The major findings of the study were as follows: 1. Rats ate significantly more coumatetralyl than brodifacoum, and rats in an area of physiological resistance (central southern England) ate greater quantities of rodenticide than rats in the east midlands. Coumatetralyl failed to achieve rat control on farms in central southern England. 2. Physiological resistance was suspected on two farms in the east midlands (near Lincoln) where rats ate excessive quantities of coumatetralyl and the control programme was unexpectedly extended. 3. GCMS analyses performed on extracts of 169 whole rodents revealed that some rats had eaten levels of coumatetralyl that far exceeded a lethal dose for susceptible animals. Excessive bait consumption occurred mostly in the area of physiological resistance, but also on the two sites in the east midlands where resistance was suspected. Brodifacoum consumption by some rats was also high, but complete control was usually achieved with brodifacoum and there was no evidence of any resistance to brodifacoum. 4. HPLC analyses carried out on 10 rats from coumatetralyl sites (five from each region), revealed that trapped, physiologically-resistant animals are capable of carrying 50 times the LD50 of coumatetralyl without any obvious ill effect. Rats from the east midlands carried a significantly lower load of coumatetralyl. 5. Video observations gave no evidence to support a bait point monopoly theory. Interactions at feeding sites were common. 6. This study has revealed that the potential exposure of non-target predators and scavengers to rodenticides is considerably higher in areas where rats show physiological resistance. This fact is discussed with reference to the regulation and monitoring of pesticides in the UK.

No description available.

No description available.

Cytological and genetical studies were carried out on the possible mechanism(s) involved in the production of new physiologic races of the yellow rust pathogen, Puccinia struformis. The size and structure of dividing and non-dividing nuclei in hyphae and basidiospores were ascertained using light and electron microscopy. The parallel observations of the dividing somatic nucleus by the two techniques permitted a relatively complete sequence of events to be established for mitosis. Chromosome studies revealed that P. striiformia has a basic chromosome number of three, although some nuclei from basidiospores were seen to possess four chromosomes. Preparations in which hyphae were possibly fusing were observed using the light and electron microscope. The results of the cytological work are discussed with reference to the physical basis of genetic variation in the yellow rust fungus. Genetical studies included tests on the progeny of a mixture of two races, following the inoculation of susceptible and differentially resistant varieties. A new race, designated 105E137, was identified following the mixed inoculation of races 104E137 and 33E32. Induced mutation experiments established that the rate of mutation in the pathogen for specific virulence factors was less than one in 2 x 107. The results of the study are discussed and genetic models are presented to explain how race 105E137 may have evolved. In a series of experiments on quantitative variation in P. striiformis differences between isolates in spore production and colony grown rate were established. An isolate of race 40E8 was the most prolific genotype in experimental studies on a range of host varieties. Isolates tended to rank similarly on the test varieties although there was some evidence of isolate-host interactions. Detailed studies on the various stages of infection revealed that the prolificacy of the race 40E8 isolate could be detected at all stages of pathogen development.

This study aimed to contribute to the understanding of the morphogenesis and structure of signal grass (<i>Brachiaria decumbens</i> Stapf.) pastures, and how they are affected by the environment and cattle defoliation. A diagrammatic model was adopted to identify and relate the major morphogenetic and structural characteristics at different levels of organisation of the sward: phytomer, tiller, plant and population. The morphogenetic characteristics studied were leaf elongation rate (LER) and internode elongation rate (IER) at the phytomer and tiller level, leaf senescence rate (LSR) and leaf appearance rate (LAR) at the tiller level, and tiller birth rate (TBR) and tiller death rate (TDR) at the plant and population level. The structural characteristics analysed were length of phytomer components (blade, sheath and internode), number of phytomers per tiller, total length and number of live blades per tiller, number and size-age distributions of tillers per plant, and number and size distribution of plants and tillers per area. Experiments to analyse the effects of temperature, nitrogen application, season and defoliation intensity on sward structural and morphogenetic variables were performed under controlled and field conditions, and using destructive and non-destructive measurements. An initial experiment under controlled environments showed highest LAR, LER and IER values at 30°C, and strong mutual regulations between morphogenetic and structural characteristics at the phytomer and tiller levels. The effects of cattle grazing and nitrogen fertilisation on the dynamics of signal grass morphology, and the patterns of defoliation were monitored during one year in long established pastures in Santa Cruz, Bolivia. Plant population demography were mainly defined by the season: high number of seedlings and new vegetative tillers at the beginning of summer; new vegetative tillers, plant fragmentation, and flowering during summer (wet season); and low tiller and plant densities with low proportions of live leaf towards the following winter. Defoliation affected only the phytomer level of organisation.

