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Thesis: Ph. D., Joint Program in Biological Oceanography (Massachusetts Institute of Technology, Department of Biology; and the Woods Hole Oceanographic Institution), 2015., Cataloged from PDF version of thesis., Includes bibliographical references., Zooplankton, such as copepods, are highly abundant environmental reservoirs of many bacterial pathogens. Although copepods are known to support diverse and productive bacterial communities, little is understood about whether copepods are affected by bacterial attachment and whether they can regulate these associations through mechanisms such as the innate immune response. This thesis investigates the potential role that copepod physiology may play in regulating Vibrio association and the community structure of its microbiome. To this end, the intrinsic ability of oceanic copepod hosts to transcriptionally respond to mild stressors was first investigated. Specifically, the transcriptional regulation of several heat shock proteins (Hsps), a highly conserved superfamily of molecular chaperones, in the copepod Calanusfinmarchicus was examined and demonstrated that Hsps are a conserved element of the copepod's transcriptional response to stressful conditions and diapause regulation. To then investigate whether copepod hosts respond to and regulate their microbiota, the transcriptomic response of an estuarine copepod Eurytemora affinis to two distinct Vibric species, a free-living strain (V. ordalii 12B09) and a zooplankton specialist (V. sp. F10 9ZB36), was examined with RNA-Seq. Our findings provide evidence that the copepod E. affinis does distinctly recognize and respond to colonizing vibrios via transcriptional regulation of innate immune response elements and transcripts involved in maintaining cuticle integrity. Our work also suggests that association with E. affinis can significantly impact the physiology of Vibrio colonists. Finally, the inter-individual variability of the C.finmarchicus microbiome was examined to identify how specifically and predictably bacterial communities assemble on copepods and whether host physiology influences the bacterial community structure. Our findings suggest that copepods have a predictable "core microbiome" that persists througho, by Amalia Aruda Almada., Ph. D.

Thesis: Ph. D., Joint Program in Biological Oceanography (Massachusetts Institute of Technology, Department of Biology; and the Woods Hole Oceanographic Institution), 2015., Cataloged from PDF version of thesis., Includes bibliographical references., Zooplankton, such as copepods, are highly abundant environmental reservoirs of many bacterial pathogens. Although copepods are known to support diverse and productive bacterial communities, little is understood about whether copepods are affected by bacterial attachment and whether they can regulate these associations through mechanisms such as the innate immune response. This thesis investigates the potential role that copepod physiology may play in regulating Vibrio association and the community structure of its microbiome. To this end, the intrinsic ability of oceanic copepod hosts to transcriptionally respond to mild stressors was first investigated. Specifically, the transcriptional regulation of several heat shock proteins (Hsps), a highly conserved superfamily of molecular chaperones, in the copepod Calanusfinmarchicus was examined and demonstrated that Hsps are a conserved element of the copepod's transcriptional response to stressful conditions and diapause regulation. To then investigate whether copepod hosts respond to and regulate their microbiota, the transcriptomic response of an estuarine copepod Eurytemora affinis to two distinct Vibric species, a free-living strain (V. ordalii 12B09) and a zooplankton specialist (V. sp. F10 9ZB36), was examined with RNA-Seq. Our findings provide evidence that the copepod E. affinis does distinctly recognize and respond to colonizing vibrios via transcriptional regulation of innate immune response elements and transcripts involved in maintaining cuticle integrity. Our work also suggests that association with E. affinis can significantly impact the physiology of Vibrio colonists. Finally, the inter-individual variability of the C.finmarchicus microbiome was examined to identify how specifically and predictably bacterial communities assemble on copepods and whether host physiology influences the bacterial community structure. Our findings suggest that copepods have a predictable "core microbiome" that persists througho, by Amalia Aruda Almada., Ph. D.

