A major challenge in agroecosystems is how to manage the systems so that it reduces crop pests and enhances natural pest control. This thesis investigates patterns of crop pests and top-down effects of birds and arthropod predators in relation to land-use composition across spatial scales. In paper (I) I examined the crop distribution and land-use types in relation to the crop raiding patterns in 15 transectsin sites close to and far from forests along with a questionnaire survey at household level. I found severe crop raiding close to forests, but it had no impact on crop composition growing between the two sites. In paper (II) I examined the effect of forest and tree cover, at local and landscape scales, on the abundance of arthropod predators by collecting specimens from 40 home gardens. My result showed higher abundance of arthropod predators when either the home garden or the surroundings had a high tree-cover, compared to when tree-cover at both scales was similarly either high or low. In paper (III) I investigated the effect of excluding birds and arthropod predators on leaf damage on rape seed in 26 home gardens. I found stronger top-down impacts from arthropod predators on crop pests in tree-poor gardens than in tree-rich gardens. There was no effect of birds. In paper (IV) I explored the effect of landscape complexity on bird and arthropod predation using plasticine caterpillars in 36 home gardens across landscapes. The rate of arthropod predation on caterpillars was higher in simple than in complex landscapes. The rate of bird predation did not vary between complex and simple landscapes. In simple landscapes, arthropod predation was higher than that of birds. The overall results suggest that simplified gardens/landscapes still have enough habitat heterogeneity to support arthropod predators for the significant top-down controlling effect on crop pests. However, I did not find clear effect of complexityon the top-down effect of birds. / <p>At the time of the doctoral defence the following papers were unpublished and had a status as follows: Paper 1: Manuscript; Paper 3: Manuscript; Paper 4: Manuscript</p>

Doctor of Philosophy / Department of Agronomy / Allan K. Fritz / In this research, a recombinant inbred line (RIL) population derived from cross between a leaf rust- and stripe rust-susceptible spring wheat ‘Avocet S’ and a slow leaf- and stripe-rusting resistant spring wheat ‘Amadina’ was used to postulate and map leaf rust seedling resistance genes, identify quantitative trait loci (QTL) for slow-rusting resistance against leaf and stripe rust, and study slow leaf-rusting components, latent period and infection frequency. Two known Lr genes (Lr23, and Lr26) were identified to be present in ‘Amadina’ through gene postulation, pedigree, cytogenetic, and polymerase chain reaction analyses. One unknown gene associated with seedling resistance was also mapped on chromosome 1BL. In greenhouse experiment, it was estimated that at least five genes conditioning final disease severity (FS) and latent period (LP), and four genes conditioning infection frequency (IF), segregated in the population. Correlations between LP and FS, and LP and IF were moderately negative, and that between IF and FS was moderately positive, indicating inter-dependence of the traits. Two QTL on chromosomes 1BL and 6BL were associated with LP and FS, and three QTL on chromosomes 1BL, 6BL and 2DS were associated with IF. Segregation of the RIL population in field experiment indicated that there were at least four and three adult plant resistance (APR) genes involved in resistance for leaf and stripe rust. Six QTL on chromosomes 3AL, 4AL, 1BL, 5BL, and 7BL were associated with APR for leaf rust, and seven QTL on chromosome 4AL, 5AL, 1BL, 2BL, 4BL, 5BL, 2DL, and 4D were associated with APR for stripe rust. Our results indicated that the major portion of genetic variability for slow-rusting resistance was additive gene action, and, to some extent, epistasis. In this research, we also explored the utility of remote sensing and geographic information systems (GIS) and analytical operations to discriminate leaf rust pustules from other parts of leaf and to accurately determine pustule size in ‘Amadina’ and ‘Avocet S’.

