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Quantification of apple replant pathogens from roots, and their occurrence in irrigation water and nursery trees

Moein, Shahrooz (2016-03)

Thesis (MSc)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: Apple replant disease (ARD), an economically important disease of apple, occurs when young apple trees are planted on soil previously cultivated to apple or closely related species. Soil is considered as the main inoculum source of pathogens causing ARD, which include some Rhizoctonia spp., Cylindrocarpon-like spp., parasitic nematodes (Pratylenchus spp.) and oomycetes (Pythium, Phytopythium and Phytophthora). However, additional inoculum sources might be nursery trees and irrigation water. Investigations into inoculum sources, and effective disease management strategies, require knowledge on relationships between pathogen quantification techniques and the extent of plant damage. The relationship between quantification techniques [percent root infection, and pathogen DNA biomass (absolute and relative)] and disease in apple seedlings were investigated for the ARD pathogens Pythium sylvaticum, Pythium irregulare, Pythium ultimum, Phytopythium vexans and Phytophthora cactorum in glasshouse trials. Quantification data from natural P. irregulare nursery infections were also investigated. In glasshouse trials, the percent root infection and pathogen DNA biomass quantities (absolute and relative) were good predictors of apple seedling growth reductions for P. sylvaticum, Pht. vexans and P. ultimum, when targeting the fine feeder root system. Significant and high correlations were also found for these pathogens between pathogen DNA biomass and percent infected roots. This, however, was not true for Ph. cactorum and P. irregulare. For P. irregulare only a low, but significant correlation was found between percent root infection and seedling stunting. In the nursery where P. irregulare was investigated, good and significant correlations were found between percent root infection and pathogen DNA biomass (absolute and relative). Apple nursery trees and irrigation water (only oomycetes) were investigated as ARD inoculum sources over two seasons in the Western Cape province of South Africa. Nursery trees were a large inoculum source of P. irregulare, Cylindrocarpon-like spp. and Pratylenchus spp., and to a lesser extent of P. ultimum. Pythium sylvaticum was rarely detected, whereas Ph. cactorum and Pht. vexans were absent in nursery trees. The nurseries differed in the occurrence of trees infected with specific pathogens, but none were free of ARD pathogens. Almost all trees were infected with P. irregulare (95%) and Cylindrocarpon-like species (100%). Pythium ultimum was present in 60% of nurseries and 41% of trees. An average of 35% of trees were infested with Pratylenchus spp., with some trees (6-22%) having unacceptably high infestation levels. Irrigation water analyses in dams, and at the exit point of irrigation lines over a 5 month period in each of two seasons revealed the presence of a few ARD pathogens. Only P. irregulare was considered as important and was detected in each of the sampled months in 31 to 82% of the orchard water samples, and the pathogen also occurred in dam water. Pythium ultimum, Phytopythium litorale and Pythium spp. complex B2A were rarely detected in orchard water. The study demonstrated that percentage root infections and pathogen DNA biomass quantity (absolute and relative) can provide a good prediction of apple seedling growth reduction for P. sylvaticum, Pht. vexans and P. ultimum, but not Ph. cactorum and P. irregulare. Oomycete pathogen DNA biomass in roots can be quantified equally well using relative or absolute qPCR amplification. Knowledge was also gained on the colonization pattern of the root system by different oomycete pathogens. Apple nursery and irrigation water of apple orchards were shown to be inoculum sources of apple replant pathogens by using two approaches, conventional isolation and qPCR assay. qPCR significantly improved detection of ARD pathogens relative to conventional isolations. For nurseries, rootstock management strategies will be required. The biological significance of ARD oomycete pathogens in irrigation water requires further investigation.

