In interactions between plants, insect herbivores and natural enemies, resistant plant varieties and soil abiotic/biotic factors can affect natural enemies through changes in the plant or host insect. Interactions can be negative, neutral or positive in relation to herbivore populations. The majority of studies have used above-ground plant–insect–parasitoid systems and relatively few studies have included below-ground effects on above-ground tritrophic interactions. The aim of the present project was to understand how soil-based stress factors (nutrient availability and root pathogens) influence multitrophic interactions in a potato-aphid (Myzus persicae)-parasitoid (Aphidius colemani) system. Specific objectives were: to assess aphid performance on different potato cultivars under laboratory and field conditions; use aphid resistant and susceptible cultivars to assess the effects of soil nutrition and a plant pathogen (root-knot nematode) on aphid performance; conduct multifactorial ditrophic (aphid performance) and tritrophic (parasitism success) experiments with selected combinations of soil treatments; use olfactometry to determine whether plant volatiles are involved where significant tritrophic interactions occur between treatments. Potato cv. Anya, a cv derived from Desiree and Pink Fir Apple, was found to be consistently resistant to M. persicae compared with the three other cultivars tested (Desiree, Santé and Pink Fir Apple) in both laboratory experiments (UK) and field trials (Pakistan and UK). For Anya (resistant) and Desiree and Pink Fir Apple (susceptible), high levels of soil nutrients enhanced aphid performance, their parasitism by A. colemani, and adult parasitoid emergence. Root-knot nematodes had a negative effect on aphid performance at both low and high soil nutrition but had a positive effect on parasitism at low soil nutrition. Olfactometry showed Desiree to be more attractive to M. persicae compared with Anya. The work is discussed in relation to the development of pest management strategies for the control of M. persicae on potato.

This study presents the theoretical and experimental results of natural ventilation rates induced by stack, wind and the combination of both stack and wind effects for a typical crop protection structure suitable for the tropics. The structure consists of simple structural frame, transparent roofing and insect screen side walls. It was found the relative importance of the stack and wind effects is dependent on the ratio between wind speed and the square root of the inside-outside temperature difference (u/AT°.5). In this study, the wind effect dominates over the stack effect when the ratio u/AT" becomes greater than 0.5. Ventilation rate induced by the stack effect was found to increase with increasing temperature difference between inside and outside of the crop protection structure according to a power law, with an index of 0.5. The wind effect ventilation rate was found to increase linearly with increasing outside wind speed measured at eaves level. In addition, the combination of the stack and wind effects could be represented as the vectorial sum of two the independent effects (40sw = (43)k2 + (1)„,nd21 0.5). However, the result of the wind effect in the combined effects was insignificant when the ratio of ventilator opening to the total wall area is higher than 20 %. Different methods have been used to determine the natural ventilation rates. The dynamic tracer gas was used as the control; direct airspeed measurement, energy balance and neutral plane methods were used to quantify ventilation induced by the stack effect. Pressure field measurements were used to quantify ventilation by wind effect. In addition, the dynamic tracer gas, energy balance, and stack and wind methods were used to quantify ventilation induced by the combined effects. However, these methods have their constraints and limitations because of statistically significant differences in the comparison between the methods. The tracer gas method was found very difficult to use in the highly porous structure. In addition, the ventilation rate measured by this method was 30-40 % less than the other methods. The energy balance method has the advantage that it estimates many important climatic and crop parameters, however, the errors were found to be the highest. The neutral plane method was suitable for measuring ventilation induced by stack effect, the simplest method, requiring only the measurement of the inside and outside temperatures. The direct airspeed measurement method was much easier to handle and the result was comparable to other methods suitable for determining the ventilation induced by the wind effect. The physical properties of the covering materials, namely light transmission, coefficient of discharge and airflow characteristics were also determined in this study. It was found that the light transmissions of transparent polythene film and insect screens were close to each other. The coefficient of discharge and light transmission were dominant parameters in the ventilation rate calculation. It was found that when air flows through a screen, the pressure drop increases linearly with the square of approach airspeed. Airflow distributions inside the crop protection structure induced by the stack and wind effects are also presented in this study. Finally, this study presents information on natural ventilation for tropical greenhouses that was not previously available.

The study was carried out to clarify the nature of symptomless infection by Botrytis cinerea and to what extent it differs from aggressive necrotic infection in Lactuca sativa (lettuce) and Arabidopsis thaliana. Symptomless plants were produced by dry spore inoculation in plants growing in controlled environmental conditions or in glasshouses. Plating out of surface-disinfected and non-surface-disinfected samples of inoculated, apparently healthy, plants on selective medium revealed that the fungus was spreading from the initial inoculation site to newly developing plant organs both internally and externally. Similar findings were obtained in microscope experiments in which host plants were inoculated with GFP labelled B. cinerea and symptomless spreading was monitored under confocal laser scanning microscope. Spore germination on leaf surface was followed by development of sub-cuticular vesicles and plant cell damage in the infected epidermal cell and a few nearby cells. Sparsely branched long hyphae arose from the vesicles and spread on the leaf surface; spread was mostly on the outer surface of the epidermal layer but occasionally below the cuticle or epidermal cells. In the late symptomless phase, mycelium arising from single vesicles formed several mycelial networks on leaves. Experiments were carried out to compare the extent of gene expression in symptomless and necrotic infections, using RT-qPCR. Expression of selected genes was quantified in tissue samples based on the amount of mRNA of the respective genes found. In both host species, the mRNA concentration of signalling genes bcg1, bmp1 and calcineurin, and the pathogenicity genes bcsod1 and bcpg1 were similar to or slightly greater in symptomless samples than in necrotic samples. The mRNA of the signalling gene bac and pathogenicity genes bcbot1 and bcnep1, were not detected or detected in lower abundance than in necrosis. In lettuce, the leaves developing distant from the site of inoculation showed similar results to A. thaliana, but in healthy leaves close to the site of inoculation mRNA concentrations of bac and bcnep1 were similar to necrotic samples. Thus, in both host species, the fungus grew along with the plant and moved to newly growing plant parts without producing symptoms; during this growth some pathogenicity genes were less expressed than in necrotic infection.

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