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2015, Copepods are highly abundant environmental reservoirs of many bacterial pathogens. This thesis investigates whether copepod physiology influences the abundance and community structure of its microbiome. To this end, we first examined the ability of the oceanic copepod Calanus finmarchicus to transcriptionally respond to mild stressors and demonstrated that heat shock proteins (Hsps) are a conserved element of the copepod’s regulation of stressful conditions and diapause. We then investigated the transcriptomic response of an estuarine copepod Eurytemora affinis to two distinct Vibrio species, a free-living (V. ordalii 12B09) and a zooplankton specialist (V. sp. F10 9ZB36), and found that E. affinis distinctly responds to colonizing Vibrios. Finally, we probed how specifically and predictably bacterial communities assemble on C. finmarchicus. Our findings suggest that C. finmarchicus has a predictable “core microbiome” that persists throughout the host’s entrance into diapause. Furthermore, the structure of the copepod microbiome may be driven by a combination of the copepod’s feeding history, body size, and bacterial interactions. This thesis work highlights the role of copepods as dynamic reservoirs of diverse microbial communities and implicates copepod host physiology as an important contributor to the activity, abundance, and community structure of its often pathogenic microbiota., This work has been supported by a National Science Foundation Graduate Research Fellowship, an Environmental Protection Agency Science to Achieve Results Fellowship, National Science Foundation Grants (OCE-1132567 to AM Tarrant and MF Baumgartner), student awards from the WHOI Ocean Venture Fund, and private donation from Richard and Susan Hill.

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2015, Copepods are highly abundant environmental reservoirs of many bacterial pathogens. This thesis investigates whether copepod physiology influences the abundance and community structure of its microbiome. To this end, we first examined the ability of the oceanic copepod Calanus finmarchicus to transcriptionally respond to mild stressors and demonstrated that heat shock proteins (Hsps) are a conserved element of the copepod’s regulation of stressful conditions and diapause. We then investigated the transcriptomic response of an estuarine copepod Eurytemora affinis to two distinct Vibrio species, a free-living (V. ordalii 12B09) and a zooplankton specialist (V. sp. F10 9ZB36), and found that E. affinis distinctly responds to colonizing Vibrios. Finally, we probed how specifically and predictably bacterial communities assemble on C. finmarchicus. Our findings suggest that C. finmarchicus has a predictable “core microbiome” that persists throughout the host’s entrance into diapause. Furthermore, the structure of the copepod microbiome may be driven by a combination of the copepod’s feeding history, body size, and bacterial interactions. This thesis work highlights the role of copepods as dynamic reservoirs of diverse microbial communities and implicates copepod host physiology as an important contributor to the activity, abundance, and community structure of its often pathogenic microbiota., This work has been supported by a National Science Foundation Graduate Research Fellowship, an Environmental Protection Agency Science to Achieve Results Fellowship, National Science Foundation Grants (OCE-1132567 to AM Tarrant and MF Baumgartner), student awards from the WHOI Ocean Venture Fund, and private donation from Richard and Susan Hill.

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2015, Copepods are highly abundant environmental reservoirs of many bacterial pathogens. This thesis investigates whether copepod physiology influences the abundance and community structure of its microbiome. To this end, we first examined the ability of the oceanic copepod Calanus finmarchicus to transcriptionally respond to mild stressors and demonstrated that heat shock proteins (Hsps) are a conserved element of the copepod’s regulation of stressful conditions and diapause. We then investigated the transcriptomic response of an estuarine copepod Eurytemora affinis to two distinct Vibrio species, a free-living (V. ordalii 12B09) and a zooplankton specialist (V. sp. F10 9ZB36), and found that E. affinis distinctly responds to colonizing Vibrios. Finally, we probed how specifically and predictably bacterial communities assemble on C. finmarchicus. Our findings suggest that C. finmarchicus has a predictable “core microbiome” that persists throughout the host’s entrance into diapause. Furthermore, the structure of the copepod microbiome may be driven by a combination of the copepod’s feeding history, body size, and bacterial interactions. This thesis work highlights the role of copepods as dynamic reservoirs of diverse microbial communities and implicates copepod host physiology as an important contributor to the activity, abundance, and community structure of its often pathogenic microbiota., This work has been supported by a National Science Foundation Graduate Research Fellowship, an Environmental Protection Agency Science to Achieve Results Fellowship, National Science Foundation Grants (OCE-1132567 to AM Tarrant and MF Baumgartner), student awards from the WHOI Ocean Venture Fund, and private donation from Richard and Susan Hill.

