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A Decision Aid to Support the Use of Curative Late Blight Fungicides Kyran Maloney Doctor of Philosophy 2 2 ii Declaration This thesis has been authored by myself, and is substantially my own work. The work has not been previously submitted as part of any other degree of professional qualification. Kyran Maloney February, 2020 iii iv Table of Contents Acknowledgements.................................................................................................ix Abstract...................................................................................................................xi Lay summary........................................................................................................xiii Chapter 1 General introduction, background and literature review.........................1 1.1 Introduction...................................................................................................1 1.2 Pathogen biology and infection cycle............................................................2 1.3 Population structure.......................................................................................6 1.4 Epidemiology and risk prediction...............................................................10 1.5 Host resistance.............................................................................................15 1.6 Chemical control.........................................................................................19 1.7 Curative activity and its evaluation.............................................................24 1.7.1 Carboxylic acid amides (CAAs)..........................................................31 1.7.2 Cyazofamid..........................................................................................31 1.7.3 Cymoxanil............................................................................................31 1.7.4 Oxathiapiprolin....................................................................................32 1.7.5 Propamocarb........................................................................................32 1.7.6 Phenylamides.......................................................................................33 1.7.7 Fluopicolide.........................................................................................33 1.8 Diagnostics..................................................................................................34 1.9 Decision aids and support systems..............................................................35 1.10 Structure of project....................................................................................40 1.11 Project objectives.......................................................................................47 Chapter 2 General methods....................................................................................49 2.1 Maintenance of P. infestans cultures...........................................................49 2.2 Growth and maintenance of plants..............................................................51 2.3 Growth and maintenance of plants in the field............................................51 2.4 Production of inoculum...............................................................................52 2.5 Bioassay incubation conditions...................................................................53 2.6 Temperature monitoring..............................................................................54 2.7 Visual assessment of disease symptoms......................................................54 2.8 Biomass estimation using qPCR.................................................................55 2.9 Statistical analysis.......................................................................................58 Chapter 3 Selection of a temperature-dependent development model for describing growth of P. infestans in planta............................................................59 Abstract..............................................................................................................59 3.1 Introduction.................................................................................................60 3.2 Modelling temperature dependence in biological processes.......................61 3.2.1 Heat units and accumulated thermal time............................................61 v 3.2.2 Non-linear models of temperature-dependent development................65 3.3 Temperature-dependent development in P. infestans..................................67 3.4 Estimating P. infestans biomass accumulation at different temperatures....69 3.4.1 Temperature bioassay: methods and materials....................................70 3.4.2 Temperature bioassay: statistical analysis...........................................74 3.4.3 Temperature bioassay: symptom measurement results........................74 3.4.4 Temperature bioassay: biomass estimation results..............................76 3.4.5 Correlation between biomass and symptom measurement..................79 3.5 Model selection...........................................................................................81 3.5.1 Candidate models of temperature-dependent development.................81 3.5.2 Model fitting........................................................................................82 3.5.3 Model assessment for linear lesion growth rates.................................84 3.5.4 Model assessment for incubation period biomass accumulation.........90 3.6 Discussion and final physiological-time model selection...........................96 Chapter 4 Determining the effect of pathogen development on the level of curative control.....................................................................................................105 Abstract............................................................................................................105 4.1 Introduction...............................................................................................106 4.2 Curative activity and pathogen development............................................106 4.3 Characterisation of curative opportunity window.....................................108 4.3.1 Fungicide selection............................................................................108 4.3.2 Detached leaf bioassays: methods and materials...............................109 4.3.3 Detached leaf bioassays: data handling and statistical analysis.........113 4.3.4 Detached leaf bioassays: results.........................................................114 4.3.5 Leaf disc bioassay: methods and materials........................................116 4.3.6 Leaf disc bioassay: data handling and statistical analysis.................120 4.3.7 Leaf disc bioassay: results.................................................................121 4.3.8 Curative field trial: methods and materials........................................125 4.3.9 Curative field trial: data handling and statistical analysis.................127 4.3.10 Curative field trial: results...............................................................127 4.4 Relationship between curative control and pathogen development..........128 4.5 Discussion..................................................................................................134 Chapter 5 Modifying effect of host resistance on fungicide curative activity.....137 Abstract............................................................................................................137 5.1 Introduction...............................................................................................137 5.2 Modifying effect of cultivar resistance field trials....................................146 5.2.1 Natural inoculum whole plant trial....................................................146 5.2.2 Natural inoculum trial: materials and methods..................................146 5.2.3 Natural inoculum trial: statistical analysis.........................................150 5.2.4 Natural inoculum trial: results...........................................................150 5.3 Artificial inoculum trial.............................................................................156 5.3.1 Artificial inoculum trial: materials and methods...............................156 vi 5.3.2 Artificial inoculum trial: statistical analysis......................................157 5.3.3 Artificial inoculum trial: results.........................................................157 5.4 Plot scale field trial....................................................................................159 5.4.1 Plot scale field trial: materials and methods......................................159