François Delmotte 

Researcher at INRAE, Director of the SAVE INRAE-ISVV research unit

Published in "Terre de vins" No. 88 – September 2023

 

Today, downy mildew is present in all French vineyards. How did it arrive in France and how does it spread?

Like phylloxera, which caused a major health crisis in the wine industry, downy mildew was imported from America with wild grapevine species selected for their ornamental qualities. This brown algae spreads very rapidly. Following an asexual phase in spring, the pathogen spreads via spores on the vine’s leaves, bunches and tendrils. After the harvest, in autumn, the algae begin their sexual reproduction. The organisms mate and produce winter eggs within the leaves. When the leaves fall and decompose, the eggs are incorporated into the soil, creating a reservoir from which a new outbreak will emerge the following spring.

In the face of downy mildew, research is developing sustainable solutions based on prevention, genetics and the reduction of inputs.

What methods are used to control downy mildew?

The disease is mainly controlled through plant protection treatments applied to the leaves in spring and summer, such as Bordeaux mixture or synthetic fungicides. However, one of the problems with these treatments is the copper they contain, which accumulates in the soil and kills off all life there.

Are there any alternatives to these plant protection treatments?

Nowadays, it is difficult to guarantee a harvest – whether in organic or conventional farming – without using pesticides. But we want to try to move away from these chemical solutions by staying one step ahead of the disease, using two approaches. The first is to break the pathogen’s sexual cycle in order to reduce egg populations. As with the sexual confusion technique used against insects, we aim to act pre-emptively to prevent downy mildew from mating by deciphering its hormonal signalling. We are also working on the removal and composting of leaves bearing eggs. The second approach involves the use of naturally resistant grape varieties obtained by crossing wild vines – which possess genes enabling them to withstand attacks by the algae – with cultivated varieties. Extensive selection work is being carried out through successive backcrossing to produce resistant vines with a genotype similar to our Vitis vinifera varieties. INRAE and the IFV have already developed varieties with long-lasting resistance to downy mildew and powdery mildew. These varieties are now being used as parent stock in regional programmes aimed at creating varieties adapted to climate change whilst preserving the regional character of the wines.