AFRIKAANSE OPSOMMING: Appel herplant siekte (AHS), ‘n ekonomies belangrike siekte van appels, kom voor wanneer jong appelboompies geplant word op grond waarop daar voorheen appels of naby-verwante spesies geplant was. Grond word gesien as die hoof inokulumbron van patogene wat AHS veroorsaak, wat sommige Rhizoctonia spp., Cylindrocarpon-agtige spp., parasitiese nematodes (Pratylenchus spp.) en oömisete (Pythium, Phytopythium en Phytophthora) insluit. Addisionele inokulumbronne kan egter kwekerybome en besproeiingswater wees. Ondersoeke na inokulumbronne, en effektiewe siektebestuurstrategieë, vereis kennis oor die verhoudings tussen patogeen kwantifiseringstegnieke en die mate van plantskade. Die verhouding tussen kwantifiseringstegnieke [persentasie wortel-infeksie, en patogeen DNS biomassa (absoluut en relatief)] en siekte in appelsaailinge, is vir die AHS patogene Pythium sylvaticum, Pythium irregulare, Pythium ultimum, Phytopythium vexans en Phytophthora cactorum in glashuisproewe ondersoek. Kwantifiseringsdata vanaf natuurlike P. irregulare kwekery-infeksies is ook ondersoek. In glashuisproewe was die persentasie wortel-infeksie en patogeen DNS biomassa hoeveelhede (absoluut en relatief) goeie voorspellers van appelsaailinggroei vermindering vir P. sylvaticum, Pht. vexans en P. ultimum, wanneer die fyn voedingswortelsisteem aangeval word. Betekenisvolle en hoë korrelasies is ook vir hierdie patogene gevind tussen patogeen DNS biomassa en persentasie geïnfekteerde wortels. Dit was egter nie waar vir Ph. cactorum en P. irregulare nie. Vir P. irregulare is slegs ‘n lae, maar betekenisvolle korrelasie tussen wortel-infeksie en saailing verdwerging waargeneem. In die kwekery waar P. irregulare ondersoek is, is goeie en betekenisvolle korrelasies tussen persentasie wortel-infeksie en patogeen DNS biomassa (absoluut en relatief) waargeneem. Appel kwekerybome en besproeiingswater (slegs oömisete) is as AHS inokulumbronne, oor twee seisoene in die Wes-Kaap Provinsie van Suid-Afrika, ondersoek. Kwekerybome was ‘n groot inokulumbron van P. irregulare, Cylindrocarpon-agtige spp. en Pratylenchus spp., en tot ‘n minder mate van P. ultimum. Pythium sylvaticum is bykans nooit waargeneem nie, terwyl Ph. cactorum en Pht. vexans afwesig in kwekerybome was. Die kwekerye het verskil in die voorkoms van bome geïnfekteer met spesifieke patogene, maar geen een was vry van AHS patogene nie. Amper alle bome was geïnfekteer met P. irregulare (95%) en Cylindrocarpon-agtige spesies (100%). Pythium ultimum was teenwoordig in 60% van die kwekerye en 41% van die bome. ‘n Gemiddeld van 35% van die bome was geïnfesteer met Pratylenchus spp., met sommige bome (6-22%) wat onaanvaarbare hoë infestasie-vlakke gehad het. Besproeiingswater analises in damme, en by die uitgangspunte van besproeiingslyne oor ‘n 5 maand periode, in elk van twee seisoene, het op die teenwoordigheid van ‘n paar ARD patogene gedui. Slegs P. irregulare is as belangrik beskou en is in elk van die monsterneem maande in 31 tot 82% van die boordwatermonsters waargeneem, en die patogeen het ook in damwater voorgekom. Pythium ultimum, Phytopythium litorale en Pythium spp. kompleks B2A is selde in boordwater waargeneem. Die studie het getoon dat persentasie wortel-infeksies en patogeen DNS biomassa hoeveelheid (absoluut en relatief) ‘n goeie aanduiding kan verskaf vir appelsaailing groeivermindering vir P. sylvaticum, Pht. vexans en P. ultimum, maar nie vir Ph. cactorum en P. irregulare nie. Oömiseet patogeen DNS biomassa in wortels kan ewegoed gekwantifiseer word deur die gebruik van relatiewe en absolute qPCR amplifisering. Kennis is ook opgedoen oor die kolonisasie-patroon van die wortelsisteem deur verskillende oömiseet patogene. Appelkwekerye en besproeiingswater van appelboorde is aangedui as inokulumbronne van appel herplant patogene deur die gebruik van twee benaderings, konvensionele isolasie en qPCR toetse. qPCR het waarneming van AHS patogene betekenisvol verbeter relatief tot konvensionele isolasies. Onderstam bestuurstrategieë sal in kwekerye toegepas moet word. Die biologiese belang van AHS oömiseet patogene in besproeiingswater benodig verdere ondersoek.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/98329
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