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2015, Copepods are highly abundant environmental reservoirs of many bacterial pathogens. This thesis investigates whether copepod physiology influences the abundance and community structure of its microbiome. To this end, we first examined the ability of the oceanic copepod Calanus finmarchicus to transcriptionally respond to mild stressors and demonstrated that heat shock proteins (Hsps) are a conserved element of the copepod’s regulation of stressful conditions and diapause. We then investigated the transcriptomic response of an estuarine copepod Eurytemora affinis to two distinct Vibrio species, a free-living (V. ordalii 12B09) and a zooplankton specialist (V. sp. F10 9ZB36), and found that E. affinis distinctly responds to colonizing Vibrios. Finally, we probed how specifically and predictably bacterial communities assemble on C. finmarchicus. Our findings suggest that C. finmarchicus has a predictable “core microbiome” that persists throughout the host’s entrance into diapause. Furthermore, the structure of the copepod microbiome may be driven by a combination of the copepod’s feeding history, body size, and bacterial interactions. This thesis work highlights the role of copepods as dynamic reservoirs of diverse microbial communities and implicates copepod host physiology as an important contributor to the activity, abundance, and community structure of its often pathogenic microbiota., This work has been supported by a National Science Foundation Graduate Research Fellowship, an Environmental Protection Agency Science to Achieve Results Fellowship, National Science Foundation Grants (OCE-1132567 to AM Tarrant and MF Baumgartner), student awards from the WHOI Ocean Venture Fund, and private donation from Richard and Susan Hill.

Increasing concern over adverse environmental effects and decreasing efficacy of synthetic fungicides has brought about the urge for the development of new and natural control alternatives for plant diseases. Thus,extensive exploration on exploitation of plants as natural commercial biopesticides is highly desired. Cosmos caudatus Kunth or locally known as ‘Ulam Raja’ in Malaysia, is a traditional herb to cure and improve human ailments. However, antifungal activity of C. caudatus on plant pathogens has not yet been studied. Crude leaf ethanol extract of C. caudatus was separated into three fractions; hexane, ethyl acetate (EtOAc) and aqueous fractions and screened for their antifungal activity against selected plant pathogens using agar cup method. The EtOAc fraction was shown to be the most active in inhibiting mycelia growth of Phytophthora palmivora, the causal pathogen of black pod of cocoa with percentage inhibition in radial growth (PIRG) of 52.0%. Sporangia germination was recorded the lowest in EtOAc fraction with values of 15.62%. This was further observed in the scanning electron micrographs of P. palmivora treated with EtOAc fraction showing stunted growth and abnormalities in the mycelium with reduced sporangia production. The bio-efficacy of the fractions was further tested on detached cocoa pods. The EtOAc fraction gave the highest inhibition of 57.46% on diameter of lesions. These findings indicated that the EtOAc fraction of C. caudatus leaves possesses fungistatic properties against P.palmivora. Identification of the active antifungal compound(s) was carried out using bioassay guided fractionation by column chromatography technique. The EtOAc fraction was first fractionated into thirteen fractions, in which the first fraction (F1) showed highest inhibition against P. palmivora (PIRG of 54.73%). Fraction F1 was further fractionated into eight sub-fractions (F1-1 to F1-8) and it was found that F1-4, F1-5, and F1-6 as the most active with PIRG of 52.5

