Grape downy mildew risk analysis in Bordeaux vineyards based on regional survey data and local expert knowledge analysis

Supervisors: David Makowski et François Brun (ACTA)

Doctoral thesis of Paris-Saclay University prepared at AgroParisTech
Doctoral school n° 581 Agriculture, Food, Biology, Environment and Health (ABIES)
PhD speciality: Agronomic sciences
Thesis presented and defended in Paris, on December 12, 2019

Abstract

Pesticides reduce yield losses but have negative environmental consequences. It is important to provide precise information on the epidemic risks concerning harmful organisms in order to reason the use of pesticides, in particular in the case of grape downy mildew, which is responsible on average for 43% of pesticides used in Bordeaux vineyards.The objective of this work is to estimate the benefits of using downy mildew onset date to avoid unjustified sprays in the control of this disease. Based on regional observations and local expertise, we show that in Bordeaux, the first treatments are applied on average four weeks before the first symptoms appear.We show that postponing the date of the first downy mildew spray to disease onset reduces fungicide use by an average of 56% compared to current practices in this region. For operators, our results show that combining this strategy with the use of personal protective equipment reduces exposure by more than 70%.By using machine learning methods, we also show that the precocity and severity of downy mildew epidemics are strongly linked. Our predictions can be used to trigger disease treatments only in high-risk cases, resulting in a reduction of more than 50% in mildew treatments compared to current practices. Our results and the used methods are discussed and compared with other methods for reducing the use of pesticides in viticulture.