The aim of this project was to investigate the hypothesis that aphid-infested potato plants produce elevated amounts of the main secondary metabolites that occur in potatoes compared to the amounts produced by non-infested plants. Peach potato aphids <I>Mysuz persicae </I>(Sulzer) (Homoptera: Aphididae) were reared on artificial diets containing the steroidal glycoalkaloids (GAs) α-solanine and α-chaconine in concentrations lower or similar to those observed in potato leaves. The parameters used to measure the aphid performance were the total individual adult cumulative fecundity, diet uptake and mortality the mean relative growth rate (MRGR) and the intrinsic rate of natural increase (IRI). The adults proved to be susceptible to high concentrations (80-160 mg GA /100ml of diet) showing reduced fecundity, diet uptake and increased mortality in comparison to controls. Ingestion of these artificial diets by nymphs delayed maturity and decreased the intrinsic rate of natural increase. GAs in low concentrations marginally stimulated the reproductive performance and diet acceptability of this aphid. The effects of nitrogen fertilisation and pesticide (Aldicarb) application on the foliar total and individual GA production of two potato cultivars King Edward and Maris Piper were examined using a semi-hydroponic culture system. Nitrogen deficient potato plants produced elevated concentrations of GAs. The two cultivars reacted in a different way on pesticide application. Cv. King Edward produced elevated amounts of GAs in the low pesticide treatment when no differences were observed between pesticide treatments on cv. Maris Piper. Glycoalkaloid production of potato plants subjected to stress induced by peach potato aphid <I>M. persicae</I> infestation was investigated in a glasshouse, a plant growth room, and a field experiment. In both glasshouse and plant growth room experiments aphid-infested potato plants produced reduced amounts of total and individual GAs compared to those produced by non-infested plants. This reduction was attributed to a decrease of sugar moities possibly induced to plants heavily infected by aphids.

A new selective medium containing pentachloronitrobenzene and 2-aminobutane - the PAB medium - was developed for use with the soil-dilution plate method for the enumeration of fungal propagules of F. solani var. coeruleum and F. sulphureum in field soil. These fungi cause a dry rotting of potatoes in storage. The efficiency of the PAB medium in the measurement of levels of soil contamination was compared with that of other methods. Also described is the PM70 medium, suitable for the isolation of a number of pathogens, including P. exigua. var. foveata from diseased tuber tissue. The incidence of dry rot after grading was related logarithmically to the number of F. solani var. coeruleum propagules in progeny tubersphere soil. Highest levels of progeny tuber contamination with F. solani var. coeruleum were associated with the planting of infected seed. Planting of contaminated seed sometimes gave high levels of transmission, possibly because dry rot developed after planting. Propagule production by infected seed varied between seasons and may be related to soil temperature. Tuber factors, eg variety and seed size, also influenced propagule production. Of the seed treatments tested, only thiabendazole reduced consistently the transmission of F. solani var. coeruleum. Levels of soil contamination increased during the growing season but removal of the seed tuber prevented further increase. Highest numbers of propagules were in a 5cm diam. zone of soil surrounding the seed and spread of propagules was mainly lateral and downwards forming a decreasing gradient of inoculum with increasing distance from the seed tuber. Propagule distribution on progeny tubers followed the same pattern but harvesting by elevator digger disturbed the soil inoculum, making all progeny tubers highly contaminated. F. solani var. coeruleum survived a 6 year rotation in field soil and this soil-borne inoculum is possibly important in the re-contamination of clean seed stocks. Most varieties, of those tested, were resistant to infection by F. solani var. coeruleum in November but susceptible by February. Tubers were more susceptible if incubated initially at 40°C rather than at 15°C. Transmission of F. solani var. coeruleum was compared with that of F. sulphureum. Propagules of F. solani var. coeruleum were produced in cavities in the tuber and in pustules on the tuber surface but F. sulphureum showed little surface sporulation. Thus thiabendazole, which inhibits surface sporulation, was inconsistent in reducing transmission of F. sulphureum. Moreover, F. sulphureum sporulated on stems growing from infected seed tubers in the field. F. sulphureum infected seed usually produced less inoculum than did F. solani var. coeruleum but in one season the reverse was true and was possibly related to high soil temperatures. F. sulphureum does not appear to survive in field soil as well as F. solani var. coeruleum. Although F. sulphureum seems less well adapted than F. solani var. coeruleum for propagule transmission, none of the varieties tested was resistant to infection by F. sulphureum.

