This thesis was designed to address three seldom studied aspects of the relationship between herbivores and vegetational diversity. 1. Interactions between vegetational diversity and herbivore mortality due to predation were assessed by experimentally manipulating both the species diversity of plants and the densities of a common generalist predator, the spider Enoplagnatha ovata. 2. The importance of plant size to herbivore densities was examined by quantifying plant size (measured as plant height, width, leaf area and growth rates) and adjusting for it through covariate analysis. 3. Differences in population responses of several species of herbivores to both vegetational diversity and a predator, were compared by concurrently studying four lepidopterans. The main experiment used a two factor design, with two planting treatments and two predator treatments. The planting treatments consisted of plots planted with monocultures of brussels sprouts (Brassica oleraceae) and dicultures of brussels sprouts intercropped with peppermint (Mentha piperita). The natural enemy treatments involved augmentations of E ovata in some plots and untreated controls. Two of the herbivores studied, Plutella xylostella and Pieris rapae are monophagous lepidopterans, specializing on crucifers, while the others, Autographa californica and Mamestra configurata are polyphagous. For two species, P. xylostella and M. configurata responses to augmentations of the spider, E. ovata. were different in monocultures and dicultures. Reduced densities of these two species were found in monoculture plots with added spiders; in dicultures increased densities were found in plots with added spiders. This interaction effect points out that generalist predators can be effective in monocultures. I suggest that the importance of natural enemies in monocultures is often overlooked because only the initial colonization phase is being studied. By augmenting predator populations I was able to simulate densities equivalent to those in more established cropping systems. The increased herbivore densities in dicultures with added spiders might be explained by possible predation by E. ovata on other natural enemies of P. xylostella and M. configurata in dicultures but not in monocultures. Supporting evidence for this interpretation lies in the fact that percent parasitism of P. xylostella by the ichneumonid, Diadegma insulare was lower in plots with added spiders than in control plots. Furthermore, parasitism of P. xylostella by D. insulare increased with host density in diculture plots, but not in monoculture plots. Mamestra configurata was not subject to parasitism in this study, precluding assessment of a similar relationship. No A. californica larvae were found in plots with additional spiders. In contrast, P. rapae larvae were not affected by the experimental treatments. Plant size was a crucial determinant of both herbivore populations and percent parasitism of those herbivores. Most importantly, had plant size not been accounted for, the importance of vegetational diversity to both herbivore densities and percent parasitism would have been overestimated. For example, the incorrect conclusion, that vegetational diversity alone was important in determining the abundance of both of the generalist feeders would have been reached. The greater densities of A. californica in monocultures and M. configurata in dicultures were accounted for by plant size. Without plant size adjustments, percent parasitism of P. xylostella by D. insulare would have been misinterpeted as being greater in monocultures than dicultures. With plant size adjustments, the importance of E. ovata augmentations on lowering percent parasitismwas unmasked. All important interaction effects were discovered only after adjustments for plant size had been made. Despite the low densities of all herbivore species, significant responses to experimental treatments were found in three of the four species studied. Only P.l rapae was unaffected by any of the treatments. However, conclusions based on the feeding ng habits of the herbivores could not be made. The polyphagous feeders were affected by generalist predation as much as the crucifer specialist. Parasitism was found in only two of the species, P. xylostella and A. californica. Of these two species parasitism of the specialist, P. xylostella was affected by both vegetational diversity and generalist predation, whereas parasitism of A. californica was not. My study emphasizes multifaceted interactions between the size and diversity of a primary resource and several trophic levels of consumers. Multifactor models, involving several aspects of a cropping system, are required to uncover the important mechanisms behind variable herbivore responses to vegetational diversity. / Land and Food Systems, Faculty of / Graduate

