What is Botrytis cinerea?

What is in a name? Botrytis cinerea is ignoble rot when it infects grapes resulting in bunch rot and grey mold, but under the right conditions, Botrytis cinerea can develop into noble rot capable of producing wines of immense sugar concentration. So what is Botrytis cinerea?^1,2

• Botrytis cinerea is a fungus
• The initial inoculum probably develops from overwintering mycelia which are strands of the living fungus that can survive on previously diseased tissues or from resting structures called sclerotia
• Grapevine flowers, immediately after capfall, and berries post veraison are particularly susceptible to botrytis infection.
• Infected flower parts located in the developing cluster may initiate fruit infection later during the season
• As the fruit reaches maturity, resistance to fungal growth declines and under moist conditions, new infections, incited by spores from external sources become important
• Botrytis relies on damaged tissue to begin an infection so any wounded green tissue is susceptible
• Grape varieties that produce thin-skinned, tightly packed bunches are prone to splitting and retain moisture, providing conditions that suit botrytis
• Botrytis susceptible varieties include Chardonnay, Chenin Blanc, Grenache, Muller Thurgau, Pinot Gris, Pinot Noir, Riesling, Sauvignon Blanc, Semillon, Traminer, and Sultana

Here is the lifecycle of Botrytis cinerea:¹

What are the conditions that can lead to noble rot?²

• Noble rot develops late in the season under conditions of fluctuating humidity, specifically humid nights followed by dry, sunny days which limits fungal growth
• Fall infection of Botrytis leads to the release of pectolytic enzymes that degrade the pectinaceous component of the cell wall leading to the dehydration of the grape under dry conditions
• This drying of the berry is crucial to the concentration of the juice
• Drying also limits secondary infection by bacteria and fungi including Penicillium, Aspergillus, and Mucor
• A distinctive feature of noble rotting is a loss in varietal aroma due to the destruction of terpenes that give these varieties their distinctive fragrance.

So, Botrytis cinerea, ignoble during the growing season producing bunch rot and grey mold, but noble under certain rare conditions which produces a desirable transformation into a dehyrated, highly concentrated grape enhancing it's quality.
References:
1. AWRI, Viti-Notes, Botrytis, 2010.
2. Ronald Jackson, Wine Science Third Edition, Elsevier, 2008, pg. 520- .