Increasing concern over adverse environmental effects and decreasing efficacy of synthetic fungicides has brought about the urge for the development of new and natural control alternatives for plant diseases. Thus,extensive exploration on exploitation of plants as natural commercial biopesticides is highly desired. Cosmos caudatus Kunth or locally known as ‘Ulam Raja’ in Malaysia, is a traditional herb to cure and improve human ailments. However, antifungal activity of C. caudatus on plant pathogens has not yet been studied. Crude leaf ethanol extract of C. caudatus was separated into three fractions; hexane, ethyl acetate (EtOAc) and aqueous fractions and screened for their antifungal activity against selected plant pathogens using agar cup method. The EtOAc fraction was shown to be the most active in inhibiting mycelia growth of Phytophthora palmivora, the causal pathogen of black pod of cocoa with percentage inhibition in radial growth (PIRG) of 52.0%. Sporangia germination was recorded the lowest in EtOAc fraction with values of 15.62%. This was further observed in the scanning electron micrographs of P. palmivora treated with EtOAc fraction showing stunted growth and abnormalities in the mycelium with reduced sporangia production. The bio-efficacy of the fractions was further tested on detached cocoa pods. The EtOAc fraction gave the highest inhibition of 57.46% on diameter of lesions. These findings indicated that the EtOAc fraction of C. caudatus leaves possesses fungistatic properties against P.palmivora. Identification of the active antifungal compound(s) was carried out using bioassay guided fractionation by column chromatography technique. The EtOAc fraction was first fractionated into thirteen fractions, in which the first fraction (F1) showed highest inhibition against P. palmivora (PIRG of 54.73%). Fraction F1 was further fractionated into eight sub-fractions (F1-1 to F1-8) and it was found that F1-4, F1-5, and F1-6 as the most active with PIRG of 52.5

Rhodococcus fascians is a Gram-positive phytopathogenic bacterium which induces the development of leafy galls, local amplifications of multiple buds, on most infected plants. This process is linked to the production of phytohormones along with the presence of essential virulence-associated genes like the plasmid loci att and fas and the chromosomal gene vicA. However, the presence of these genes is not sufficient to ensure the infection phenotype development, indicating that other genes play a role in R. fascians pathogenicity. In this work, we studied the metabolic modifications occurring when the bacterium interacts with its host using a proteomic approach. A comparison between virulent and avirulent strains showed variations in the expression of catalases. In the virulent strain, besides the transitory induction of the att locus expression, the bacterium changes its metabolism from the Krebs cycle to the glyoxylate shunt, a process which is frequently observed in bacteria confronted to a hostile environment. The expression of the shunt-specific enzyme isocitrate lyase increased, while expression of fumarate hydratase and pyruvate dehydrogenase decreased. Hence, we focused on the link between the glyoxylate shunt and virulence. A screening of a R. fascians mutant library based on the capacity of bacteria to use acetate as the sole carbon source, a metabolic pathway depending on the glyoxylate shunt, resulted in the identification of a new gene essential for R. fascians pathogenicity. This gene encodes a glycosyl transferase, an enzyme known to be involved in the bacterial cell wall biosynthesis but possibly also implicated in cytokinin secretion. A mutant in this gene harboured an altered colony phenotype and could not induce malformations on infected plants. Accordingly, our results were integrated in the leafy gall pathology model recently presented by Stes et al. (2011). Finally, the several questions that are raised by this work, allowed us to suggest further, Doctorat en Sciences, info:eu-repo/semantics/nonPublished

Rhodococcus fascians is a Gram-positive phytopathogenic bacterium which induces the development of leafy galls, local amplifications of multiple buds, on most infected plants. This process is linked to the production of phytohormones along with the presence of essential virulence-associated genes like the plasmid loci att and fas and the chromosomal gene vicA. However, the presence of these genes is not sufficient to ensure the infection phenotype development, indicating that other genes play a role in R. fascians pathogenicity. In this work, we studied the metabolic modifications occurring when the bacterium interacts with its host using a proteomic approach. A comparison between virulent and avirulent strains showed variations in the expression of catalases. In the virulent strain, besides the transitory induction of the att locus expression, the bacterium changes its metabolism from the Krebs cycle to the glyoxylate shunt, a process which is frequently