Thesis (MScAgric)--University of Stellenbosch, 2001. / ENGLISH ABSTRACT: Apple replant disease (ARD) is one of the major impediments to the establishment of
an economically viable apple orchard on sites previously planted to apple. In spite of
extensive research on ARD, the etiology remains to be fully elucidated. A possible
biological origin of ARD etiology in South Africa was investigated by the dilution of
replant field soil with sterilised soil. Commercial orchards with ARD were selected for
use in pot trials and disease severity evaluated after three months, by measuring
shoot length, dry mass of plants as well as root discolouration. Although diluting
replant soil to 25 and 50% (v/v) significantly reduced the effects of ARD, symptoms
were only absent in 0% replant soil. It was clear that seedlings planted in any
mixture containing replant soil, even only 25% replant soil, consistently exhibited
symptoms of stunted growth and root discolouration similar to those seedlings grown
in 100% replant soil. This indicates that ARD in South Africa is primarily of a
biological nature.
As an initial step in formulating sustainable disease control alternatives to replace
methyl bromide, pot trials were conducted to assess the impact of compost
treatments as well as biological control products on ARD. Compost as well as
sterilised and unsterilised compost teas (compost extract) significantly increased
seedling growth even under optimum nutrient conditions when compared to the
control, suggesting that they negate the effects of ARD. Results also indicated that
applying high concentrations of compost does not necessarily provide additional
growth benefits compared to lower concentrations. Results with biocontrol
formulations were less favourable. Only one of the biocontrol formulations, a
combination of Bacillus spp. (Biostart®) improved growth significantly compared to
the control. There was, however, some inconsistency with results for the different
trials conducted using this product.
Fungal as well as nematode populations associated with ARD soils were
characterised to the generic level to get a clearer understanding of the etiology of
ARD in South Africa. Pythium and Cylindrocarpon spp. were consistently isolated
from all six replant soils in all trials that formed part of this study, indicating that these
fungi may have a role in ARD etiology in South Africa. Nematodes implicated in ARD
development were inconsistently associated with ARD soils used in these studies.
This suggests that nematodes do not have a primary causal role in ARD etiology in
South Africa. Field trials were conducted in commercial orchards to assess the impact of organic
amendments and promising biological control products, as indicated by the pot trials,
on ARO severity under field conditions. These biological soil amendments were also
compared with the standard chemical control methods for ARO, methyl bromide and
chloropicrin. In all three trials established, compost and mulch as well as manure
and mulch, consistently increased growth to the same extent as the standard
chemical treatments and by combining these chemical treatments with organic
amendments a significant, additional growth increase could be attained. Biocontrol
formulations evaluated in field studies gave variable results. Biostart® improved
growth when applied on its own, but not in combination with the chemical Herbifume
(metham-sodium). Inoculating soil with effective microorganisms (EM), consisting
primarily of photosynthetic bacteria, had no significant effect on growth.
Results from this study indicate that application of organic amendments could
possibly substitute for soil fumigation in replanted apple orchards. However, compost
quality standards need to be implemented and because few types of compost are
universally effective, different types of composts should be compared in specific soil
environments before recommendations can be made. Oue to variable results with
biocontrol products, ARO management with these biological soil amendments cannot
be guaranteed at this stage and further studies are recommended. / AFRIKAANSE OPSOMMING: EVALUERING VAN DIE SPESIFIEKE APPELHERVESTIGING-PROBLEEM
IN BOORDE IN DIE WES-KAAP
Appelhervestiging-siekte (AHS) skep 'n groot probleem in die vestiging van jong
appelbome op grond waar daar reeds voorheen appels verbou is. Ten spyte van
omvangryke navorsing is die oorsaak van die probleem nog hoofsaaklik onbekend.
'n Moontlike biologiese oorsaakleer is in Suid-Afrika ondersoek deur die
hervestigings-effek te probeer verminder deur die vermenging van hervestigingsgrond
met gesteriliseerde grond. Kommersiële boorde met 'n appelhervestigingsprobleem
is geselekteer en gebruik in potproewe. Die ernstigheidsgraad van die
siekte is na drie maande se groei geevalueer deur lootlengte, droë massa en
wortelverkleuring te meet. Alhoewel verdunning van die hervestigingsgrond tot 50 en
25% (vlv) die effek van AHS op groei betekenisvol verminder het, kon die skadelike
effek van die veroorsakende faktor slegs uitgeskakel word deur saailinge in 100%
gesteriliseerde grond te plant. Dit was duidelik dat saailinge wat in enige
grondmengsel geplant is waarin hervestigingsgrond voorgekom het, selfs al was dit
net 25%, konsekwent simptome van vertraagde groei en wortelverkleuring getoon
het. Dit is 'n aanduiding dat AHS in Suid-Afrika hoofsaaklik biologies van aard is.
