Main symptoms 

• Berries

The rot acid starts frequently near the stem attaches bays or at various injuries. The altered tissues then take on a color which varies according to the state of development of the rot and the grape varieties. It is rather light brown on white varieties (figures 1 to 7) and purple to reddish on red varieties (figures 8 to 10). Once initiated, the rot spreads fairly quickly to the entire berry which, as a rule, retains its shape. Its skin gradually weakens and juice flows, giving a shiny appearance to the adjoining berries located below. More rarely, whitish mucous masses are observed following the formation of yeast colonies (figure 5). These clusters should not be confused with the many eggs that fruit flies lay on the wounds of rotten berries. Some clusters may be entirely affected (Figure 11).

A vinegar odor characterizes the disease; sometimes very strong, it emanates from particularly affected bunches and vines. The presence of numerous fruit flies (figures 12 and 13) is also a distinguishing factor of this decay.

• Confusion : Several rots can appear on the same bunch (figure 14), and diagnostic confusion is possible when the symptoms of acid rot are still little developed, with:
  • gray rot, recognizable in the long term thanks to the numerous conidiophores and conidia of Botrytis cinerea forming on the damaged tissues;
  • Rhizopus stolonifer . This little-known fungus occurs very occasionally in the vineyard; it is characterized by the production of black "pinhead" fruiting bodies.
    
    

• Organoleptic impact

This disease is very detrimental because it leads to an increase in the volatile acidity of the musts. Considering yeasts, K. apiculata is capable of producing a considerable amount of secondary compounds especially acetic acid. P. membranifaciens and M. pulcherrima are also known to oenologists because they lead to an increase in the content of acids and esters.

Acetic bacteria are undesirable at all stages of wine making because they notably have the property of oxidizing ethanol to acetic acid. Generally, Gluconobacter oxydans predominates on grape berries, Acetobacter aceti rather contaminates wines whereas Acetobacter oxydans is more occasional there.