In supervised control of pests and diseases real-time information on pest and disease intensity and cost-benefit analysis of decision alternatives are used to support farmers' decisions on pesticide application. This thesis focuses on improving supervised control in the pathosystem aphids - brown rust - winter wheat. First, effects of the grain aphid, Sitobion avenae , on crop physiology are studied to arrive at simple relations between aphid density and damage. Second, the consequences of uncertainty about the costs of alternative strategies for optimal timing of chemical control of aphids and/or brown rust ( Pucciniarecondita ) are evaluated.Uptake of assimilates by S. avenae and reduction of the rate of net leaf photosynthesis by honeydew were quantified and introduced into a model which simulates growth and development of winter wheat from flowering till ripeness. Attainable yields from 4000 to 10000 kg ha -1and damage at the higher yield levels were simulated satisfactorily. Honeydew caused more damage than assimilate uptake at crop development stages until milky ripeness. Attainable yield, crop development stage, and timing and intensity of infestation were of major importance in explaining variation in damage. The results of the simulation model were summarized in regression models which predicted damage at given infestation levels as good as the best published empirical model.Sources of uncertainty in a decision model for evaluating costs of chemical control strategies were quantified using field data. Assessment of the contribution of various sources of uncertainty to model output uncertainty showed white noise in the relative growth rates of both aphids and brown rust to be the major cause of uncertainty. Risk-neutral damage thresholds representing pest intensities at which expected costs of control just exceed expected costs of no control, were considerably lower than the deterministic damage thresholds based on average input values. At the risk-neutral damage thresholds only 30% of chemical applications was found to be cost-effective. To support pest control under uncertainty a framework is proposed in which alternative strategies are evaluated in terms of 'profitability' and 'risk', operational measures of the economic objectives 'return on investment' and 'insurance', respectively.

In supervised control of pests and diseases real-time information on pest and disease intensity and cost-benefit analysis of decision alternatives are used to support farmers' decisions on pesticide application. This thesis focuses on improving supervised control in the pathosystem aphids - brown rust - winter wheat. First, effects of the grain aphid, <em>Sitobion avenae</em> , on crop physiology are studied to arrive at simple relations between aphid density and damage. Second, the consequences of uncertainty about the costs of alternative strategies for optimal timing of chemical control of aphids and/or brown rust ( <em>Puccinia</em><em>recondita</em> ) <em>are</em> evaluated.Uptake of assimilates by <em>S. avenae</em> and reduction of the rate of net leaf photosynthesis by honeydew were quantified and introduced into a model which simulates growth and development of winter wheat from flowering till ripeness. Attainable yields from 4000 to 10000 kg ha <sup>-1</SUP>and damage at the higher yield levels were simulated satisfactorily. Honeydew caused more damage than assimilate uptake at crop development stages until milky ripeness. Attainable yield, crop development stage, and timing and intensity of infestation were of major importance in explaining variation in damage. The results of the simulation model were summarized in regression models which predicted damage at given infestation levels as good as the best published empirical model.Sources of uncertainty in a decision model for evaluating costs of chemical control strategies were quantified using field data. Assessment of the contribution of various sources of uncertainty to model output uncertainty showed white noise in the relative growth rates of both aphids and brown rust to be the major cause of uncertainty. Risk-neutral damage thresholds representing pest intensities at which expected costs of control just exceed expected costs of no control, were considerably lower than the deterministic damage thresholds based on average input values. At the risk-neutral damage thresholds only 30% of chemical applications was found to be cost-effective. To support pest control under uncertainty a framework is proposed in which alternative strategies are evaluated in terms of 'profitability' and 'risk', operational measures of the economic objectives 'return on investment' and 'insurance', respectively.

