Grapevine - Biology, epidemiology

Biology, epidemiology

Storage and sources of inoculum

Guignardia bidwellii can be preserved on mummified berries (unharvested grapples, which have remained attached to the trellis, or fallen to the ground), tendrils, infected leaves that have fallen to the ground and on cankers present on the vine shoots, in the form of undifferentiated conceptacles that evolve in perithecia during winter and spring (Figures 1 to 3).

Pycnidia can sometimes be a form of winter conservation (Figures 4 and 5). Remember that heaving in spring can bring mummified berries buried during fall plowing to the soil surface and therefore bring up a potential inoculum. Let us add that the abandoned vines allow G. bidwellii to multiply and be preserved; they permanently maintain a potential inoculum near the cultivated plots.

Plant penetration and host invasion

In the spring, the increase in temperature, associated with high humidity, induces the production of ascospores which are projected for several months from the mature perithecia; these pollute the vines, in particular the leaves and young berries, and are responsible for primary contaminations. They are ejected from the perithecium after a rainfall of at least 0.3 mm, and the process can continue for up to 8 hours after the precipitation stops. The spores are then exposed to the wind which can carry them over long distances to surrounding plants.

They germinate in the presence of free water or high relative humidity for several hours, if the temperatures are high enough. Infections occur in the presence of humidity, in 6 hours at 27 ° C, in 24 hours at 10 ° C and in 12 hours at 32 ° C. The germ tube gives rise to a mycelium that directly pierces the cuticle, penetrates and invades leaf tissue. The first symptoms appear after about ten days in summer, 20 to 30 days in spring.

Sporulation and dissemination of the fungus

Subsequently, brown to black pits appear on the altered tissues, it is the pycnidia that gradually form (Figures 6 and 7). They contain conidia (figure 8) which will ensure secondary contamination especially on the young berries located below. G. bidwellii also produces other globular structures, but smaller: the spermogonia. These form conidia or sperm in the shape of a rod which reveal some difficulties in germinating and whose epidemiological role is not clearly defined. Note that new contaminations could be initiated by ascospores or conidia. The outlines of the perithecia would rather form at the end of the season from the already formed and empty pycnidia.

Ascospores are carried by the wind over long distances. The conidia are dispersed in a more limited way on the occasion of the rains and the splashing generated by this one.

Factors influencing the development of the fungus

The length of the incubation period depends on both the temperature and the age of the tissue at the time of infection. The climate particularly influences the development of G. bidwellii , and in particular the frequent and lasting rains.

These also condition the maturation of the perithecia, the projection of ascospores, the dissemination of conidia, and their germination. It is for this reason that black rot is serious in humid temperate regions, and less active or even absent in drier Mediterranean areas. Let us add that G. bidwellii develops over a temperature range going from 9 ° C to a maximum of 32 ° C, its optimum being around 26 ° C.

The foliage of the vine is receptive to G. bidwellii from the emergence of the first leaves (stage D) to a few days after flowering (stage Y). The severity of leaf damage increases from the non-spreading stage to row 5 leaf. When plant growth stops, no attack is observed except on the regrowth. Young leaves in the growing phase are more susceptible than adult leaves. The resistance linked to the age of the organ is set up more quickly in the hot season and has the effect of reducing the sensitivity to the pathogen and in particular of extending the incubation time.

The cluster is very vulnerable from the flowering stage until the cluster closure stage; thereafter its sensitivity gradually decreases until veraison. Note that no infection would be possible before the fall of the floral caps; maximum sensitivity would be reached within two to three weeks of this stage. Let us add that the symptoms obtained by artificial contaminations, from ascospores or pycniospores, are identical under favorable conditions of temperature and humidity.

Several grape varieties are very sensitive to G. bidwellii : Aligoté, Garignan, Colombard, Cinsaut, Folle blanche, Gamay, Grenache, Merlot blanc, Mondeuse, Muscadelle, Muscadet, Négrette, Piquepoul, Poulsard, Sauvignon, Sémillon, Syrah, Ugni blanc. Other grape varieties are less sensitive to the fungus, they are: Cabernet Sauvignon, Jurançon blanc, Malbec, Marsanne, Portuguese blue, Roussanne and Tannat. Finally, some grape varieties are not very receptive to the disease, such as Chasselas and Clairette.