When it comes out of the latency phase, it develops preferentially towards the skin, due to the presence of antagonist enzymes (chitinase and B. glucanase) in the flesh cells. An exogenous nutrient supply facilitates the elongation of the germinative tube.When the mycelial hypha reaches a microfissure, it penetrates the grape. Thus B. cinerea development occurs mainly in the grape’s superficial cell walls.More precisely, the mycelial filaments are located in the middle lamella of the pectocellulosic cellnwalls. The latter are degraded by the enzymesnof the fungus (pectinolytic, cellulasic complex, protease and phospholipase enzymes).nWhen the mycelium has totally overrun the pectocellulosic cell walls of the epidermal and immediate sub-epidermal cells, white grapes take on a chocolate-brown color that is characteristic of the pourri plein stage. The mycelium thenproduces filaments that emerge from the skin surface by piercing the cuticle or by taking advantage of the diverse fissures used for the initial penetration. The filament extremities differentiate by producing conidiophores, whose conidia later detach and contaminate nearby berries.The cell walls of vegetal tissue are so greatly modified that they can no longer ensure theirfunctions. In particular, berry cell hydration is no longer regulated. It can vary with respect to climatic conditions and, in ideal conditions, should lead to a characteristic desiccation accompanied by the cytoplasmic death of the epidermal cells. The sugar concentration of these cells is considerable. Due to the high osmotic pressure, the fungus can no longer subsist and stops developing. This shriveled confit stage, known as pourri rˆoti,isused for elaborating sweet white wines (Table 10.11).The infection process, from healthy grape to pourri rˆoti grape, lasts from 5 to 15 days, depending on environmental conditions. This overripening period must be characterized by an alternation between short humid periods (3–4 days), favoring conidial germination, and longer dry periods (about 10 days) permitting grape concentration and chemical transformations to occur.
For quality noble rot development, the phenomenon must be rapid and occur near maturity, but
the berries must reach this stage intact.
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