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Spectrum of in-vitro activity and efficacy of phosphonates for management of apple replant disease and Oomycete root rot pathogens in South Africa

dc.contributor.advisorMcLeod, Adeleen_ZA
dc.contributor.advisorMazzola, M.en_ZA
dc.contributor.authorNyoni, Makomboreroen_ZA
dc.contributor.otherStellenbosch University. Faculty of AgriSciences. Dept. of Plant Pathology.en_ZA
dc.date.accessioned2018-02-28T13:58:02Z
dc.date.accessioned2018-04-09T11:49:03Z
dc.date.available2020-01-01T03:00:11Z
dc.date.issued2018-03
dc.identifier.urihttp://hdl.handle.net/10019.1/103961
dc.descriptionThesis (PhDAgric)--Stellenbosch University, 2018.en_ZA
dc.description.abstractENGLISH ABSTRACT: In South Africa, apple replant disease (ARD) and Phytophthora root rot are two soilborne diseases that are important in apple production. ARD occurs when old apple orchards are replanted, causing a reduction in tree growth. Several biotic agents are involved. In South Africa, a few Pythium spp. and Phytophthora cactorum were previously shown to be prominent in the development of ARD, with Pratylenchus spp. occasionally being involved. Phytophthora root rot, caused by P. cactorum, most often becomes problematic in the 2nd or 3rd year post-plant causing tree death and reduced tree growth. The pathogen is most likely introduced through nursery trees, irrigation water and residual soil populations (unfumigated inter-row strips). Management of ARD mainly consists of preplant fumigation of tree rows with chloropicrin/1,3-dichloropropene. Phytophthora root rot can be controlled using phosphonate fungicides, but these are not registered for apples in South Africa. Phosphonates, which breaks down to phosphite in plants, are highly mobile in plants and can reduce disease through a direct toxic effect towards pathogens or the induction of host plant defences. Three orchard trials were conducted to determine whether ARD can be managed using semi-selective chemicals and different chloropicrin formulations. In all three trials, tree growth (trunk diameter and shoot growth) was improved significantly relative to the control by preplant fumigation with either of two formulations of chloropicrin/1,3-dichloropropene (formulations containing chloropicrin at 60.8% or 57.0%), or with a postplant semi-selective treatment programme that included applications of fenamiphos, phosphite, imidacloprid and metalaxyl. Yield increases did not always accompany the tree growth increases. In one orchard, yield was only increased significantly by combining semi-selectives with a fumigation treatment, whereas in the other two orchards all fumigation treatments significantly increased yield. Phytophthora cactorum and Pratylenchus spp. likely interacted synergistically and were important ARD pathogens. In a second set of two orchard trials, the temporal nature of root phosphite concentrations in asymptomatic apple trees [trees where oomycete pathogens were present in roots, but no foliar symptoms were evident] was examined following different methods of application of phosphonates (foliar sprays, stem sprays, soil drenching and trunk paints) applied in summer and fall. A trunk paint application, was the best application method based on root phosphite concentrations. Foliar sprays, which were only applied in summer, also showed potential based on root phosphite concentrations. Phytophthora cactorum and Pythium irregulare DNA quantities in the roots of trees receiving phosphonate treatments were significantly lower than the quantities in the control treatment. In vitro studies showed that medium type (liquid or solid) and phosphate concentration significantly influenced the percentage mycelial growth inhibition of P. cactorum and P. irregulare by phosphite. This made it problematic to assess the relative effect of root