Decision aid to support the use of curative late blight fungicides
Late blight, caused by the oomycete pathogen Phytophthora infestans (Mont.) de Bary remains a severe threat to potato production in temperate regions and necessitates a high volume of fungicide inputs. A recent focus of research has been to improve the application of Integrated Pest Management (IPM) principles to the control of late blight. Extensive monitoring programmes and warning systems are currently delivered via online platforms, and growers also have access to information on fungicide properties and the relative resistance of different cultivars to the pathogen. Growers and agronomists would benefit from additional tools which aid the decision-making process and allow improved integration of available control strategies. A strong example of this is the use of curative fungicides. Fungicides which can act curatively (within the incubation period of pathogen development) are an increasingly important component of late blight control, and there is scope to improve their deployment. The aim of this study was to produce a simple decision aid that can be used by growers and agronomists to inform their decision to use a fungicide with curative properties following weather conditions associated with a high risk of infection. Guidance available before the development of this decision aid was somewhat subjective, and did not take into account factors that may modify the efficacy of curative fungicides for which there is very little published information. Several contemporary P. infestans isolates were characterised in this study for their growth rates, both visually and sub-clinically using a qPCR assay. These data were then used to test a range of potential pathogen growth models which have been used by previous authors to model temperature-dependent growth in other biological systems. Many of these models provided good descriptions, and the best performing was used to predict pathogen development with the decision aid. Characterisations of the curative effect for a representative curative fungicide (propamocarb-HCl + fluopicolide) were generated for selected isolates in both laboratory assays and under field conditions. Within the laboratory bioassay, more frequent sampling (4 hour intervals) than is usually reported in experiments of this nature was used to assess the nature of the curative effect over a time frame of up to 72 hours post inoculation. Curative control declined rapidly with increasing pathogen development, with the relationship best described by a logistic function. This function, and the parameters generated from the bioassays, were used within the decision aid to predict the likely outcome of curative treatments.