Ramets of 14 clones of western black cottonwood Populus trichocarpa Torr. & Gray collected from the interior and coast of British Columbia were tested in the nursery for their growth performance after infection by the leaf rust Melampsora occidentalis Jacks. Linear relationships were demonstrated between various growth parameters (total dry weight, stem dry weight, root dry weight, volume, diameter and height) and disease severity rated as diseased leaf-weeks divided by total leaf-weeks. Losses due to rust infection included reduced total dry weight and volume growth in the year of heavy disease, the death of severely infected ramets during the following winter and reduced initial volume increment in the following growing season. The percentage reduction in yield (total dry weight) was greater than the cumulative percent leaf area infected, suggesting that the rust infected leaf parts act as sinks for photosynthate. The normal pattern of photosynthate allocation was altered in favour of the top growth of ramets. Ratios of stem/root dry weight increased rapidly as disease level increased. A threshold infection level, below which no loss occurred, was not detected in this pathosystem. Significant variation in rust resistance of black cottonwood clones was detected both within and between the two geographic areas. Clones from the coast or warm, moist areas were, on average, more resistant than clones from the interior or cold, dry climates. The phenomenon of induced resistance was not detected at either the local or the systemic levels in black cottonwood challenged by the rust. Variation in rust tolerance among black cottonwood clones was demonstrated. Rust tolerance was defined as the slope of the relationship between yield of ramets (expressed as a proportion of controls) and disease severity (expressed as the proportion of the total number of leaf-weeks infected). A negative correlation between rust tolerance and rust resistance was found. In the collection of clones tested, the positive effect of disease resistance on the growth of cottonwood clones was partially counterbalanced by the negative effect of reduced tolerance on the growth. The importance of this relationship in both natural and artificial selection of superior trees against disease is indicated. / Forestry, Faculty of / Graduate

The distribution, population ecology, behavior and host interactions of the root weevil, Hylobius warreni Wood were investigated in lodgepole pine forests in Alberta. Highest incidence of the weevil occurs in the Lower Foothills Section of the Boreal Forest Region, between 2,500 and 4,000 feet in elevation. In even-aged forests weevil numbers are distributed according to stand maturity, stand density, tree size and duff depth. Interactions between these variables modify the patterns of abundance in different stands. Attack incidence upon the host varies, being highest in the collar zone and least on lateral roots. As tree size increases the ratio of weevils on roots/weevils on collar tends to increase. During normal stand development initial invasion of weevils occurs at age 6-10 years, and persists with successive attacks throughout the life of the stand. Weevil populations are highly aggregated in mature stands; "k" values of the negative binomial varied from 0.09 to 0.68, while Taylor's power law gave an aggregation index "b" value of 1.92. Estimates of weevil populations indicated that low levels are characteristic of this species and are maintained, mostly within the range 200-1200 weevils per acre. Estimates of absolute numbers indicate similar levels of abundance occur in young and old stands alike, while population intensity values increase with stand maturity. The highest rate of increase of attack density per tree appears to occur during the ages of 30-45 years. The structure of weevil populations was described and mortality factors were identified and measured for larval, pupal and teneral stages. The main mortality factor of these stages appeared to be from excess moisture in the larval gallery and pupal cell. Only the first 3 larval instars are definable by head capsule width measurement. The feeding behavior of larvae varies with its maturity. In the early instars the feeding pattern relates to bark thickness, but damage is insignificant. Damage of late instars may consist of decorticated gallery lengths up to 24 cm. Larval and pupal habitats are described to indicate the special adaptations for survival. Adults live at least 3 years but lay their eggs during the second and third summers of adulthood. Their seasonal peak of activity occurs in June and early July. Dispersion in the forest tends to be random, commencing about 2 hours after sunset and when temperatures exceed 36-40 °F. Host trees are located partly by vision, the pattern of selection being related to host size. Maximum fecundity per female per season may be 36 or more eggs, but in the field the actual number may not exceed 12.0. Most eggs are deposited singly in niches excavated by the female in the root-collar bark, and are subsequently covered over with excreta. The egg requires a moist environment maintained for up to 42 days for successful hatch. During stand development up to 100 percent of trees may sustain larval feeding damage accumulated to various degrees of intensity. Young trees up to 30 years of age show less resistance to girdling damage than older trees, and reasons are given for this. Estimates of mean height losses of 20-25-year old tree stems sustaining 50 percent girdling were 11.5 and 10.9 percent over 2- and 3-year periods respectively. The total impact of the weevil in the stand as a whole appears to hasten successional changes during stand development. A method of regulation of weevil abundance is postulated and takes into account the behavior of the female during oviposition, host selection, larval feeding habits, cumulative damage and host interactions. Overall numerical restraint and stability of numbers are considered to be effected largely through the inherent behavior of adults. Several weevil control measures are suggested through forest management. Clearcutting of mature timber in alternate strips reduced a weevil population by an estimated 67 percent, but some larvae developed to adults in the cut stumps one and two years after tree removal. The effect of cutting resulted in a concentration of weevils on adjacent trees along stand peripheries, 3-5 years after cutting. / Forestry, Faculty of / Graduate