The geographical distribution and phenology of WBF are matched more closely with those of couch than with those of other hosts. These factors suggest that couch, and not wheat, is the preferred host. Aspects of this hypothesis were tested in the laboratory, glasshouse and field. In choice test bioassays neonate larvae preferred couch seedlings and their exudates to wheat seedlings and their exudates, and couch rhizome exudates to controls. Couch seedling exudates had attractant properties, whereas wheat exudates had attractant and arrestant properties, when compared with controls. The larvae were photophobic and positively geotactic. In a pot trial, symptoms of infestation appeared earlier in couch than in wheat. At first, larvae encouraged production of extra shoots, especially of couch, which they kill. After 5 weeks, infested plants suffered a relative reduction in number of shoots, but uninfested neighbouring plants, especially wheat, compensated for this by producing more shoots themselves. Larvae raised on couch emerged as adults earlier than those raised on wheat. They thus develop more rapidly, and use more resources, on couch than on wheat, i.e. they are better adapted to couch as a food source. Earlier eclosion in the field would allow adults to make better use of favourable weather conditions, and to live longer, mate more often, and produce more eggs. In the laboratory and the field adult WBF preferred to rest on couch than on wheat. They also preferred taller plants and those nearer the edge of a stand. Buried couch rhizomes did not encourage oviposition by gravid WBF females, either in the laboratory or the field. These findings support the hypothesis that couch is the preferred host of WBF, provide a partial explanation of high larval mortalities on wheat, and suggest that attractants isolated from couch and arrestants isolated from wheat could be used to control WBF larvae if incorporated into buried granules.

The main purpose of this study was to further an understanding of host-pathogen interactions and the role of phytotoxins in the host relationships of <i>Alternaria brassicae</i> (Berk.) Sacc. and <i>Alternaria brassicicola</i> (schw.) Wilts., using microscopical, biochemical and statistical approaches. A further aim was to assess the taxonomic positions of <i>A.brassicae</i> and <i>A.brassicicola</i> within the <i>Alternaria</i>, along with their attributes and behaviour patterns in relation to other members of the anamorph-genus. On the leaf surfaces of host plants <i>A.brassicae</i> and <i>A.brassicicola</i> showed broadly similar patterns of development but with some features which distinguished them. The larger spores of <i>A.brassicae</i> typically produced two to three germ-tubes whereas <i>A.brassicicola</i> gave rise to only one. <i>A.brassicae</i> generally produced more extensive extra-matrical growth with hyphal branching and appressoria in intercalary as well as terminal positions; hyphae of <i>A.brassicicola</i> produced very few branches and usually terminated in appressoria. For both fungi appressoria were formed most frequently near to or over anticlinal walls of epidermal cells. Smaller numbers of appressoria were formed over periclinal walls and, in the case of <i>A.brassicae</i>, over stomata. The extra-matrical development of both species was more or less similar on contrasting host leaf surfaces, although <i>A.brassicae</i> showed somewhat reduced germ-tube numbers and stomatal penetration on leaves with a pronounced waxy bloom. Marked differences in behaviour on different hosts or between hosts and non-hosts were evident only after penetration. In the case of <i>A.brassicae</i> unsuccessful penetrations were associated with only localised deposition of callose in the cell wall of the challenged cell, while with <i>A.brassicicola</i> unsuccessful penetration events were associated with cell wall responses which occurred in whole single cells. In comparing <i>A.brassicae</i> and <i>A.brassicicola</i> with other <i>Alternaria</i> species varying in their host range and degree of parasitism, all exhibited essentially similar patterns of extra-matrical behaviour on leaves, culminating in the formation of appressoria. Distinctive host relationships were evident only in the post-penetration phase. From successful penetrations <i>A.brassicae</i> produced a short sub-cuticular phase whereas <i>A.brassicicola</i> appeared to penetrate to intracellular position in the epidermis. Subsequent colonisation involved intercellular hyphal growth and extensive callose formation in host cell walls in response to both species.