Potproewe is uitgevoer as 'n eerste stap in die formulering van volhoubare
siektebeheer-strategieë, om die impak van kompos-behandelings en biologiese
beheer produkte op AHS te ondersoek. Kompos sowel as gesteriliseerde en
ongesteriliseerde kompos-tee (kompos-water) het, selfs onder optimale voedingsomstandighede,
die groei van saailinge betekenisvol verbeter. Dit dui aan dat hierdie
behandelings die effek van AHS kan teenwerk. Resultate het ook daarop gedui dat
hoër kompos konsentrasies nie noodwendig enige addisionele voordele vir groei
inhou in vergelyking met laer konsentrasies nie. Resultate met biologiese beheer
produkte was minder gunstig. Slegs een van die produkte wat geëvalueer is, 'n
kombinasie van Bacillus spp. (Biostart®), het groei betekenisvol verbeter in
vergelyking met die kontrole. Resultate was egter inkonsekwent vir die verskillende
proewe waarin hierdie produk gebruik is.
Swampopulasies sowel as aalwurmpopulasies wat met hervestigingsgrond
geassosieer word, is geïdentifiseer tot op generiese vlak om vas te stel waardeur AHS in Suid-Afrika veroorsaak word. Pythium en Cylindrocarpon spp. is konsekwent
van al ses hervestigingsgronde geïsoleer wat daarop dui dat hierdie twee swamgenera
'n beduidende rol in AHS ontwikkeling in Suid-Afrika mag hê. Aalwurms wat
aangedui is in die literatuur om 'n moontlike rol in AHS te hê, was slegs in enkele
gevalle geassosieer met hervestigingsgronde waarvan in hierdie studie gebruik
gemaak is. Die gevolg-trekking is dus gemaak dat aalwurms nie 'n betekenisvolle rol
speel as hoof-veroorsakende organisme onder Suid-Afrikaanse toestande nie.
Veldproewe is uitgevoer in kommersiële appelboorde om vas te stel wat die effek van
organiese materiaal, asook belowende biologiese beheermiddels, soos aangedui
deur potproewe, op AHS onder veldtoestande is. Die biologiese grondtoedienings is
ook vergelyk met die standaard chemiese beheermiddels (metielbromied en
chloorpikrien). In al drie proewe wat gevestig is, het kompos met 'n deklaag, sowel
as kraalmis met 'n deklaag, groei betekenisvol verbeter tot dieselfde mate as
chemiese middels. Daar kon ook 'n beduidende, addisionele groeitoename gemeet
word in gevalle waar chemiese middels met organiese materiaal gekombineer is.
Resultate met biologiese beheer formulasies wat onder veldtoestande geëvalueer is,
het gevarieer. Biostart® het groei verbeter wanneer dit alleen toegedien is, maar in
kombinasie met die chemiese middel Herbifume (metham-sodium) het dit geen effek
gehad nie. Die inokulering van grond met 'n oplossing van effektiewe mikroorganismes
(EM) wat hoofsaaklik uit fotosinterende baterieë bestaan, het ook geen
betekenisvolle effek op groei gehad nie.
Die gevolgtrekking is gemaak dat toediening van organiese materiaal moonlik as
plaasvervanger vir metielbromied-beroking kan dien in die beheer van AHS. Die
nodige komposkwaliteit-standaarde moet egter eers geïmplimenteer word. Omdat
feitlik geen kompos universeel effektief kan wees nie, is dit ook nodig dat verskillende
tipes kompos met mekaar vergelyk moet word in spesifieke grondtoestande voordat
verdere aanbevelings gemaak kan word. As gevolg van variërende resultate met
biologiese beheer produkte kan AHS beheer met hierdie middels nie gewaarborg
word op hierdie stadium nie en verdere studies word aanbeveel.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/52108 |
| Date | 12 1900 |
| Creators | Rabie, Louise |
| Contributors | Cook, N. C., Denman, S., Stellenbosch University. Faculty of AgriSciences. Dept. of Horticulture. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | 106 p. : 1 map |
| Rights | Stellenbosch University |
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