Call number: LD2668 .T4 1979 C66 / Master of Science

Master of Science / Department of Agronomy / Jianming Yu / Tan spot, caused by Pyrenophora tritici-repentis (Ptr), is an economically important foliar disease worldwide. Race 1 of the fungus, which produces the necrosis toxin Ptr ToxA and the chlorosis toxin Ptr ToxC, is the most prevalent race in the Great Plains of the United States. The purposes of this study are to 1) identify and map novel quantitative trait loci (QTL) involved in resistance to tan spot race 1 in common wheat (Triticum aestivum L.) and 2) explore the inverse gene-for-gene interaction in the wheat-P. tritici-repentis pathosystem. A population of 288 F2:6 recombinant inbred lines (RILs) developed from the cross between Chinese landrace WSB (resistant) and Ning7840 (highly susceptible) was firstly used to identify genomic regions harboring novel sources of resistance. Two QTLs associated with resistance to chlorosis were mapped to the short arm of chromosome 1A and 2B in the WSB/Ning7840 population. No interaction was found between the two QTL. To further explore the specific wheat-ToxC model, three other populations were developed based on two susceptible parents, Ning7840 and Wheaton. QTL analysis revealed that common QTL were detected in populations shared with the same susceptible parents. The observations suggested that susceptibility rather than resistance for tan spot chlorosis is specific and presented evidence for the inverse gene-for-gene theory in the WSB-ToxC pathosystem. Leaf rust, caused by Puccinia triticina Eriks., is another important foliar disease of common wheat worldwide. The rust-resistance genes Lr41 and Lr42 from T. tauschii accessions TA2460 (Lr41) and TA2450 (Lr42) have been used as sources of rust resistance in breeding programs. Molecular markers linked to these genes are essential tools for gene pyramiding. Two BC3F2:6 mapping populations were evaluated for leaf rust resistance at both seedling and adult plant stages and analyzed with simple sequence repeat (SSR) markers. Both genetic and physical mapping confirmed that markers linked to Lr41 and Lr42 were on chromosome arm 2DS and 1DS, respectively. Marker analysis in a diverse set of wheat germplasm indicated that tightly linked markers for Lr41 and Lr42 can be used for marker-assisted selection (MAS) in breeding programs.

Freshwater is a resource under threat due to anthropogenic actions. Stream restoration is a common method for mitigating disturbance. Inconsistent methodologies used for evaluating restorations have drawn criticism. Limited use of baseline data for guiding stream restoration activities is of particular concern. This study was developed to elucidate metrics that differentiate reference and disturbed sites in Upper Coastal Plain streams. This information could improve resource use and successes of restorations. Structural and functional variables were examined in 10 reference and 10 streams that meet the traditional definition of disturbance and would be restoration priorities. Disturbed streams were classified into two regimes, temporal, based on time since disturbance, and categorical, based on disturbance cause. Some metrics of geomorphology, water chemistry and macroinvertebrates differentiated reference from disturbed regimes and while other metrics separated streams within disturbance regimes. Surprisingly, leaf decay rate was not an effective metric for determining disturbance. However, macroinvertebrate leaf pack colonizers were found to be useful for differentiating reference sites and disturbance regimes. Of the 10 disturbed streams this study examined, my data suggests that only three are in immediate need of restoration. This study emphasizes the importance of baseline data and its potential benefits for guiding stream restoration.

The C₄ photosynthetic cycle acts to concentrate CO₂ around the enzyme Rubisco. By doing so, C₄ photosynthesis leads to increased radiation, water and nitrogen use efficiencies. As such, C₄ photosynthesis is the most productive form of photosynthesis known. Because it enables such high levels of productivity there are large international efforts to introduce C₄ photosynthesis into non-C₄ crop species such as rice. Kranz anatomy is a characteristic leaf cellular arrangement of concentric rings of bundle sheath and mesophyll cells around closely spaced veins and is crucial to C₄ photosynthesis in almost all known examples. Despite the fact that Kranz has evolved on over 60 times independently little is known about the genetic regulation of Kranz development, as attempts to elucidate Kranz regulators using conventional mutagenesis screens have provided few insights. However, the advent of next generation DNA sequencing technologies has enabled the interrogation of genetic networks at a previously unprecedented scale. The work in this thesis describes a genome-wide transcriptomic analysis of leaf development in maize, a C₄ species, that develops both Kranz-type and non-Kranz-type leaves. Detailed bioinformatics analyses identified candidate regulators of both Kranz development and additional aspects of maize leaf development. Three of the identified Kranz candidates were functionally characterised in both C₄ and non-C₄ species. Furthermore, expression and phylogenetic analyses of GOLDEN2-LIKE (GLK) genes, a small transcription factor family previously implicated in C₄ development in maize, were extended to determine the generality of GLK function in C₄ evolution.