Commercialised herbicide safeners (also known as protectants or antidotes) are synthetic chemicals used to enhance herbicide tolerance in cereal crops. They do this by causing an up-regulation in xenobiotic detoxifying enzymes such as glutathione transferases (GSTs). Seedlings of wheat (Triticum aestivum cv ‘Einstein’) were sprayed with the safeners cloquintocet mexyl, fenchlorazole ethyl and mefenpyr diethyl. All three compounds caused an identical up regulation of GSTs from the phi, tau and lambda classes, despite their differences in chemistry. Using cloquintocet mexyl as a classic wheat safener treatment, it was found that GST induction was both dose and time dependent. Safening was found to be associated with the rapid hydrolysis of the parent ester to cloquintocet acid. When the free acid was tested, the GST-induction response obtained was identical to that determined with the parent ester, suggesting that cloquintocet itself is the active safener. GST induction was found to be tissue specific within the wheat shoots, with the lambda GSTs being preferentially expressed in the meristematic tissue. Proteomic 2 D gel analysis revealed that the tau TaGSTU3 was a major up-regulated GST. In addition, six GSTs that were previously shown in literature to be up-regulated by herbicide safeners in wheat were cloned, expressed and characterized as the respective recombinant enzymes and renamed to bring them in line with existing nomenclature. The GSTs cloned included TaGSTU3, TaGSTU6, TaGSTF4, TaGSTF10 and TaGSTL1. Metabolism studies showed that following the hydrolysis of cloquintocet mexyl, no further down- stream metabolites could be identified and none of the up-regulated GSTs showed any activity toward the safener. However TaGSTU3 was found to bind and be inhibited by cloquintocet free acid as determined by isothermal titration calorimetry. Safener treatment also led to a transient inhibition of GST activity in crude wheat extracts after spraying the seedlings. In addition to the induction of GSTs, safener treatments also resulted in an enhanced growth of wheat seedlings. The work presented in this thesis confirms that very different compounds can induce apparently identical downstream events at the level of GST enhancement and that these induction events underpin wider changes in plant physiology.

Parasitic nematodes cause enormous public health, agricultural and economic problems worldwide, as pathogens of humans, livestock and crops. Their impact is increasing due to lack of full efficacy of current anthelmintics and development of resistance by the nematodes, therefore there is an urgent need for an alternative. Plant cysteine proteinases (CP) from papaya (“papain”), fig (ficin), and pineapple (bromelain) have been shown to be effective on gastrointestinal (GI) nematodes. The enzymes attack by digesting the cuticle leading to rupture and death of the nematode. The nematode cuticle is composed of collagens and cuticlins but the specific molecular target(s) of the proteinases have yet to be identified. There are about 160 collagen genes and 8 identified functional cuticlins genes in the C. elegans genome. This study identified some of the molecular targets and thereby began to define the mechanism of action of this new class of anthelmintics, through imaging, proteomic, immunohistochemical, and automated motility assay techniques using a free-living nematode, Caenorhabditis elegans and murine GI nematode Heligmosomoides bakeri as target organisms. Through proteomic approach, Col-87 and Cut-19 were identified as CP targets on C. elegans and H. bakeri cuticles respectively, cuticle globin and chemosensory protein that localise in the cuticle were also target for CPs. Immunohistochemical staining indicated that DPY-7 collagen is also a target for CPs on the cuticle of C. elegans. Imaging showed that there was a marked difference in degree of damage done to the two model worms used. H. bakeri was most affected by CPs as its entire cuticle was digested more rapidly when compared to the two strains of C. elegans. Motility of the three strains of C. elegans was affected by exposure to CPs, in a concentration-, time- and CP type-dependent manner. In papaya latex supernatant (PLS), there was no detectable statistically significant difference in susceptibility between wild-type and cystatin-null mutants. Papain affected the motility of worm types and was more effective than PLS. CP's mechanism of attack on the nematode cuticle is by degrading the structural proteins, leading to loss of integrity, motility and finally death of the nematode.

Virulence of E. amylovora, the causal agent of fireblight, depends on the synthesis of harpin and amylovoran, an acidic exopolysaccharide and is enhanced by levan, produced from sucrose by the secreted enzyme levansucrase. Amylovoran synthesis and capsule formation were influenced by the presence of various carbon sources. Sorbitol, the major transport sugar of rosaceous plants, showed the strongest induction. Amylovoran synthesis by E. amylovora in the presence of sorbitol or sucrose was dependent on the ionic strength of the medium.;The importance of sorbitol for E. amylovora was further shown by mutations in genes involved in its uptake leading to a non-virulent phenotype on apple seedlings. Sorbitol uptake in E. amylovora is achieved via a phosphoenolpyruvate:carbohydrate phosphotransferase system (PTS). Sequence analysis identified an operon encoding six proteins that shared a high similarity to the PTS enzyme II complex for sorbitol uptake in E. coli.;The genetic regulation of amylovoran synthesis by RcsV required the expression of rcsV as a fusion protein, which could also suppress an rcsA mutation. The intact rcsV promoter sequence separated from the gene induced amylovoran synthesis via the RcsA/RcsB pathway. The titration effect and the lack of transcription from the rcsV promoter indicated that rcsV was negatively regulated by several proteins.;The gene encoding for H-NS, a global negative regulator, was isolated from a genomic library of E. amylovora and found to have significant homology to hns genes of several bacteria. Overexpression of hns reduced both amylovoran and levan production. A mutation of the E. amylovora hns gene did not alter amylovoran production or virulence but levan formation was increased. The study of the E. amylovora hns allele is the first time this gene has been characterised from a plant pathogen.

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