Thesis (MScAgric)--University of Stellenbosch, 2002. / ENGLISH ABSTRACT: Historically root diseases have been a production-limiting problem for the strawberry
industry worldwide. In the Western Cape Province of South Africa the most serious
root disease is black root rot, which causes losses of up to 30%, annually. The aims
of this study were to investigate aspects of the etiology and epidemiology of this
disease in the Western Cape, and to provide information that can be incorporated in an
integrated disease management strategy. In Chapter I a summary of published
information on this disease is presented. Disease symptoms include severe stunting of
plants, which have black, rotted, reduced root systems. Even though this disease is of
great economic importance, the etiology remains unresolved. However, soilborne
fungal root pathogens, particularly Pythium and Rhizoctonia spp. have been
implicated as major role players. Control of this disease, as well as the other root
diseases affecting strawberries, has relied on soil fumigation with broad spectrum
chemical fumigants, in particular methyl bromide. However, due to the ozone
depleting action of methyl bromide it was decided at the signing of the Montreal
Protocol to remove this chemical from the market. This action has caused great
demand for alternative measures to control root diseases on many crops including
strawberries. Development of integrated disease management strategies is dependent
upon a more complete understanding of the etiology, biology and ecology of the
disease complex.
In Chapter 2 fungal pathogens associated with diseased plants were isolated
and Koch's postulates were carried out. The most frequently isolated fungal
pathogens were Pythium irregulare, Rhizoctonia spp. and Cylindrocarpon
destructans. Two morphotypes of Rhizoctonia were isolated viz. a brown and a white
type. Pythium irregulare was isolated more frequently in July than in September, and
was not isolated at all in November. Rhizoctonia spp. were present at all sampling
dates but were more frequently isolated in November than at the other times. All the
fungi that were tested were pathogenic and caused root lesions. Cylindrocarpon
destructans and Coniella fragariae did not have a stunting effect on the plants. These
results confirm a major role for Pythium and Rhizoctonia in the black root rot complex and suggest that there is a complimentary seasonal variation in occurrence
between these two pathogens.
The Rhizoctonia species and anastomosis groups of isolates obtained from
diseased strawberries in the Western Cape Province were determined, and their
pathogenicity and relative virulence assessed. Both binucleate and multinucleate
types were recovered from diseased roots and identified as R. fragariae and R. so/ani,
respectively. All isolates of R. solani were members of anastomosis group (AG) 6,
whereas three AG types were identified among isolates of R. fragariae, viz. AG-A,
AG-G and AG-I at a relative occurrence of 69%, 25%, 6% respectively. All
Rhizoctonia isolates were pathogenic to strawberry, but R. solani (AG 6) was the most
virulent causing severe stunting of plants. This is the first species confirmation and
AG type identification of Rhizoctonia taxa causing root rot of strawberries in South
Africa.
An assessment of the presence and quantity of black root rot pathogens
associated with soils prior to fumigation and post fumigation with methyl bromide,
was made in Chapter 4. Isolations were also made from nursery plants to determine
whether any black root rot pathogens were in the plants before transplanting. Results
demonstrated that after fumigation the soil was free of all pathogenic fungi associated
with the disease. However, the main pathogens involved in black root rot, viz.
Rhizoctonia fragariae, R. solani and Pythium spp. were isolated from nursery plants.
The fact that the plants are already infected with these pathogens renders the prospects
for control of this disease difficult. Further studies are urgently required to develop
production practices that can be included in disease management programmes.
In vitro studies were carried out to determine the ECso values of different
fungicides for isolates of Pythium irregulare, Rhizoctonia fragariae AG-A, AG-G and
AG-I and R. solani AG 6. Benomyl, fludioxonil and tolc1ofos-methyl were used in
these tests. Field trials were also conducted using these fungicides. In Chapter 5 it is
shown that in general application of fungicides improved the yield and did not affect
the survival rate of strawberry plants. Fludioxonil showed potential for short-term
use. Applications of fungicides that targeted specific fungal genera were not
sufficient to control the disease. Seasonal fluctuation of Pythium and Rhizoctonia
spp. became apparent with the occurrence of Pythium being relatively high early in
the season but low late in the season. Conversely, the occurrence of Rhizoctonia was
low at the beginning of the season but high late in the season. In the short-term there is potential for fungicide applications as part of an integrated disease management
strategy, but the economic feasibility of this practice needs to be assessed.
In this study the major pathogens causing black root rot were identified in the
Western Cape Province of South Africa, and important information regarding the
epidemiology of the disease was reported. These results can be incorporated in an
integrated management strategy to reduce losses of strawberry production, attJibutable
to black root rot. / AFRIKAANSE OPSOMMING: Wortelsiektes is wêreldwyd 'n produksie-beperkende probleem vir die aarbeibedryf. .
Swartwortelvrot, wat jaarliks verliese van tot 30% veroorsaak, is die belangrikste
wortelsiekte in die Wes-Kaap Provinsie van Suid-Afrika. Die doelwitte van hierdie
studie was om aspekte van die etiologie en epidemiologie van die siekte in die Wes-
Kaap te ondersoek en inligting wat in geïntegreerde siektebestuurstrategië ingesluit
kan word, te voorsien.
