Disease Decision Support Systems: Their Impact on Disease Management and Durability of Fungicide Effectiveness

In modern agriculture, fungicides play a major role in maintaining crop health and reliable yields of products that meet the demand for high quality. Hence, fungicides represent a key component of most integrated disease management programs, and their effectiveness should be sustained as much as possible. However, fungicide effectiveness has been significantly affected by the development of resistance in some fungal pathogen populations. Moreover, environmental considerations are becoming increasingly important and, as a consequence, it is less acceptable to apply fungicides needlessly. In this chapter we intend to demonstrate that disease decision support systems could help reduce fungicide usage and as a consequence maintain the durability of fungicides effectiveness. This will be done using apple scab, caused by the fungal pathogen Venturia inaequalis as a model system. In most apple producing areas of the world, apple scab is the most important disease and its control depends almost exclusively on frequent use of fungicides; in some areas up to 75% of all pesticides used are applied to control apple scab. Apple scab is a complex disease that develops following two phases; a monocyclic phase caused by ascospores produced in pseudothecia originating from leaf lesions present the previous fall, and a polycyclic phase produced by conidia originating from lesions on leave and fruits. Various management tactics to manage apple scab were developed during the last half century. These tactics are either biologically/culturally or chemically based. However, in most IPM programs, the control of apple scab is based on the use of fungicides and these programs are developed to improve fungicide efficacy using procedures to estimate potential ascospore dose, weatherbased models to estimate ascospore maturity, leaf growth and risk of infection periods. These tools were integrated into various disease decision support systems (DDDS). Almost all of the DDSS focus on management of primary infections (monocyclic phase) and much less attention has been paid to secondary infections (polycyclic phase) that develop during the summer, despite that up to 6-8 additional fungicide sprays are often used to manage