Grapevine - Biology, epidemiology

Biology, epidemiology

Storage and sources of inoculum

Depending on the genetic group to which it belongs, Erysiphe necator can be conserved during the winter at least in one of the 2 following forms: dormant mycelium remaining in the buds, or thanks to cleistothecia (figures 1 and 2), structures from its sexual reproduction forming on the surface of herbaceous organs at the end of the season. These cleistothecia spend the winter on the leaves that have fallen to the ground, on the twigs, but especially in the crevices of the bark of the vines.

Penetration and invasion

In spring, depending on how stored E. necator is , the primary inoculum can be of a different nature:
- mycelium dormant , present in the buds, reactivating as soon as the temperature becomes milder. Subsequently, it spreads to the surface of organs, and emits specialized hyphae which enter epidermal cells after having pierced the cuticle and the wall of plant cells. These suckers (called haustoria ) allow E. necator , once it has entered the cells, to collect the food it needs to grow. In the presence of such a primary inoculum, symptoms of powdery mildew of the "flag" type will appear subsequently (Figure 3);
- ascospores from mature and dehiscent cleistothecia (Figures 4 and 5). These germinate once present on an organ and initiate primary contaminations (figure 6). Subsequently, several branching mycelial filaments develop as well as haustoria at regular intervals, which allows the fungus to superficially colonize the tissues and live at their expense.

Sporulation and dissemination

Whatever the nature of the primary inoculum, the mycelium formed fairly quickly produces perpendicular conidiophores at the end of which chain conidia are born (figure 7). These are mainly disseminated by air - by the wind - and pollute new organs or new vines. Secondary contaminations will occur once they have germinated and formed an appressorium which will allow them to attach to plant tissues (figure 8).

Sexual reproduction occurs when two compatible mycelia meet (of opposite sexual signs called "MAT 1" and "MAT 2" for MAting Types 1 and 2). After fusion and passage through a diploid phase, cleistothecia are produced at the end of the season in particular. They can be found from July-August in a temperate climate, on leaves and berries, and fall on the ground or on the bark of old wood where they remain fixed by their fulcers.

These reproductive organs are likely to withstand significant negative temperatures (-12 ° C to -28 ° C). In spring, after maturation, they release ascospores which form a primary inoculum largely in the majority in European vineyards.

We should add that certain materials intended for the application of pesticides, generating air currents and turbulence in the plots, contribute to the dispersion of E. necator .

Powdery mildew is a polycyclic disease : several asexual cycles follow one another during the growing period of the vine.

Factors influencing the development of the fungus

- This fungus does not require liquid water to germinate and develop, however it requires high humidity and low light. This is why he likes vigorous vines in which the light penetrates less well. However, the light favors the germination of the spores, but on the other hand UVB would have negative effects on the development of E. necator .
- The spores germinate under natural conditions at temperatures between 4 ° C and 35-40 ° C, with an optimum of the order of 20 to 25 ° C. Germination begins in 1 to 2 hours, or even several hours depending on climatic conditions. The germination rate increases with temperature over a range of 15 to 27 ° C.
- The first leaf sporulations appear within 5 days at 22 ° C, and stop after 35 days.
- The high relative humidities are favorable to the development of the fungus, as well as the irrigation. The same is true for fine rains, unlike heavy rains which leach out the conidia.
- The wind promotes the dissemination of conidia and limits summer temperatures. High winds (3m / s) are necessary to ensure the dispersal of the spores.
- All the grape varieties produced from Vitis vinifera and V. labrusca are more or less susceptible to E. necator . This sensitivity varies depending on the cultivars and species of Vitaceae. The grape varieties have been classified into three groups according to their sensitivity to powdery mildew: very sensitive (chasselas, carignan, cabernet sauvignon ...), sensitive (merlot, pinot noir ...) and not very sensitive (syrah, grenache. ..).
- Sources of resistance exist, in particular in Muscadinia rotundifolia ( Vitis rotundifolia ) which has several genes including RUN involved in resistance to powdery mildew. Their introgression into Vitis vinifera by crossing is possible and constitutes an interesting avenue for providing an alternative to chemical control against this pathogen. Let us add that a large number of hybrids have been created, but unlike the United States, they are little used in France.