phosphite concentrations as a determinant of pathogen suppression in orchard tree roots. A third set of trials were conducted, aimed at evaluating the curative efficacy of phosphonates in three apple orchards with Phytophthora root symptoms. Different phosphonate application methods (foliar sprays, trunk sprays and trunk paints), yielded similar levels of shoot growth in trees, which was significantly better than the control in two trials in the Grabouw region after 11-months, but not in the Koue Bokkeveld trial. Yield data could only be obtained in the latter trial, which was also not significantly increased by phosphonate applications. In the two Grabouw trials, all application methods yielded relative high root phosphite concentrations for fall phosphonate applications 13-weeks postapplication, but not in the Koue Bokkeveld trial. In all three trials, P. cactorum root quantities were not reduced by any of the phosphonate treatments. The study showed that phosphonates have potential for managing Phytophthora root rot in apple orchards. Phosphonates combined with other semi-selective chemicals (fenamiphos, imidacloprid and metalaxyl), can also be used to manage ARD. The relationship between phosphite concentrations required in tree roots for suppression of P. cactorum and P. irregulare, and phosphite concentrations required for pathogen suppression in vitro is unclear due to (i) various factors influencing the in vitro sensitivity of isolates and (ii) the seasonal fluctuation of root phosphite concentration in apple trees. Future work should focus on determining whether root phosphite concentrations are important for direct pathogen suppression by co-quantification of root phosphite and pathogens in time course studies in orchard trials. Furthermore, the effect of root phosphite concentrations on host plant defence induction must be investigated.en_ZA
dc.description.abstractAFRIKAANSE OPSOMMING: In Suid-Afrika is appel herplantsiekte (AHS) en Phytophthora wortelvrot twee grondgedraagde siektes wat belangrik is in appelverbouing. AHS kom voor wanneer ou appelboorde herplant word, en veroorsaak ‘n vermindering in boomgroei. Verskeie biotiese agente is betrokke. In Suid-Afrika was ‘n paar Pythium spp. en Phytophthora cactorum voorheen aangewys as prominent in die ontwikkeling van AHS, met Pratylenchus spp. soms betrokke. Phytophthora wortelvrot, veroorsaak deur P. cactorum, word meestal problematies in die tweede en derde jaar ná plant, en veroorsaak boom-afsterwing en verminderde boomgroei. Die patogeen word heel waarskynlik deur kwekerybome, besproeiingswater en oorblywende grondpopulasies (nie-berookte tussen-ry stroke) ingebring. Bestuur van AHS bestaan hoofsaaklik uit vóór-plant beroking van boomrye met chloropikrien/1,3- dichloropropeen. Phytophthora wortelvrot kan beheer word deur die gebruik van fosfonaat fungisiedes, maar dit is nie vir appels in Suid-Afrika geregistreer nie. Fosfonate, wat in plante na fosfiet afbreek, is hoogs beweeglik in plante en kan siekte verminder deur ‘n direk toksiese effek teenoor die patogene, óf deur die induksie van gasheer plantverdedigings. Drie boordproewe is uitgevoer om te bepaal of AHS bestuur kan word deur die gebruik van semi-selektiewe chemikalieë en verskillende chloropikrien formulasies. In al drie proewe is boomgroei (stamdeursnit en lootgroei) betekenisvol verbeter relatief tot die beheer deur vóór-plant beroking met enige van twee formulasies van chloropikrien/1,3- dichloropropeen (formulasies bevattende chloropikrien teen 60.8% of 57.0%), of met ‘n náplant semi-selektiewe behandelingsprogram wat toedienings van fenamifos, fosfiet, imidacloprid en metalaksil insluit. Opbrengs toenames het nie altyd met die boomgroei toenames saamgegaan nie. In een boord is opbrengs slegs betekenisvol verhoog wanneer semi-selektiewe chemikalieë met ‘n berokingsbehandeling