Coutinho, TeresaWingfield, Michael J.Wolfaardt, FrancoisMaseko, Bongani2017-06-062017-06-061999-03http://hdl.handle.net/11660/6333English: Research conducted in this thesis explores the role that Phytophthora and Pythium species play in citrus and eucalypt root rot in South Africa. The first chapter presents an extensive literature review with special emphasis on the importance of Phytophthora and Pythium spp. to the South African Citrus and Forestry Industries. This chapter is divided .into two parts. Part one deals with biotic and abiotic factors that contribute to citrus decline. Part two focuses on recently published work on root diseases of exotic forest trees species planted 'commercially in South Africa. Special emphasis was placed on P. cinnamomi since it is thought to be the most important pathogen in the forestry industry. Results obtained in Chapter Two indicate that a wide variation in tolerance and susceptibility to P. cinnamomi exits amongst half-sib families of E. fraxinoides. Field mortality of 52 % and 30% was recorded at two trial sites following natural infection. Seven disease tolerant families were identified with the potential to be used for commercial propagation. Eucalyptus fraxinoides families tolerant to P. cinnamomi were more reliably identified using stem inoculation of young trees relative to selecting families that survived the disease after planting in the .field. In Chapter Three half-sib families of E. fraxinoides and E. smithii were evaluated for tolerance to P. cinnamomi using stem inoculations in the greenhouse. Three replicate trials for each species were conducted at different times of the year. Disease tolerance and susceptibility varied between and among the different E. fraxinoides and E. smithii families. No correlation was found between the results obtained from the replicate trials of E. fraxinoides and E. smithii. The technique used was found to be unreliable for screening E. fraxinoides and E. smithii families for tolerance to P. cinnamomi. Results of a preliminary survey in selected citrus nurseries and orchards in the Northern and Mpumalanga provinces of South Africa are presented in Chapter Five. A total of 320 Phytophthora and Pythium isolates were recovered from the rhizosphere of diseased citrus trees and diseased plant material. Phytophthora nicotianae and Pythium irregulare where the most frequently isolated species. Sixteen isolates of P. citrophthora were successfully recovered. This species has previously been presumed to be absent from the sampled regions. In the last Chapter of this thesis, pathogenicity tests were conducted on Phytophthora and Pythium spp. associated with citrus root rot. Phytophthora isolates proved to be pathogenic when inoculated into citrus fruit while Pythium spp. where either weakly or non pathogenic. Contrary results were found when the lupin assay was conducted since all Pythium spp. proved pathogenic. Phytophthora isolates were also found to be pathogenic on Rough Lemon (RL) and Troyer Citrange (RTC) rootstocks whereas Pythium spp were either weakly pathogenic or avirulent. No difference in susceptibility was observed between RL and RTC. The fruit _assay and stem inoculation techniques were found to be reliable, inexpensive and rapid results were obtained. Results obtained in this study indicated that Phytophthora spp. play an important role in citrus and eucalypt root rot. Artificial stem inoculation using a single virulent isolate of P.cinnamomi proved to be a reliable technique for screening young eucalyptus trees in the field. However, contrary results were obtained when this technique was used to screen half-sib seedlings of E. fraxinoides and E. smithii in the greenhouse. Eight Pythium and two Phytophthora spp. were isolated from the rhizosphere soil and declining citrus trees. Phytophthora species proved to be pathogenic when inoculated into citrus fruit and on Rough Lemon and Troyer Citrange rootstocks. On the other hand Pythium species screened proved to be nonpathogenic or only weakly pathogenic and thus probably play an insignificant role in citrus decline.Afrikaans: Die navorsing in hierdie tesis handeloor die rol wat Phytophthora en Pythium spesies in sitrus en Eucalyptus wortelvrot speel. Die eerste hoofstuk behels 'n uitgebreide literatuur-oorsig met spesifieke verwysing na die-belangrikheid van Phytophthora en Pythium spesies in die Suid- hulle op sitrusvrugte geïnokuleer is, terwyl Pythium spesies swak of glad nie patogenies was nie. Teenstrydig met 19. resultate het die lupin analiese egter gewys dat alle Pythium spesies patogenies is. Phytophthora isolate was- patogenies op Rough Lemon (RL) en Troyer Citrange (RTC) wortelstok variëteite, terwyl Pythium spesies swak patogenies of avirulent was op hierdie variëteite. Daar was geen verskil in vatbaarheid tussen RL en RTC nie. Die vrugte- en stam-inokulasie tegnieke het