Thesis (PhD)--Stellenbosch University, 2000. / ENGLISH ABSTRACT: Gray leaf spot (GLS) of maize, caused by the fungus Cercospora zeae-maydis, can reduce grain yields by up to 60% and it is now recognized as one of the most significant yield-limiting diseases of maize in many parts of the world. The most sustainable and long-term management strategy for GLS will rely heavily on the development of high-yielding, locally adapted GLS resistant hybrids. Molecular markers could be useful to plant breeders to indirectly select for genes affecting GLS resistance and to identify resistance genes without inoculation and at an early stage of plant development. Only two studies in the USA have examined quantitative trait loci (QTL) association with GLS resistance. The aim of this study was to map GLS resistance genes in a resistant Seed Co LTD, Zimbabwean inbred line. Molecular markers linked to the GLS resistance QTL were identified by using the amplified fragment length polymorphism (AFLP) technique together with bulked segregant analysis. Eleven polymorphic AFLP fragments were identified and converted to sequence-specific PCR (polymerase chain reaction) markers. Eight of the 11 converted AFLP markers were added to the maize marker database of the University of Stellenbosch. Five of the 8 converted AFLP markers were polymorphic between the resistant and the susceptible parent. They were amplified on the DNA of 230 plants of a segregating F2 population and linkage analysis was performed with MAPMAKER/EXP. Two linkage groups consisting of two markers each, with a linkage distance of 10.4 cM (LOD 22.83) and 8.2 cM (LOD 55.41) between the two markers, were identified. QTL mapping with MAPMAKER/QTL confirmed the presence of QTL in both linkage groups. Two publicly available recombinant inbred families (Burr et a/., 1988) were used to localize the converted AFLP markers on the genetic map of maize. The QTL, which were identified with the AFLP markers, were mapped to chromosomes 1 and 5. Another AFLP marker was mapped to chromosome 2 and a further to chromosome 3. To obtain more precise localizations of the QTL on chromosomes 1 and 5, sequence-tagged site markers and microsatellite markers were used. The markers were amplified on the DNA of the 230 plants of the F2 population and linkage analysis was performed with MAPMAKER/EXP. The order of the markers was in agreement with the UMC map of the Maize Genome Database. Interval mapping using MAPMAKERlQTL and composite interval mapping using QTL Cartographer were performed. The QTL on chromosome 1 had a LOD score of 21 and was localized in bin 1.05/06. A variance of 37% was explained by the QTL. Two peaks were visible for the QTL on chromosome 5, one was localized in bin 5.03/04 and the other in bin 5.05/06. Both peaks had a LOD score of 5 and 11% of the variance was explained by the QTL. To test the consistency of the detected QTL, the markers flanking each QTL were amplified on selected plants of two F2 populations planted in consecutive years and regression analysis was performed. Both the QTL on chromosome 1 and the QTL on chromosome 5 were detected in these populations. Furthermore, the presence of a QTL on chromosome 3 was confirmed with these populations. A variance of 8 -10% was explained by the QTL on chromosome 3. In this study, a major GLS resistance QTL was thus mapped on chromosomes 1 and two minor GLS resistance QTL were mapped on chromosomes 3 and 5 using a resistant Seed Co LTD, Zimbabwean inbred line. Markers were identified which could be used in a marker-assisted selection program to select for the GLS resistance QTL. / AFRIKAANSE OPSOMMING: Grys blaarvlek (GBV) van mielies, veroorsaak deur die swam Cercospora zeaemaydis, kan graanopbrengs met tot 60% verlaag en word beskou as een van die vernaamste opbrengs-beperkende siektes wêreldwyd. Die toepaslikste langtermyn stragtegie vir GBV beheer sal wees om plaaslike mieliebasters met hoë opbrengs en GBV weerstand te ontwikkel. Molekulêre merkers kan nuttig deur plantetelers gebruik word om weerstandsgene te selekteer. Seleksie is moontlik in die afwesigheid van inokolum en op 'n vroeë stadium van plant ontwikkeling. Slegs twee vorige studies (in die VSA) het kwantitatiewe-kenmerk-Iokusse (KKL), vir GBVweerstand ondersoek. Die doel van hierdie studie was om die GBV weerstandsgene in 'n weerstandbiedende ingeteelde lyn (Seed Co BPK, Zimbabwe) te karteer. Molekulêre merkers gekoppel aan die GBV weerstands KKL is geïdentifiseer deur gebruik te maak van die geamplifiseerde-fragmentlengte-polimorfisme- (AFLP-) tegniek en gebulkte-segregaat-analise. Elf polimorfiese merkers is geïdentifiseer en omgeskakel na volgorde-spesifieke PKR (polimerase kettingreaksie) merkers. Agt van die elf omgeskakelde AFLP-merkers is by die mieliemerker databasis van die Universiteit van Stellenbosch