A correct approach to crop protection must consider the effect of disease on plant activity and its impact in terms of economical losses. These evaluations can drive the farmers to a quantification of plant responses to pathogen attack and to the application of economical thresholds for fungicide sprays. For grapevine (Vitis vùlÎfera L.), the knowledge of plant responses to disease is still very limited independently from the level of investigation (physiology, growth, development, etc.). The aim of this research was to evaluate the impact of downy mildew (Plasmopara viticola Berl. et De Toni) on grapevine photosynthesis under controlled and field conditions. Downy mildew was shown to reduce to a negative value net photosynthesis in the oilspot, as well as to affect the physiology of gas exchanges of the tissues surrounding the oilspot. The daily trend under field condition pointed out a normal trend of assimilation, without differences between green portions of healthy and diseased leaves. The consequences on source-sink relationships were discussed, as well as the relation between visible symptoms and physiological alterations.
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