gekombineer is, terwyl in die ander twee boorde alle berokingsbehandelings opbrengs betekenisvol verhoog het. Phytophthora cactorum en Pratylenchus spp. het moontlik sinergisties op mekaar gereageer en was belangrike AHS patogene. In ‘n tweede stel van twee boordproewe, is die temporele aard van wortelfosfietkonsentrasies in asimptomatiese appelbome [bome waar oömiseet patogene in wortels teenwoordig was, maar geen blaarsimptome sigbaar was nie] ondersoek, volgende op verskillende metodes van toediening van fosfonate (blaarspuite, stamspuite, gronddrenkings en stamverwe) toegedien in die somer en herfs. ‘n Stamverf toediening was die beste toedieningsmetode gebaseer op wortelfosfietkonsentrasies. Blaarspuite, wat slegs in die somer toegedien is, het ook potensiaal getoon gebaseer op wortelfosfietkonsentrasies. Phytophthora cactorum en Pythium irregulare DNS hoeveelhede in die wortels van bome wat fosfonaat behandelings ontvang het, was betekenisvol laer as die hoeveelhede in die kontrole behandeling. In vitro studies het getoon dat mediumtipe (vloeistof of vastestof) en fosfaatkonsentrasie betekenisvol die persentasie miseliumgroei inhibisie van P. cactorum en P. irregulare deur fosfiet beïnvloed. Dit maak dit moeilik om die relatiewe effek van wortelfosfietkonsentrasies as ‘n determinant van patogeen onderdrukking in boord boomwortels vas te stel. ‘n Derde stel proewe is uitgevoer met die doel om die kuratiewe effektiwiteit van fosfonate in drie appelboord met Phytophthora wortelsimptome te evalueer. Verskillende fosfonaat toedieningsmetodes (blaarspuite, stamspuite en stamverwe) het soortgelyke vlakke van lootgroei in bome opgelewer, wat betekenisvol beter was as die kontrole in twee proewe in die Grabouw area ná 11 maande, maar nie in die Koue Bokkeveld proef nie. Opbrengs data kon slegs in die laaste proef verkry word, wat ook nie betekenisvol deur fosfonaat toedienings verhoog is nie. In die twee Grabouw proewe, het alle toedieningsmetodes relatiewe hoë wortelfosfietkonsentrasies vir herfs fosfonaat toedienings 13 weke ná toediening opgelewer, maar nie in die Koue Bokkeveld proef nie. In al drie proewe is P. cactorum wortel hoeveelhede nie deur enige van die fosfonaat behandelings verminder nie. Die studie het getoon dat fosfonate potensiaal het om Phytophthora wortelvrot in appelboorde te bestuur. Fosfonate, gekombineer met ander semi-selektiewe chemikalieë (fenamifos, imidacloprid en metalaksil), kan ook gebruik word om AHS te bestuur. Die verhouding tussen fosfietkonsentrasies nodig in boomwortels vir die onderdrukking van P. cactorum en P. irregulare, en fosfietkonsentrasies nodig vir patogeen onderdrukking in vitro is onduidelik weens (i) verskeie faktore wat die in vitro sensitiwiteit van isolate beïnvloed en (ii) die seisoenale fluktuasie van wortelfosfietkonsentrasie in appelbome. Toekomstige werk moet daarop fokus om vas te stel of wortelfosfietkonsentrasies belangrik is vir direkte patogeen onderdrukking deur ko-kwantifisering van wortelfosfiet en patogene in tydsverloopstudies in boordproewe. Verder moet die effek van wortelfosfietkonsentrasies op gasheerplant verdediging induksie ondersoek word.af_ZA
dc.format.extentx, 180 leaves : illustrations (some color)
dc.language.isoenen_ZA
dc.publisherStellenbosch : Stellenbosch Universityen_ZA
dc.subjectPhosphonatesen_ZA
dc.subjectReplant diseaseen_ZA
dc.subjectRoot rotsen_ZA
dc.subjectApples -- Diseases and pests -- Control -- South Africa -- Western Capeen_ZA
dc.subjectUCTDen_ZA
dc.subjectOomycetesen_ZA
dc.titleSpectrum of in-vitro activity and efficacy of phosphonates for management of apple replant disease and Oomycete root rot pathogens in South Africaen_ZA
dc.typeThesisen_ZA
dc.description.versionDoctoralen_ZA
dc.rights.holderStellenbosch Universityen_ZA
dc.embargo.terms2020-01-01


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