Saprophytic fungi and bacteria were recovered from healthy foliage of Douglas fir (Pseudotsuga menziesii (Mirb.) Franco). The populations of microorganisms on foliage were variable and changed through the growing season. Three species of the bacteria isolated from Douglas fir foliage were used in attempts to control the development of the rust, Melampsora medusae Thum., on Douglas fir seedlings in the greenhouse. Effective control was obtained when Bacillus cereus Frankland and Frankland and B. mycoides Flugge were applied to the seedlings in pure cultures of nutrient broth. The most effective control was obtained from the application of a mixture of these two species and a third unidentified species of Baci1lus in nutrient broth. Less effective control resulted from the application of bacteria suspended in water and cell free filtrates of the bacterial cultures. The numbers of bacteria on the foliage of Douglas fir were estimated using a modification of the soil dilution and bacterial plate count technique. The bacterial populations on foliage of Douglas fir in the greenhouse and in the field were increased by the application of bacteria in nutrient broth and sterile nutrient broth. / Science, Faculty of / Botany, Department of / Graduate

Volatile constituents of bark from the two species of pine hosts of the mountain pine beetle, (Dendroctonus ponderosae Hopkins) were investigated as postulated factors in host tree discovery and selective attack by the insect. The anemo-olfactory, klinokinetic and klinotactic responses of pedestrian adult flown females to odors of different aged pines of the species Pinus ponderosa Laws, (ponderosa pine) and Pinus monticola Dougl. (western white pine) were used as criteria to test for evidence as to possible involvement of host tree volatiles in host selection. Volatile substances from pine bark were collected through sublimation under vacuum from frozen state. The volatile extracts were analyzed by Gas-Liquid Chromatography. Total volatile extracts and their ether soluble fraction from mature pine trees were "attractive" to beetles, whereas those from saplings were "repellent." No qualitative differences were found in the ether-soluble fraction of mature trees and saplings respectively, but proportions of individual constituents in the extracts differed. Ethanol at various concentrations caused arrestance of the anemo-olfactory response of pedestrian beetles. As a constituent of trees under stress ethanol may thus play an important role in the programme of responses which lead ultimately to attack. Problems encountered during the storage of tree samples and their extracts have been discussed. / Forestry, Faculty of / Graduate

Polyclonal antisera produced against spores, soluble protein and the whole mycelium fractions of Didymella lycopersici reacted with the homologous and heterologous antigens. The most sensitive antiserum was that raised against the whole mycelium, the soluble protein and the spore, in decreasing order of sensitivity. Using the antiserum raised against the whole mycelium it was possible to detect D. lycopersici on diseased plants and infested seeds. Cross reactivity was observed between the antisera produced to D. lycopersici and D. applanata, D. bryoniae and other tomato fungal pathogens including Fusarium spp. and B. cinerea. ELISA was most sensitive and reliable compared to double immunodiffusion, and latex tests. No reactions were obtained using the latex agglutination procedure and no antiserum detected spores in double diffusion tests. Protein profiles on SDS-PAGE revealed that the total number of protein bands decreased with increased age of cultures of D. lycopersici incubated in liquid media. Western blots probed with the antiserum raised against the whole mycelium showed that protein bands from extracts of both D. lycopersici and D. applanata were antigenic. / Land and Food Systems, Faculty of / Graduate

Blueberry bushes with scorch symptoms were found during a survey of blueberry fields in British Columbia, Washington, and Oregon. Some of these bushes were infected with blueberry scorch virus (BBScV) while others contained a second virus which was sap transmissible to Nicotiana clevelandii, N. benthamiana, and N. tabacum cv. 'Havana 425' . This virus was purified from N. clevelandii and had isometric particles of approximately 30 nm in diameter, a coat protein subunit of 27,500 daltons and a tripartite genome. I was unable to transfer the virus from either infected N. clevelandii or infected blueberry to healthy N. clevelandii with Myzus persicae or Fimbriaphis fimbriata. Serological tests of this blueberry virus with antisera against members of the ilar-, cucumo-, bromo-, or nepovirus groups failed to indicate any relationship. In a subsequent survey using enzyme-linked immunosorbent assay, this isometric virus was found in blueberry plants from northern Washington state to central Oregon but has not yet been found in B.C. Of the established members of the carlavirus group examined, BBScV is most closely related to potato virus S (PVS) and less closely related to carnation latent virus (CLV) and potato virus M (PVM). The difference in host range between BBScV and PVS would indicate that the BBScV is not a strain of PVS but is a separate virus that is related to PVS. Therefore, BBScV should be renamed blueberry scorch carlavirus (BBSCV). BBSCV was also compared to a carlavirus isolated from blueberry in the Sheep Pen Hill blueberry growing area of New Jersey (referred to as SPHV). These viruses were compared serologically and by use of nucleic acid hybridizations. BBSCV and SPHV were found to be closely related and were concluded to be strains of the same virus. SPHV should be named the New Jersey strain of BBSCV. / Land and Food Systems, Faculty of / Graduate