gevoeg. Vyf van die 8 omgeskakelde AFLP-merkers was polimorfies tussen die bestande en vatbare ouers. Hulle is geamplifiseer op die DNA van 230 plante van 'n segregerende F2-populasie en is gebruik in 'n koppelingstudie met MAPMAKER/EXP. Twee koppelingsgroepe, elk bestaande uit twee merkers, met onderskeidelik koppelingsafstande van 10.4 eM (LOD 22.83) en 8.2 eM (LOD 55.41) tussen die merkers, is geïdentifiseer. KKL-kartering het getoon dat KKL in albei koppelingsgroepe aanwesig is. Twee kommersieël beskikbare, rekombinant-ingeteelde families (Burr et aI., 1988) is gebruik om die omgeskakelde AFLP-merkers op die mielie genetiese kaart te plaas. Die KKL wat met die AFLP-merkers geïdentifiseer is, is gekarteer op chromosome 1 en 5. 'n Verdere AFLP-merker is op chromosoom 2 gekarteer en 'n ander op chromosoom 3. Ten einde die KKL op chromosome 1 en 5 meer akkuraat te karteer, is volgordege- etikeerde en mikrosatelliet merkers gebruik. Die merkers is geamplifiseer op die DNA van die 230 plante van die F2-populasie en koppelings-analises is uitgevoer. Die volgorde van die merkers was dieselfde as die van die UMC-kaart in die Mielie Genoom Databasis. Interval kartering met MAPMAKER/QTL en komposiet interval kartering met QTL Cartographer is uitgevoer. Die KKL op chromosoom 1 het 'n LOD-telling van 21 gehad en is in bin 1.05/06 geplaas. Die KKL was verantwoordelik vir 37% van die variansie. Twee pieke was onderskeibaar vir die KKL op chromosoom 5, een in bin 5.03/04 geleë en die ander in bin 5.05/06. Elke piek het 'n LOD-telling van 5 gehad en die twee KKL was verantwoordelik vir 11% van die variansie. Om die herhaalbaarheid van die effek van die KKL te toets is die merkers naaste aan elke KKL geamplifiseer op geselekteerde plante van twee F2-populasies wat in opeenvolgende jare geplant is. Regressie analise is op die data gedoen. Beide die KKL op chromosoom 1 en die KKL op chromosoom 5 kon in hierdie populasies geïdentifiseer word. Verder kon die aanwesigheid van 'n verdere KKL op chromosoom 3 in hierdie populasies bevestig word. Laasgenoemde KKL was verantwoordelik vir 8-10% van die totale variansie. In hierdie studie is daar dus 'n hoof GBV-weerstands KKL gekarteer op chromosoom 1 en twee kleiner GBV-weerstands KKL gekarteer op chromosome 3 en 5. Merkers is geïdentifiseer wat moontlik in merker-gebaseerdetelingsprogramme gebruik kan word om plante te selekteer wat die GBVweerstands KKL het.

Thesis (PhD)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: This project involved the analysis of genes in Solanaceae species that have previously been shown to be involved in the phosphorylation of starch or its subsequent dephosphorylation. Both these processes are essential for normal starch mobilization. A tomato conditional mutant lacking the starch phosphorylating enzyme glucan water dikinase was analyzed. It is known that starch accumulates transiently in tomato fruit and is degraded throughout the ripening process. The study aimed to determine the effect of inhibited starch degradation on fruit development. Unfortunately no effect on starch mobilisation was found in the fruit of the mutant. Immunoblot analysis revealed expression of Glucan Water Dikinase (GWD) within the fruit of the tomato mutant indicating that the conditionality of the mutation was compromised. The second set of experiments analyzed the roles of Starch Excess4 (SEX4), Like Sex Four-1 and Like Sex Four-2 (LSF1 and LSF2) in starch degradation in potato and Nicotiana benthamiana. These enzymes have, thus far, only been studied in Arabidopsis, with the proposed role for SEX4 and LSF2 being that they are involved in dephosphorylation of the C-6 and C-3 positions of starch breakdown products. The role of LSF1 is unclear, although it is not thought to be a phosphatase. SEX4, LSF1 and LSF2 were repressed individually while the expression of SEX4 and LSF2 were also inhibited simultaneously. Using a transient repression system in N. benthamiana it was shown that all of the genes play a role in leaf starch degradation. The SEX4 and LSF2 enzymes were shown to influence the proportion of phosphate located on the starch which contained an altered ratio of C-3/C-6 phosphate. Stably transformed potato plants were produced where SEX4 and LSF2 were successfully repressed in potato leaves and tubers. Although AtLSF2 had been shown not to be essential for normal starch degradation on its own, in potato plants when LSF2 was repressed, the plants developed a starch-excess phenotype. Taken together with the N. benthamiana data this indicates that LSF2 plays a bigger role in leaf starch degradation in Solanaceae than in Arabidopsis. The ratio of C-3/C-6 phosphate was also altered in tuber starch from some of the silenced plants. Starch from SEX4 repressed potato plants contained increased amounts of glucose-6-phosphate