Differences in morphology may predispose certain cultivars to disease by influencing the duration free water remains on the head and the availability of sites for infection. Head shape (doming), head size, bud number, bud prominence and stomatal number were assessed. Whilst these factors differed significantly between cultivars, two sets of factors correlated positively at a significant level: head size and disease susceptibility, and head doming and disease resistance. Biosurfactants, produced by pathogenic bacterial strains, have been implicated in the development of head rot on unwounded heads, by aiding bacterial establishment and spread. Their role in disease, as virulence or pathogenicity factors, was assessed using a mutant approach. Initially, five biosurfactant detection tests were evaluated to allow the mutants produced to be tested for loss of production. The most suitable proved to be monitoring changes in the surface tension of a water droplet following the addition of bacterial cells removed from a colony. Thirty-five mutants, defective in biosurfactant production (surf <SUP>-</SUP>), were produced by Tn5 mutagenesis. Because some surf<SUP> -</SUP> mutants were unaffected in their ability to cause disease, it was concluded that biosurfactant production in <I>P. fluorescens</I> is neither a virulence nor a pathogenicity factor. Biosurfactants are probably produced, in this bacterium, to assist initial establishment and colonisation of the plant surface. Further work is required to elucidate the nature of the mutated genes to define the precise role of biosurfactants in the host/pathogen interaction.

No description available.

<i>In vitro</i> tests on the response of different <i>Fusarium</i> species to triadimenol fungicides showed a greater sensitivity in <i>F.nivale</i> and <i>F.dimerum</i> compared with other species which may be linked with their distinctive taxonomic grouping. The low incidence of <i>F.nivale</i> in the survey relative to that reported by earlier workers may be attributed in part to the more frequent application of fungicides to cereal crops in the field and the sensitivity of this fungus to the widely used triadimenol-containing formulations. A further factor accounting for the unexpectedly low incidence of <i>F.nivale</i> may have been the relatively mild winters and warm summers over the period of this survey which would be unfavourable to this low temperature fungus. From artificial inoculation of ears of different cereals with <i>Fusarium</i> species, <i>F.culmorum</i>, followed by <i>F.avenaceum</i> and <i>F.graminearum</i> were shown to depress grain yield significantly, <i>F.culmorum</i> having a major effect on grain size and the other two species causing a decrease in grain numbers. Earlier inoculations during the flowering period aggravated the effects of infection. The introduction of <i>T.roseum</i> 1 week before inoculation reduced these effects. The carry-over of ear inoculated <i>Fusarium</i> on seed was generally greater with the more pathogenic species, which also showed more deep-seated infection. However, the weak pathogen <i>F.sambucinum</i> also showed a high rate of carry-over and deeper penetration of seed tissues. Seed and soil inoculation tests confirmed that <i>F.culmorum</i> was the most pathogenic species, followed by <i>F.avenaceum</i> and <i>F.graminearum</i>. The addition of <i>T.roseum</i> to <i>Fusarium</i> inoculum adversely affected seed performance but other fungi, an isolate of <i>Penicillum</i> and of <i>Cladosporium</i>, reduced disease effects.

If diseased plants (or plant units) are randomly dispersed, the frequency distribution of diseased plants (or plant units) per sample may be described by a binomial distribution, and statistical analyses may be based on the linear logistic model. Since most disease incidence data do not have a random spatial pattern, the binomial distribution can hardly ever, in practice, be used to describe observed frequencies. In this study, the use of conditional probability distributions, such as the logistic-normal binomial distribution, for such data is illustrated. Both descriptive distribution fitting and statistical modelling are discussed. The study evaluates several methods for analysis of incidence data which do not exhibit a random spatial pattern. Some of these methods are applied to plant disease data for the first time. A method of choosing between the different analyses is discussed. All the techniques are illustrated using examples and, as an application, survey data collected on pineapple wilt disease in Sri Lanka are extensively studied. As an alternative method of describing disease incidence data with a non random spatial pattern, the use of two-dimensional distance class (2DCLASS) analysis was evaluated using the same survey data. 2DCLASS analysis is widely accepted in plant disease epidemiology as a method of analysing non-random spatial patterns when the observations are made as presence or absence of the disease on individual plant basis. We demonstrate the possibility of using quadrat-based data in 2DCLASS analysis. We investigate the use of 2DCLASS analysis as a methodology and find some drawbacks with this technique, which are discussed in detail. Moreover, this study introduces a new parameter in the 2DCLASS analysis called Scaled Core Cluster size, that may be more suitable to use for comparison of datasets of different sizes.

The role epicuticular waxes play in resistance of brassicas to foliar diseases was investigated for two known brassica pathogens, <i>Alternaria brassicicola</i> and <i>Erysiphe cruciferarum</i>, a weak parasite <i>Alternaria alternata</i> and a non-pathogen <i>Erysiphe graminis</i>. Waxes of swede, oilseed rape and Brussels sprout leaves from different leaf positions were found to be composed of eight analogous compounds but in varying proportions. Epicuticular waxes from seven Brussels sprout mutant lines, in comparison to a commercial variety, again had eight similar groups of compounds, however the variations in proportions were more distinct. Correlations could be made betweeen prevalence of certain compound groups and wax crystal configuration. Waxes containing high ketones and hydrocarbons had crystals in the form of rods which projected from the cuticle and these surfaces tended to be waxy. Temperature was seen to transform the crystalline configuration of wax crystals, whereas light intensity altered their dimensions. Waxy surfaces had the lowest wettability and permeability, and also tended to be most resistant to <i>Alternaria</i> infection. Infection by <i>Erysiphe cruciferarum</i> seemed to be related to the genetic background of the host and not surface phenotype. Treatment of the surface with surfactant, which reduced the wettability, and environmental conditions which increased wettability had equivalent effects on <i>Alternaria</i> infection but variations in <i>Erysiphe</i> infections could not be explained by changes in leaf surface wettability.