betroubare, ekonomiese en vinnige resultate gelewer. Die resultate van hierdie studie het gewys dat Phytophthora spesies 'n belangrike rol speel in sitrus en Eucalyptus wortelvrot. Stam-inokulasies met 'n enkele virulente P. cinnamomi isolaat kon suksesvol gebruik word om te selekteer vir toleransie in jong bome in die veld. Die tegniek het egter onbetroubare resultate gelewer tydens die ondersoek na toleransie in E. fraxinoides en E. smithii in glashuise. Agt Pythium en Phytophthora spesies is geïsoleer van die risosfeer van siek sitrus-plante en die plantmateriaal self. Stam-inokulasies het gewys dat Phytophthora spesies patogenies is op Rough Lemon en Troyer Citrange wortelstok variëteite. Pythium spesies aan die ander kant was nie patogenies nie of swak patogenies op hierdie wortelstok variëteite en speel waarskynlik 'n minder belangrike rol in sitrusboom agteruitgang. Afrikaanse sitrus- en bosbouindustrieë. Hierdie hoofstuk bestaan uit twee dele. Die eerste deel fokus op die biotiese en abiotiese faktore wat aanleiding gee tot die agteruitgang van sitrusbome. Die tweede deel fokus op onlangse publikasies oor wortelsiektes van eksotiese bome wat kommersieel in Suid-Afrika geplant word. Spesiale aandag is aan P. cinnamomi gegee, aangesien dit as die belangrikste wortelpatogeen in die bosbou-industrie gesien word. Resultate in hoofstuk twee bewys dat daar aansienlike variasie in toleransie en vatbaarheid is in half-sib families van Eucalyptus fraxinoides teenoor P. cinnamomi. In twee E. fraxinoides proewe is onderskeidelik 52% en 30 % mortaliteit aangeteken na natuurlike infeksie deur P. cinnamomi. Sewe weerstandbiedende families is vir :potensiële gebruik in kommersiële propagering geïdentifiseer. Weerstandbiedendheid van E. fraxinoides families teenoor P. cinnamomi is beter geïdentifiseer met stam-inokulasies van jong bome, as met oorlewing na natuurlike infeksie in die veld. In hoofstuk drie word half-sib families van E. fraxinoides en E. smithii geëvalueer vir hul toleransie vir P. cinnamomi met behulp van stam-inokulasies. Drie herhalings van die proewe is gedoen vir elke spesie op verskillende tye van die jaar. Siekte toleransie en vatbaarheid het gevarieer tussen en binne E. fraxinoides en E. smithii families. Geen korrelasie kon egter gevind word tussen die resultate van die herhalings van die proewe met E. fraxinoides en E. smithii. Inokulasies in glashuise was dus nie betroubaar vir selektering van E. fraxinoides en E. smithii families vir toleransie vir P. cinnamomi nie. Die resultate van 'n voorlopige ondersoek na die invloed van wortelsiektes in geselekteerde sitruskwekerye en -boorde in die Noordelike Provinsie en Mpumalanga in Suid Afrika word in hoofstuk vyf gegee. 'n Totaal van 320 Phytophthora en Pythium isolate is geïsoleer vanuit die risosfeer van siek sitrusbome en plant-materiaal. Phytophthora nicotianae en Pythium irregulare was die mees algemeen geïsoleerde spesies. Sestien isolate van P. citrophthora is ook gevind. Hierdie spesie is vroeër as afwesig in die gemelde gebiede aangeteken. In die laaste hoofstuk van die tesis is patogenisiteits toetse gedoen met Phytophthora en Pythium spesies wat geassosieer word met sitrus wortelvrot. Phytophthora isolate was patogenies wanneer hulle op sitrusvrugte geïnokuleer is, terwyl Pythium spesies swak of glad nie patogenies was nie. Teenstrydig met 19. resultate het die lupin analiese egter gewys dat alle Pythium spesies patogenies is. Phytophthora isolate was- patogenies op Rough Lemon (RL) en Troyer Citrange (RTC) wortelstok variëteite, terwyl Pythium spesies swak patogenies of avirulent was op hierdie variëteite. Daar was geen verskil in vatbaarheid tussen RL en RTC nie. Die vrugte- en stam-inokulasie tegnieke het betroubare, ekonomiese en vinnige resultate gelewer. Die resultate van hierdie studie het gewys dat Phytophthora spesies 'n belangrike rol speel in sitrus en Eucalyptus wortelvrot. Stam-inokulasies met 'n enkele virulente P. cinnamomi isolaat kon suksesvol gebruik word om te selekteer vir toleransie in jong bome in die veld. Die tegniek het egter onbetroubare resultate gelewer tydens die ondersoek na toleransie in E. fraxinoides en E. smithii in glashuise. Agt Pythium en Phytophthora spesies is geïsoleer van die risosfeer van siek sitrus-plante en die plantmateriaal self. Stam-inokulasies het gewys dat Phytophthora spesies patogenies is op Rough Lemon en Troyer Citrange wortelstok variëteite. Pythium spesies aan die ander kant was nie patogenies nie of swak patogenies op hierdie wortelstok variëteite en speel waarskynlik 'n minder belangrike rol in sitrusboom agteruitgang.enEucalyptus -- Diseases and pests -- South AfricaCitrus -- Diseases and pests -- South AfricaFungal diseases of plants -- South AfricaPythiumPhytophthoraDissertation (M.Sc. (Microbiology and Biochemistry))--University of the Free State, 1999DissertationUniversity of the Free State