Oomycetes associated with citrus and eucalypts root rot in South Africa

[摘要] English: Research conducted in this thesis explores the role that Phytophthora and Pythium species play incitrus and eucalypt root rot in South Africa. The first chapter presents an extensive literature reviewwith special emphasis on the importance of Phytophthora and Pythium spp. to the South AfricanCitrus and Forestry Industries. This chapter is divided .into two parts. Part one deals with bioticand abiotic factors that contribute to citrus decline. Part two focuses on recently published work onroot diseases of exotic forest trees species planted 'commercially in South Africa. Special emphasiswas placed on P. cinnamomi since it is thought to be the most important pathogen in the forestryindustry.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% wasrecorded at two trial sites following natural infection. Seven disease tolerant families wereidentified with the potential to be used for commercial propagation. Eucalyptus fraxinoidesfamilies tolerant to P. cinnamomi were more reliably identified using stem inoculation of youngtrees 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 wereconducted at different times of the year. Disease tolerance and susceptibility varied between andamong the different E. fraxinoides and E. smithii families. No correlation was found between theresults obtained from the replicate trials of E. fraxinoides and E. smithii. The technique used wasfound 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 andMpumalanga provinces of South Africa are presented in Chapter Five. A total of 320 Phytophthoraand Pythium isolates were recovered from the rhizosphere of diseased citrus trees and diseased plantmaterial. Phytophthora nicotianae and Pythium irregulare where the most frequently isolatedspecies. Sixteen isolates of P. citrophthora were successfully recovered. This species haspreviously been presumed to be absent from the sampled regions.In the last Chapter of this thesis, pathogenicity tests were conducted on Phytophthora and Pythiumspp. associated with citrus root rot. Phytophthora isolates proved to be pathogenic when inoculatedinto citrus fruit while Pythium spp. where either weakly or non pathogenic. Contrary results werefound when the lupin assay was conducted since all Pythium spp. proved pathogenic. Phytophthoraisolates 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 insusceptibility was observed between RL and RTC. The fruit _assay and stem inoculation techniqueswere 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 andeucalypt root rot. Artificial stem inoculation using a single virulent isolate of P.cinnamomi provedto be a reliable technique for screening young eucalyptus trees in the field. However, contraryresults were obtained when this technique was used to screen half-sib seedlings of E. fraxinoidesand E. smithii in the greenhouse.Eight Pythium and two Phytophthora spp. were isolated from the rhizosphere soil and decliningcitrus trees. Phytophthora species proved to be pathogenic when inoculated into citrus fruit and onRough Lemon and Troyer Citrange rootstocks. On the other hand Pythium species screened provedto be nonpathogenic or only weakly pathogenic and thus probably play an insignificant role in citrusdecline.

[发布机构] University of the Free State