The positive side of Botrytis is called Noble Rot forms when conditions are very specific with humid, cool mornings and warm, sunny, and dry afternoons which keep the infection from growing too rapidly and turning into Sour Rot. This form uses the Pectinase to break through the skin of healthy berries which leads to a natural dehydration of the fruit since the pectins in the skin are what holds in moisture. This natural dehydration concentrates the sugars, acids, and flavors within the skins which are then translated into a more concentrated must during the fermentation. Nobel rot adds its own influences in the form of additional flavors of marmalade, quince, and mushrooms.
Noble Rot also produces Gluconic acid which increases the acid content of the must. It contributes a higher Glycerol content, leading to a fuller mouth feel than the fruit would have had without the infection. These traits are particularly desirable for Sauternes where the tiny, cool Ciron river meets the larger, warmer Garonne making for cool foggy mornings and warm sunny afternoons. The wineries in this area maximize their exposure to the fungus through multiple picking times (called Tries) during which they only choose the most affected berries and leave the less affected ones to increase in concentration.
Even when the Botrytis is of the Noble variety it still comes with challenges for fermentation. The extremely concentrated juice can make it difficult for the yeast to ferment due to the high osmotic pressures to which they are exposed in such a concentrated sugar solution. Often, as is sometimes seen in Tokaji, in Hungary, the primary fermentations can take up to 6 weeks and can cause the yeast to stress and produce volatile acidity (VA). For winemakers, choosing the proper yeast strain to withstand the difficult environment.
The negative side of Botrytis is called Sour rot or Grey Rot. Sour rot forms when the humidity is high, temperature is low, and/or the airflow is low through the canopy. It also can show up when the fruit skin is opened from excess water and animal, insect, hail, or machinery damage. This opening allows not only the Botrytis to enter the fruit but also other spoilage organisms like Acetobacter leading to a moldy or sour smell in the fruit. At this point the best solution available to a winemaker is to sort out the affected fruit. This can lead to loss of yields and profits for vine growers as well as lost revenue for wineries that have purchased affected fruit but is usually the best course of action to protect the quality of the wine.
It is not desirable to produce wines from Sour Rot affected fruit due to the chemicals produced by the fungus and their affect on the fermentation and the final wines. The natural Pectinase found in affected grapes leads to juice that settles and clarifies quickly particularly in white varieties. The low level of suspended solids in the juice can inhibit yeast suspension which in turn will slow the rate of fermentation. Botrytis also uses up nutrients from the fruit which can cause deficiencies for the yeast during the fermentation leading to increased VA and stuck fermentations. From a sensory perspective Sour rot infected fruit can often show a moldy or musty character in the final wine.
The good and bad side of Botrytis depends on the winemaker’s final goal for the wine. If it is a style that is defined by Botrytis characters then it is very desirable to use Noble rot affected fruit regardless of the winemaking challenges. When the effects of Botrytis are not desired, it has the potential to be problematic for both winemakers and vineyard managers if not carefully managed.