Biology, epidemiology

• Storage and sources of inoculum

These yeasts and bacteria are very present in the vineyard; they are easily preserved in the soil on plant debris, on mummified fruits, twigs, buds. Throughout its vegetative phase, the various organs of the vine are polluted by these microorganisms which partly constitute its surface flora. Thus, the surface of mature grape berries constitutes the natural habitat of many yeasts (especially K. apiculata and its teleomorph H. uvarum ), of which about thirty species have been identified. Acetic bacteria are also part of the natural microflora of berries.
These yeasts and bacteria are hosted by many hosts and are sometimes responsible for various deteriorations, in particular acid rots as is the case on figs, tomatoes, pineapples ...

• Plant penetration and host invasion

This berry affection manifests itself after veraison, especially on ripe berries rich in sugars. These conditions promote the multiplication of yeasts and bacteria which are present in large quantities on the berries. They penetrate the latter near the peduncle attachment or via various injuries: bird pecks *, insect damage (figure 1) and hail, micro or macro-bursts of berry growth occurring as a result of hydric stress .... Subsequently, they multiply rapidly in the pulp and acidify it in a few hours. Symptoms are already visible 2 to 3 days after the contaminations (figures 2 and 3).

* A recent study showed that the beak of some birds and the nascent part of their digestive tract housed several species of contaminating yeast, such as Hanseniospora uvarum .

• Multiplication and dissemination

These microorganisms multiply very easily by budding (scissiparity) in the berries which harbor them in large quantities. Whitish mucous deposits, resulting from the formation of yeast or bacterial colonies, are sometimes visible on the berries. They can be disseminated in different ways, including:

- Drosophila are particularly involved in the dissemination of acid rot, in particular Drosophila melanogaster and to a lesser extent Drosophila simulans (figure 4).

Vection by adult fruit flies can take place in two ways: either by the presence of propagules on the surface of the insect's body, or possibly internally in their crop. Indeed, observations by scanning electron microscopy (Figures 5 and 6) make it possible to observe the presence of yeast cells on certain parts of the insect's body, such as the thorax and the legs ( K. apiculata is systematically detected and C. stellata is very common). However, these propagules are rarer on the head, abdomen or wings. An inventory of the mycoflora of the crop of these insects reveals 2 dominant species: K. apiculata and C. stellata , but also more occasionally P. membranifaciens , M. pulcherrima , Candida krusei , and various fungi such as Botrytis cinerea  or belonging to the genera Rhizopus , Penicillium . Let us add that the cells of the yeasts in particular could be expelled alive after the intestinal transit.

Of Gluconobacter spp., And less frequently Acetobacter pasteurianus, are isolated from the surface of Drosophila and their crop. It is therefore highly probable that these insects transport and transmit these different microorganisms during their movements on and in the bunches of grapes.

Finally, the transmission of these microorganisms is also ensured by:
- contact of infected berry with healthy berry, especially in the heart of very compact clusters;
- the flow of acid juice along the affected bunches;
- the hands and tools of the grape pickers;
- some nematodes associated with rotten acid berries, Panagrellus zymosiphylus (figure 7), Turbatrix aceti , which can be carriers of yeasts in particular and which are transported by fruit flies and found in their droppings;
- the birds.

• Factors influencing the development of the fungus

Acid rot is particularly damaging in vineyards where crops are harvested at an advanced stage of maturity, particularly in the case of certain grape varieties intended for the manufacture of sweet wines. It is more frequent in vigorous plots, cultivated under canvas and / or plastic. Note that the distribution of acid rot in the plots is also linked to the vigor of the vines, but also where they are crowded and compact. The climatic conditions occurring before the harvest greatly influence the development of this disease: frequent rains, humidity and high temperatures are very favorable. Remember that the damage caused by other pests ( eudemias , wasps, powdery mildew ) or various climatic or agro-cultural stresses can sensitize the berry to the expression of acid rot.