The bacterial epiphyte Pseudomonas syringae MF714R was cultured on agar or in broth or collected from colonized leaves; it was then inoculated onto greenhouse-grown bean plants incubated in a growth chamber at low relative humidity or in the field or onto field-grown bean plants. Cells cultured in liquid medium survived the least well after inoculation of leaf surfaces under all conditions. Cells cultured in solid medium exhibited the highest percent survival and desiccation tolerance in the growth chamber but generally survived less well in the field than did cells harvested from plants. Cells harvested from plants and inoculated onto plants in the field usually exhibited the highest percent survival, started to increase in population earlier, and reached a higher number than did cells cultured in vitro. Differences in field survival were apparently not attributable to differential UV tolerance. The observed effects of phenotypic plasticity on epiphytic survival and colonization should be considered in risk assessment studies, in studies of bacterial epidemiology, and in the use of microbial antagonists for biological pest control.

Perspectives d'évolution de l'emploi des engrais et des phytosanitaires dans l'agriculture française (1993)On analyse l'évolution de l'emploi des engrais et des phytosanitaires dans l'agriculture française et ses perspectives pour les prochaines années, compte tenu des changements techniques, économiques et de politique agricole. Dans un premier temps on étudie l'évolution des achats des agriculteurs en engrais et pesticides depuis 20 ans et leurs pratiques actuelles en matière de fertilisation et de protection des cultures. Puis on examine les possibilités de réduction des consommations sous l'effet de l'évolution technique de l'agriculture et de diverses mesures économiques (primes à l'extensification, taxation des engrais, quotas d'azote). Dans un troisième temps, on étudie les effets prévisibles des changements de politique agricole, gel de terres, baisse des prix et nouvelle politique communautaire définie dans le projet Mac Sharry de juillet 1991. En définitive, une réduction des consommations semble plausible dans les années à venir, ainsi qu'une plus grande rationalisation de leur emploi. | Prospects for the trends in the use of fertilizers and pesticides in French agriculture.The authors analyze the trends of fertilizers and pesticides used in French agriculture after World War II and the outlook for the next few years, taking into account technical, economic and agricultural policy changes. One of the aims of this paper is to study the possibility of reducing pollution resulting from agriculture. Three points are studied. The first is how farmers' purchases of fertilizers and pesticides evolved during the decades after World War II and what their crop practices were in the early 1990s. Following the first oil shock the rapid increase in the use of fertilizers in the fifties and the sixties has given way (i) to a slower increase for nitrogen fertilizer, (ii) to a stagnation for potash fertilizer, and (iii) to a fall for phosphate fertilizer. On the other hand pesticide use has greatly increased over the past three decades, but decreased since 1990. The evolution of the economic importance of fertilizers and pesticides in agriculture is also studied. Finally the practices and reasoning of farmers as regards fertilization and crop protection are analyzed on the basis of a Spring 1991 survey of 867 French farmers.Secondly the authors analyze possibilities of reducing fertilizer and pesticide use by technical and economic means and try to assess their practicability. A relatively wide range of technical means is available: for fertilization, there are the options of low input crops, use of green manure, better adjustment of dosage, etc.; and for crop protection, biological and integrated pest control, better risk assessment and risk forecasting, resistant crop varieties, etc. It is noteworthy that agricultural technology is evolving towards increasingly accurate matching of input application to crop needs, which could reduce waste and leaching into ground water. The range of economic solutions is also relatively wide: subsidies for extensification, taxes on fertilizers, nitrogen quotas, etc. The survey on a sample group of farmers and several simulations on farms in the Paris Basin are used to study the possible impact of such solutions. The level of extensification subsidies necessary to compensate for the difference in economic results between the most intensive farms and less intensive ones is calculated. The authors also analyze farmers' opinions on an increase in nitrogen prices, on the implementation of a nitrogen quota, and on a "nitrate pollution" tax. Thirdly the authors study the foreseeable effects of a change in agricultural policy as defined in the proposal for the Common Agricultural Policy dated July 1991: a setting aside of farmland and a decrease in agricultural prices offset by direct subsidies to farmers. To assess the effect of this reform, the cases of intensive and more extensive farms are identified. The compulsory setting aside for medium and large farms as a condition for subsidy and the adaptation of crop practices to much lower agricultural prices, should lead to an adaptation of farming systems. A decrease in fertilizer and pesticide use, a limitation of the intensification process as well as a certain trend towards extensification seem plausible depending on trends in prices of agricultural products.