Genetic studies were carried out on the fungal parasite Ustilago hordei (Pers.) Lagerh. and on its host, Hordeum vulgave L. (cultivated barley). In these studies of the host-parasite relationship, special emphasis was placed on the genetic investigation of the pathogenicity. The thesis is divided into four parts. Part I deals with multial1 elism of genes for virulence (v-genes) in the parasite, and demonstrated that four different levels of virulence (obtained on the barley cultivar Trebi) are controlled by alternative alleles at a single genetic locus in the parasite. This is the first demonstrated example of a series of multiple alleles determining different levels of virulence. Part II concerns the identification and characterization of v-genes in U. hordei and of resistance genes (R-genes) in H. vulgave. Three v-genes (two of them new, one of them previously known) were identified. It was shown that the previously identified gene was expressed either as a dominant or a recessive, dependingoon the conditions under which it was tested, and that the newly-identified genes were both recessive. Cultures possessing the newly-discovered v-genes were used in identifying two new R-genes in the barley host. A study of interactions involving the newly discovered v- and R-genes led to the conclusion that these interactions have their basis in gene-for-gene relationships. Part III deals with the synthesis of a complex biotype of u. hordei possessing v-genes at two genetic loci. Disease reaction obtained with this complex biotype were compared both qualitatively and quantitatively with those obtained with the simpler, parental biotypes. In tests on certain cultivars the complex biotype produced either the same or higher levels of disease reaction. Because the new biotype has ah extended host range it is considered that under certain conditions it would be comparatively more fit than either of the parental biotypes from which it was derived. Part IV of the thesis concerns the effects of nutritional deficiency on the action of v-genes. Dikaryons which were homozygous for arg, ad or met were in all cases non-pathogenic; for those which were homozygous for pdx, pathogenicity was unaffected. For dikaryons which were heterozygous for one or more nutritional deficiences, pathogenicity was either unimpaired or reduced, depending on the combination (deficiency: virulence gene: host cultivar) which was tested. It was concluded that the specificity of pathogen biotypes was not determined by the availability or non-availability of specific nutritional factors. However, the effects were not entirely non-specific, since changes in levels of virulence were shown only in certain tests. / Science, Faculty of / Botany, Department of / Graduate

Indigenous Rhizobium isolates from naturally-formed bean root nodules were antagonistic to some of the root rotting pathogens of snap bean. Rhizobium isolates inhibited the radial growth of Fusarium species in dual culture agar plate tests but were not inhibitory to Rhizoctonia solani or Pythium isolates. With one exception, all indigenous Rhizobium isolates showed some degree of antagonism towards F. solani f. sp. phaseoli in vitro. The level of in vitro inhibition depended upon the agar plate technique utilized. A high level of in vitro inhibitory activity was recorded in 38% of the Rhizobium isolates tested where wide zones of inhibition formed between the test isolates and persisted for more than 1 week. A similar inhibitory effect of 8/17 nodulating Rhizobium isolates to Fusarium root rot of snap bean was observed in growth pouch experiments. Protection of bean plants from severe Fusarium root rot occurred in combinations where the inoculum concentration of Rhizobium (10\ 106 cells/pouch) was equal to or greater than the inoculum concentration of F_. solani (10² , 10⁴ spores/pouch). Ten Rhizobium isolates, which were highly antagonistic in vitro, had no apparent inhibitory effect on Fusarium root rot in vivo. Two Rhizobium isolates, RCC324 and RCC607, inhibitory to Fusarium root rot did not reduce Rhizoctonia root rot of bean. Soil experiments supported the results of growth pouch experiments whereby inoculation of bean seed with a high concentration of Rhizobium (RCC106 at 10⁸ cells/seed) effectively reduced bean root rot incited by a low inoculum potential of the pathogen, F. solani (inoculum: soil, 1:10⁴ or 1:120). These results indicated the potential exists for field control of Fusarium root rot of snap bean by a highly antagonistic nodulating isolate of Rhizobium. / Land and Food Systems, Faculty of / Graduate

Please read the abstract in the section 00front of this document / Dissertation (MSc Agric (Plant Pathology))--University of Pretoria, 2006. / Microbiology and Plant Pathology / unrestricted