and increased glucose-3-phosphate in the tuber when compared to the WT. An increase in the proportion of C-6 or C-3 phosphate is not surprising with SEX4 being characterized as a phosphatase specific for C-6 position and LSF2 for the C-3 position in Arabidopsis, however the combined increase in C-3 and C-6 amounts in StSEX4 silenced plants is interesting. The differences seen in the phosphate alteration in both N. benthamiana leaves and potato tubers indicates that in Solanaceae species these proteins may have a slightly altered specificity when compared with Arabidopsis, although they are undoubtedly involved in starch degradation. The effect of silencing SEX4 or LSF2 on cold-induced sweetening was also investigated, with no effect being found. This may be because of functional redundancy between the proteins and a better approach in terms of blocking cold sweetening would be to simultaneously repress SEX4 and LSF2. Overall, these enzymes seem to play similar roles in leaves of Solanum species as has been described in Arabidopsis. The starch from the engineered plants did have an altered phosphate ratio and further analysis is needed to determine if this leads to improved or additional functionality. / AFRIKAANSE OPSOMMING: Die projek omhels die ontleding van gene van die Solanaceae spesie wat voorheengetoon het dat hulle deel neem in fosforilering of defosforilering van stysel. Altwee van hierdie reaksies is belangrik vir normale stysel metabolisme. ‘n Tamatie konditionele mutant was geanaliseer waarin die stysel fosforilering ensiem glucan water dikinase nie teenwoordig was nie. Die doel van die studie was om te ondersoek watter effek het n gebrek in stysel afbraak op die rypwording en ontwokkeling vrugte. Ongelukkig was geen effek op stysel metabolism in die munant se vrugte gesien. Immunoklad analise het getoon dat GWD protein wel uitdruk word in die vrugte en dus die mutant nie heeltemal effektief was nie. Die tweede stel van experimente het in aartappels en tabak die rol van SEX4, LSF1 en LSF2 in stysel afbraak ondersoek. Hierdie ensieme was huidiglik nog net deeglik in Arabidopsis bestudeer, waar daar gewys was dat SEX4 and LSF2 in die defosforilering van stysel by die C-6 en C-3 posisie deel neem. Die rol van LSF1 is nog onbekend, maar daar word huiglik gelgo dat dit is nie ‘n fosfatase nie. SEX4, LSF1, en LSF2 was onderdruk op sy eie, waar SEX4 en LSF2 gelyktydig onderdruk was. Met behulp van n verbygaande onderdrukking in tabak, was dit getoon dat al die bogenoemde gene n gedeeltelike rol speel in die afbraak van stysel. Dit was getoon dat SEX4 and LSF2 ensiemedie verhouding van waar fosfaat op stysel gelee is beinvloed en het n verandering in die C-3/C-6 phosphaat verhouding ook gehad. Aardappels was stabiel getransformeer en daar was suksesfol plante waar SEX4 en LSF2 onderdruk was in blare en knolle geproduseer. Alhoewel daar getoon was dat AtLSF2 op sy eie nie n groot rol speel in stysel katabolisme nie was daar wel gesien dat in aardappel wanner hierdie geen afgeskakel was dat daar n stysel oorskot fenotiepe ontwikkel. As die tabak resultate saamgevat word met die aardappel wil dit voorkom asof LSF2 n groter rol binne die stysel katabolisme in Solanaceae speel as in Arabidopsis. Daar was gevind dat die verhouding van C-3/C-6 fosfaat was in die knolle verander in perty van die lyne waar geen afskakeling wel plaasgevind het. Die verhouding van C-3/C-6 fosfaat was verander in knolle stysel van sommige stilgemaak plante. Sysel van SEX4 stilgemaak plante het hoër vlakke glukose-6-fosfaat en glukose-3-fosfaat in die knolle gehad wanner dit met die WT vergelyk was. n Toename in die persentasie van C-6 fosfaat is nie verbasend nie, SEX4 word gekenmerk as die spesifieke fosfatase verantwoordelik vir die fosfaat by die C-6 posisie en LSF2 spesifiek vir die C-3 posisie in Arabidopsis. Die gekombineerde toename in beide C-6 en C-3 bedrae in StSEX4 stilgemaak plante is wel heel interesant. Verandering in beide tabak blare and aartapple knolle dui daarop dat in solanacea spesie hierdie proteiene, n effens verandering in spesifisiteit kan hê as dit met Arabidopsis vergelyk word. Daar kan wel nie getwyfel word dat hulle wel n rol speel in stysel afbraak nie. Die effect watSEX4 of LSF2 op koue-geinduseerde soetheid het is ook ondersoek maar daar was geen effek gevind nie. Dit mag wees asgevolg van die funksionele onslag tussen die twee proteien en better benadering on die koue-soetheids effek te verhoed sou wees om beide protein op die selfde stadium aft e skakel. As daar in gegeheel gekyk word lyk dit asof hierdie protein die selfde rolle het in die Solanum spesies as in Arabidopsis.Die stysel van hierdie die ontwerpte plante het ‘n veranderde fosfaat verhouding getoon en veder analise is nodig om te bepaal of dit lei tot verbeterde einskappe of bykommende funksies.

