Masters Degrees (Microbial, Biochemical and Food Biotechnology)

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Browsing Masters Degrees (Microbial, Biochemical and Food Biotechnology) by Advisor "Coutinho, Teresa"

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    Oomycetes associated with citrus and eucalypts root rot in South Africa
    (University of the Free State, 1999-03) Maseko, Bongani; Coutinho, Teresa; Wingfield, Michael J.; Wolfaardt, Francois
    English: 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.