observed in bacteria confronted to a hostile environment. The expression of the shunt-specific enzyme isocitrate lyase increased, while expression of fumarate hydratase and pyruvate dehydrogenase decreased. Hence, we focused on the link between the glyoxylate shunt and virulence. A screening of a R. fascians mutant library based on the capacity of bacteria to use acetate as the sole carbon source, a metabolic pathway depending on the glyoxylate shunt, resulted in the identification of a new gene essential for R. fascians pathogenicity. This gene encodes a glycosyl transferase, an enzyme known to be involved in the bacterial cell wall biosynthesis but possibly also implicated in cytokinin secretion. A mutant in this gene harboured an altered colony phenotype and could not induce malformations on infected plants. Accordingly, our results were integrated in the leafy gall pathology model recently presented by Stes et al. (2011). Finally, the several questions that are raised by this work, allowed us to suggest further, Doctorat en Sciences, info:eu-repo/semantics/nonPublished

English: The successful introgression of the stripe rust seedling resistance gene YrSp into Kariega/ Avs/YrSp is towards the continued pursuit of durable resistance in this cultivar. Following the development of Kariega/ Avs/YrSp hybrid, an F2 and F3 Kariega/YrSp population was obtained and screened for virulence to pathotype 6E22A- of P. striiformis f. sp. tritici. The disease screening of the F2 population deviated from the expected single dominant gene ratio and did not fit a 3:1 gene segregation ratio determined by Chi-square analysis. Disease screening of the resistant F3 families exhibited segregation while susceptible F3 families was not considered to segregate with the observation of singular resistant plants in two susceptible F3 families. Histological studies firstly confirmed the infection pathway of P. striiformis f. sp. tritici as is described for other cereal rusts with the absence of the formation of appressoria and the observation of pseudo-SSVIs the most marked differences. The pseudo-SSVIs are thought to be due to the unsuccessful stomatal penetration by germ tubes as successful penetration results in a single SSVI that is generally larger in size than the vesicles observed but this requires further study. Secondly, quantitative analysis indicated that urediospores of P. striiformis f. sp. tritici have good germination efficiency but germ tubes were rarely observed to penetrate stomata. After penetration, the infection efficiency of the pathogen is high in susceptible cultivars due to systemic colonisation. The resistance conferred by the YrSp gene is considered posthaustorial as no significant difference in infection efficiency could be determined before haustorium formation. An increase in the number of internal infection structures after 7 dpi in the susceptible cultivar was observed while the resistant line reflected a decrease. Thirdly, fluorochrome assessment indicated uvitex 2B and the orange G probe hold the best potential of the fluorochro, Afrikaans: Die YrSp geen is suksesvol van Avocet/YrSp na Kariega, ‘n kommersiële Suid-Afrikaanse kultivar oorgedra. Na inkorporering van die geen is die F2 en F3 Kariega/YrSp populasies vir weerstand teen die patotipe 6E22A- van Puccinia striiformis f. sp. tritici geëvalueer. Resultate van die siekteweerstands-evalueering van die F2 populasie het op die teenwoordigheid van ‘n enkel dominante geen gedui en het nie voldoen aan ‘n 3:1 segregasie verhouding. Verdere evaluasie van weerstandbiedende F3 families het segregasie getoon in teenstelling met die vatbare F3 families. Die ontwikkeling van Kariega/YrSp is die volgende stap in die voortdurende strewe na volhoubare weerstand in hierdie kultivar. Histologiese studies, met behulp van transmissie-elektronmikroskopie (TEM) en skandeer-elektronmikroskopie (SEM) het bevestig dat die infeksieproses by P. striiformis f. sp. tritici dieselfde is as wat beskryf is vir ander graanroese, behalwe dat geen appressoria gevorm het nie. Verder is die inisiële fase van die pseudo-substomatale vesikel (SSVIs) van streeproes gedurende hierdie studie waargeneem en beskryf. Die vesikels vorm waarskynlik as gevolg van onsuksesvolle penetrasie van die stomata deur die kiembuis aangesien suksesvolle penetrasie ‘n enkele vesikel vorm en oor die algemeen groter is. Hierdie aspek benodig