In Hoofstuk 1 word 'n opsomming van gepubliseerde inligting aangaande die
siekte uiteengesit. Siektesimptome sluit ernstige verdwerging van plante met swart
verotte en verkleinde wortelstelsels in. Alhoewel die siekte van groot ekonomiese
belang is, is die etiologie grootliks onbekend. Grondgedraagde wortelpatogene
swamme, spesifiek Pythium en Rhizoctonia spp., is egter as belangrike rolspelers
geïdentifiseer. Tot dusver het die beheer van hierdie siekte sowel as ander
wortelsiektes van aarbeie berus op grondberoking met breë spektrum chemiese
berokingsmiddels, spesifiek metielbromied. As gevolg van die osoonafbrekende
aksie van metielbromied is daar egter tydens die ondertekening van die Montreal
Protocol besluit om dié middel van die mark te verwyder. Hierdie besluit het 'n groot
aanvraag na alternatiewe beheermaatreëls vir wortelsiektes van verskeie gewasse,
insluitende aarbeie, veroorsaak. Die ontwikkeling van geïntegreerd
siektebestuurstrategieë is egter afhanklik van 'n meer volledige begrip van die
etiologie, biologie en ekologie van die siektekompleks.
In Hoofstuk 2 is die patogene swamme wat met die siekte geassosieer word,
geïsoleer, en is Koch se postulate uitgevoer. Die mees algemeen geïsoleerde patogene
swamme was Pythium irregulare, Rhizoctonia spp. en Cylindrocarpon destructans.
Twee morfotipes van Rhizoctonia is geïsoleer, nl. 'n bruin tipe en 'n wit tipe.
Pythium irregulare is meer dikwels in Julie as in September geïsoleer, maar glad nie
in November nie. Rhizoetonia het tydens alle monstertye voorgekom, maar is meer
dikwels in November geïsoleer. Al die swamme wat getoets is, was patogenies en het
letsels op die wortels veroorsaak. Cylindroearpon des true tans en Coniella fragariae
het nie'n verdwergingseffek op plante gehad nie. Hierdie resultate bevestig die dominante rol van Pythium en Rhizoctonia in die swartwortelvrot kompleks en dui op
'n komplementêre seisoenale variasie in die voorkoms van hierdie twee patogene.
Die Rhizoctonia spesies en anastomose groepe (AG) van die isolate geisoleer
vanaf siek aarbeiplante in die Wes-Kaap Provinsie is bepaal, en die patogenisiteit en
relatiewe virulensie is beraam. Sowel tweekernige as multikernige tipes is vanaf siek
wortels geïsoleer en respektiewelik as R. fragariae en R. so/ani geïdentifiseer. Alle
isolate van R. so/ani was lede van anastomose groep 6, terwyl drie AG tipes, nl. AGA,
AG-G en AG-I onder die R. fragariae isolate geïdentifiseer is met relatiewe
voorkomste van 69%, 25%, 6% respektiewelik. Alle Rhizoctonia isolate was
patogenies op aarbeie, maar R. so/ani (AG 6) was die mees virulente en het ernstige
verdwerging van plante veroorsaak. Hierdie is die eerste bevestiging van spesies en
identifisering van AG tipes van Rhizoctonia taksa wat wortelvrot van aarbeie in Suid
Afrika veroorsaak.
In Hoofstuk 4 is 'n beraming van die voorkoms en hoeveelheid
swartwortelvrot patogene geassosieer met grond voor, en na beroking met
metielbromied, gemaak. Isolasies is ook vanaf kwekeryplante gemaak om te bepaal
of enige swartwortelvrot patogene voor oorplanting in die plante teenwoordig was.
Die resultate het getoon dat grond na beroking vry was van alle patogeniese swamme
geassosieër met die siekte. Die hoof patogene betrokke in die swartwortelvrot
kompleks, nl. Rhizoctonia fragariae, R. so/ani en Pythium spp. was egter in die
kwekery plante teenwoordig. Die feit dat plante reeds met hierdie patogene
geïnfekteer is, maak die vooruitsigte vir die beheer van hierdie siekte moeilik.
Verdere studies word dringend benodig vir die ontwikkeling van produksiepraktyke
wat by siektebestuursprogramme ingesluit kan word.
In vitro studies om die ECso waardes van die isolate van Pythium irregulare,
Rhizoctonia fragariae AG-A, AG-G en AG-I en R. so/ani AG 6 vir die fungisiedes
benomyl, fludioxonil en tolclofos-metiel te bepaal, is uitgevoer. Hierdie fungisiedes
is ook in veldproewe getoets. In Hoofstuk 5 is getoon dat aanwending van
fungisiedes die opbrengs verbeter het en nie die oorlewing van aarbeiplante beïnvloed
het nie. Fludioxonil het potensiaal vir korttermyn gebruik getoon. Die aanwending
van fungisiedes wat spesifieke swamgenera teiken, was nie voldoende om die siekte
te beheer nie. Seisoenale fluktuasies van Pythium en Rhizoctonia spp. het duidelik
geword met die relatief hoë voorkoms van Pythium vroeg in die seisoen, maar lae
voorkoms laat in die seisoen, terwyl die voorkoms van Rhizoctonia laag was aan die begin van die seisoen, maar hoog later in die seisoen. In die korttermyn is daar
potensiaal vir fungisiedtoedienings as deel van 'n geïntegreerde
siektebestuurstrategie, maar die ekonomiese haalbaarheid van hierdie praktyk moet
bepaal word.
In hierdie studie is die hoof patogene wat swartwortelvrot van aarbeie in die
Wes-Kaap Provinsie van Suid-Afrika veroorsaak geïdentifiseer, en belangrike
inligting rakende die epidemiologie van die siekte is aangeteken. Hierdie resultate
kan in 'n geïntegreerde bestuurstrategie geïnkorporeer word om verliese van
aarbeiproduksie, toeskryfbaar aan swartwortelvrot te, verminder.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/52712 |
| Date | 12 1900 |
| Creators | Botha, Adri |
| Contributors | Denman, S., Lamprecht, S. C., 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 | 76 p. : ill. |
| Rights | Stellenbosch University |
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