersoeke in die verband
word aanbeveel.
Hoewel verskeie inokulasietegnieke bestaan om jong saailinge vir weerstand te toets, is
daar tot op hierdie stadium nog nie ooreenstemming oor die beste tegniek wat toegepas moet
word om saailingseleksie te doen nie. Omdat groot getalle saailinge getoets moet tydens die
seleksieproses sal massa-inokulasie van saailinge die aangewese metode wees. Twee
verskillende inokulasie tegnieke is getoets in die studie. Deur gebruik te maak van die sandsemel
tegniek, is saailinge geplant in geinfesteerde plantmedium, waartydens die wortelmassa
van saailinge gebruik is om die reaksie op infeksie te kwantifiseer. Die soëspoor
inokulasietegniek was toegepas op saailinge wat in 'n sand en basmengsel geplant is teen
verskillende inokulurnkonsentrasies. 'n Waterverdrenkingsbehandeling is ook getoets. In
albei hierdie proewe het die aggressiwiteit van die isolate van mekaar verskil. Slegs die hoër
inokulumkonsentrasies was effektief in die ontwikkeling van die siekte. Die ouderdom van
saailinge is ook uitgewys as 'n belangrike faktor wat 'n rol speel in weerstandstoetsing.
Saailinge jonger as 5 maande word nie aanbeveel vir hierdie toetse nie. Verdrenking van
saailinge het die voorkoms van die siekte verhoog, maar die produksie van groot getalle
soëspore was 'n beperkende faktor in die uitvoering van die proef Dit word aanbeveel dat die
sand-semel inokulasietegniek verder evalueer moet word onder verskeie toestande, onder
andere deur dit met verdrenkinghte kombineer.
Die belang van die keuse van isolaat en inokulasiedatum in bepaling van relatiewe
weerstand van onderstamme teen P. cactorum is tydens die studie bevestig. Afgesien van die
beperking van die staminokulasietegnieke in soverre dit verwyderd is van natuurlike infeksie,
word die tegnieke aanbeveel om 'n indikasie te kry van die relatiewe weerstand van
onderstamme. Beide die sand-semel en soëspoor tegnieke kan gebruik word om weerstandige
saailinge te identifiseer, maar tegniese verfyning van hierdie tegnieke is nodig om saailinge in
massa te evalueer.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/52141 |
| Date | 03 1900 |
| Creators | Zondo, Patience Thembelihle |
| Contributors | Denman, Sandra, Labuschagne, Iwan, 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 | Unknown |
| Type | Thesis |
| Format | 89 p. : ill. |
| Rights | Stellenbosch University |
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