verdere navorsing. ‘n Kwantitatiewe analise van streeproesinfeksies, het aangetoon dat ten spyte van swak kiembuispenetrasie, die infeksietempo van P. striiformis f. sp. tritici hoog was in vatbare kultivars waarskynlik as gevolg van sistemiese kolonisasie. Daar is gevind dat die weerstand, oorgedra deur die YrSp-geen, na-haustoriaal is, omdat geen verskil in infeksievlak of struktuur tussen Avs/YrSp en Kariega voor haustoriumontwikkeling opgemerk is nie. Die aantal interne infeksiestrukture in die vatbare kultivars het 7 dae na infeksie toegeneem, terwyl die aantal strukture in die weerstandbiedende kultivars afgeneem het. ‘n Vergelyking van verskillende fluo, National Research Foundation (NRF), Mosanto (SA), Professional Provident Society, Eriksen and Ethel Foundation

English: Wheat and the wheat leaf rust pathogen Puccinia tritieina co-evolved for several millennia. The frequency by which host resistance genes are overcome by the pathogen has lead to a constant search for new genes, in particular in wild species related to wheat, to enlarge the resistance gene pool. For this reason four subspecies of T. turgidum that was known to have adult-plant resistance to leaf rust were crossed with the bread wheat cultivar SST55 and studied in terms of expression, inheritance and molecular markers. Seedling plants of T. turgidum ssp. dieoceoides (Kom. ex Aschers. and Graebn.) Theil. var. kotehianum, T. turgidum ssp. pyramidales, T. turgidum ssp. durum (Desf.) Husn var. libyeum and T. turgidum ssp. abyssinieum were inoculated with different races of P. tritieina, P. graminis f. sp. tritiei and P. striiformis f. sp. tntlei. A differential interaction was observed between accessions and pathotypes, indicating that the plants had race-specific resistance. All showed adult-plant resistance with leaf rust infection types ranging between ";" (flecking) and ";1eN" (flecks and small pustules associated with chlorosis and/or necrosis). Despite of low pollen viability, sterility and recessiveness of resistance genes in three of the accessions, F1, F2 and F3'Swere produced. T. turgidum ssp. durum v. libyeum was the only accession that produced resistant F1's, indicating a dominant gene or genes. Mendelian ratios in the F2 progeny suggested that resistance in this accession resulted from two major genes. The F2 of the T. turgidum ssp. abyssinicum and T. turgidum ssp. pyramidales crosses did not segregate according to Mendelian ratios and two minor genes conferred resistance of T. turgidum ssp. dicoccoides v. kotchianum. The molecular AFLP technique was used to to follow the introgression of resistance from tetraploid T. turgidum ssp. dicoccoides v. kotchianum and T. turgidum ssp. durum v. libycum to the hexaploid bread wheat cultivar SST55. In total ni, Afrikaans: Koring en die koringblaarroes patogeen Puccinia triticina het gekoëvoleer vir etlike millennia. Die frekwensie waarteen weerstandsgene deur die patogeen oorkom word, het gelei tot 'n konstante soektog na nuwe gene, veral van die wilde families van koring, om die weerstandsgeenpoel aan te vul. Om hierdie rede is vier subspesies van T. turgidum wat oor volwasseplantsweerstand beskik, gekruis met die broodkoring kultivar SST55 en bestudeerop grond van geenuitdrukking, oorerwing en molekulêre merkers. Saailingplante van T. turgidum ssp. dicoccoides (Kërn, ex Aschers. en Graebn.) Theil. var. kotchianum, T. turgidum ssp. pyramidales, T. turgidum ssp. durum (Desf.) Husn var. libycum en T. turgidum ssp. abyssinicum is geïnokuleer met verskillende rassevan P. triticina, P. striiformis f. sp. tritici asook P. graminis f. sp. tritici. Die verskeie interaksies wat waargeneem is tussen koringlyne en roesrasse het daarop gedui dat die plante oor ras-spesifieke weerstand beskik. Alle plante het volwasseplantsweerstand getoon met blaarroes infeksietipes wat gewissel het tussen ";" (vlek) en ";CN" (vlekke en klein roespuisies geassosieer met chlorose en nekrose). Ten spyte van die lae stuifmeel lewensvatbaarheid, steriliteit en resessiwiteit van weerstandsgene in drie van die koringlyne, is F1, F2 en F3 plante geproduseer. T. turgidum ssp. durum v. libycum was die enigste koringlyn wat weerstandbiedende F1 's geproduseer het. Hierdie aanduiding dat weerstand in dié lyn die resultaat van 'n dominante geen of gene is, is bevestig deur Mendeliese toetse op die F2 nageslag wat aangedui het dat twee dominante gene verantwoordelik was vir weerstand. Alhoewel die F2 plante van die kruisings T. turgidum ssp. pyramidales en T. turgidum ssp. abyssinicum nie volgens Mendeliese verhoudings gesegregeer het nie, het dieselfde toetse aangedui dat weerstand deur twee resessiewe gene veroorsaak word in T. turgidum ssp. dicoccoides v. kotchianum. Die molekulêre AFLP tegniek is gebruik om d, National Research Foundation (NRF)

