Definition - What does Botrytis mean?
Botrytis cinerea is the species name of a form of neurotrophic fungus, also called Botrytis, that affects wine grapes. It can have a negative impact on the vine when it occurs due to wet or humid conditions and results in the loss of affected grapes, in which case, it is referred to as grey mold. But it can also have a positive impact on the grapes when it occurs during drier conditions followed by wetter conditions; when this occurs, it is called noble rot or botrytis bunch rot and creates sweet dessert wines like Sauternes.
WineFrog explains Botrytis
Botrytis is a complex but weak disease that primarily attacks the tender parts of the plant, like the berries, dead or injured tissues, or tissues that are senescing (growing older/aging). Signs that a vine has been infected include: wounded tissue, fading flowers, fallen leaves, spotting, discoloration, wilting and buds failing to open. The mold takes hold in wounds on these sensitive spots, appearing as a white growth on the plant. It then darkens to smoky-gray spores that spread through the vine. Botrytis spores can remain dormant for as long as the plant continues to live, often holding on during the cold winter months to attack in the spring. Dormant spores activate when humidity increases - wet growing seasons tend to bring a higher chance of noble rot or the more dreaded gray mold.
Noble rot removes water from the grapes, leaving behind a higher ratio of juice to sugars, fruit acids and minerals. Wines made from grapes that have experienced noble rot are intense and concentrated; depending on how long the noble rot has been left on the grapes, the wine can also be considered syrupy and extremely sweet. These wines are often called dessert wines; they notably have aromas of honeysuckle and a bitter finish on the palate. The downside to using grapes that have been exposed to noble rot is that the fungus can kill yeast, resulting in a stoppage of fermentation before the wine has accumulated a sufficient level of alcohol.