The site-specific management of alfalfa has not been well-evaluated, despite the economic importance of this crop. The objectives of this work were to i) characterize the effects of soil moisture deficits on alfalfa and alfalfa yield components and ii) evaluate the use of canopy reflectance patterns in measuring treatment-induced differences in alfalfa yield. A randomized complete block design with five replicates of subsurface drip irrigation (SDI) and rainfed treatments of alfalfa was established at the University of Kentucky Animal Research Center in 2003. Potassium, as KCl, was broadcast on split-plots on 1 October 2004 at 0, 112, 336, and 448 kg K2O ha-1. In the drought year of 2005, five harvests (H1 - H5) were taken from each split-plot and from four locations within each SDI and rainfed plot. One day prior to each harvest, canopy reflectance was recorded in each plot. Alfalfa yield, yield components, and leaf area index (LAI) were determined. In 2005, dry matter yields in two harvests and for the seasonal total were increased (Pandlt;0.05) by SDI, but SDI did not affect crown density. Herbage yield was strongly associated with yield components but yields were most accurately estimated from LAI. Canopy reflectance within blue (450 nm), red (660 nm) and NIR bands were related to LAI, yield components, and yield of alfalfa and exhibited low variance (cv andlt; 15%) within narrow ( 0.125 Mg ha-1) yield ranges. Red-based Normalized Difference Vegetation Indices (NDVIs) and Wide Dynamic Range Vegetation Indices (WDRVIs) were better than blue-based VIs for the estimation of LAI, yield components, and yield. Decreasing the influence of NIR reflectance in VIs by use of a scalar (0.1, 0.05, or 0.01) expanded the range of WDRVI-alfalfa yield functions. These results indicate that VIs may be used to estimate LAI and dry matter yield of alfalfa within VI-specific boundaries.

The filamentous ascomycete fungus Magnaporthe oryzae is a pathogen of over 50 genera of grasses. Two important diseases it can cause are gray leaf spot in Lolium perenne (perennial ryegrass) and blast in Oryza sativa (rice). The telomeres of M. oryzae isolates causing gray leaf spot are highly variable, and can spontaneously change during fungal culture. In this dissertation, it is shown that a rice-infecting isolate is much more stable at the telomeres than an isolate from gray leaf spot. To determine the molecular basis of telomere instability several gray leaf spot isolates telomeres were cloned, which revealed two non-LTR retrotransposons inserted into the telomere repeats. The elements have been termed Magnaporthe oryzae Telomeric Retrotransposons (MoTeRs). These elements do not have poly-A tails common to many other non-LTR retrotransposons, but instead have telomere like sequences at their 5’ end that allow them to insert into telomeres. Intact copies of MoTeRs were restricted to the telomeres of isolates causing gray leaf spot. Surveys for the presence of these elements in M. oryzae showed they were present in several host-specialized forms including gray leaf spot isolates, but were largely absent in the rice blast isolates. The absence of MoTeRs in rice blast isolates, which are relatively stable by comparison, suggested that the telomere instability in gray leaf spot isolates could be due to MoTeRs. Analyzing spontaneous alterations in telomere restriction fragment profiles of asexual progeny revealed that MoTeRs were involved. Expansion and contraction of MoTeR arrays were observed and account for some telomere restriction profile changes. New telomere formation in asexual progeny followed by MoTeR addition was also observed. Based on this evidence, MoTeRs are largely responsible for the high variability of telomere restriction profiles observed in GLS isolates.