English: Drought, freezing and nitrogen stress are environmental factors known to predispose woody plants to infection by numerous fungal species. The main and ultimate objective of the present study was to identify possible stress factors that predispose commercially pistachio (Pistacia spp.) trees to infection and colonization by Botryosphaeria obtusa and B. dothidea in order to develop a sustainable disease control programme for South African pistachio producers. It was found that these stress conditions significantly affected the susceptibility of three genotypes of potted pistachio plants to infection and colonization by B. obtusa and B. dothidea. In the glasshouse, drought stress trials with potted Pistacia terebinthus plants artificially inoculated with B. dothidea showed significantly longer lesions than control treatments while P. integerrima displayed the greatest resistance to the pathogen. Although B. dothidea caused more severe cambial lesions in glasshouse trials, it showed a slower growth rate than B. obtusa under in vitro water stress conditions. In freezing stress trials performed in a growth chamber, there was a highly significant difference between the stressed and non-stressed plants of P. integerrima and Clone II (P. integerrima x P. atlantica). P. integerrima was the most susceptible rootstock and P. terebinthus the least susceptible. B. dothidea caused the most severe cambial lesions in all treatments. In nitrogen stress trials, most treatments showed a significant decrease in the length of cambial lesions at a higher level of applied nitrogen (28%). B dothidea was generally more pathogenic than B. obtusa for both levels of N applied. The most significant differences between nitrogen levels were recorded for P. terebinthus. From the above results it is clear that drought and freezing stress have a significant effect on the predisposition of pistachio plants to B. obtusa and B. dothidea. Drought stress occurs in pistachio orchards due to a very, Afrikaans: Droogte, koue en stikstof stremming is bekende omgewingsfaktore wat houtagtige spesies meer vatbaar maak vir infeksie deur verskeie swamme. Die hoofdoel van die huidige studie was om stremmings faktore te identifiseer wat kommersieële pistachio bome meer vatbaat maak vir infeksie deur Botryosphaeria obtusa en B. dothidea. Die doel hiervan is om 'n effektiewe siektebeheerprogram vir pistachio produksie in Suid- Afrika te ontwikkel. Droogte en koue stremming is geïdentifieseer as faktore wat die vatbaarheid verhoog terwyl stikstof stremming 'n duidelike afname in infeksie getoon het. Daar was duidelike verskille tussen verskillende pistachio genotipes. Pistacia terebinthus plante het in glashuisproewe betekenisvolle langer letsels as kontrole plante getoon terwyl P. integerrima die hoogste weerstand onder droogte toestande getoon het. B. dothidea het deurgaans die langste letsels veroorsaak al het die betrokke swam stadiger gegroei as B. obtusa onder droogte toestande in vitro. Betekenisvolle verskille het tussen gestremde en nie-gestemde plante van P. integerrima en Kloon II (P. integerrima x P. terebinthus) voorgekom, toe plante in groeikabinette aan koue stremming onderwerp is. Gestremde plante het in die bogenoemde behandelings deurgaans langer letsels getoon. P. integerrima het die langste letsels getoon terwyl P. terebinthus die beste weerstand onder die toestande getoon het. B. dothidea het weereens die langste letsels in al die behandelings getoon. Tydens die stikstof proewe was daar deurgaans -korter letsels by die verhoogde stikstof peil. B. dothidea het langer letsels as B. obtusa veroorsaak in al die betrokke behandelings. Die mees betekenisvolle verskil tussen die stikstof behandelings was in die geval van P. terebinthus. Die resultate van hierdie studie toon duidelik dat droogte en koue stremming belangrike faktore is as dit kom by predisponering van pistachio bome teenoor siektes. Droogte stremming kom op die oomblik gereeld voor in pistachio, Green Valley Nuts, Prieska