One of the scourges of the vineyard is a fungal infection known as Botrytis. The organism responsible is taxonomically known as Botrytis Cinerea, and the etymology of the name really says it all:
Botrys (ancient Greek for grapes) + itis (neolatin suffix for disease) + cinerea (neolatin for ashes, grey)
As pictured, Botrytis (henceforth known as BC) starts as a small strike on the cluster, and then spreads out over time and under the right conditions. BC infections can take various forms in different conditions, and depending on the severity, can really hurt quality of dry wines by affecting color, oxidation via the laccase enzyme, and excessive phenolic components coming from partially broken down skins. In certain instances and with certain varietals, BC infection can increase complexity and add interesting dried fruit aromatics that are highly desirable. In the Sauternes region of France, and in other locales known for the production of sweet wines, BC (favorably dubbed noble rot in these spots) is a critical element in the grape growing process, naturally concentrating the sugar and flavors in the grapes, yielding incredibly unctuous sweet wines of distinct character and quality.