A Closer Look at Botrysized Late Harvest Grapes

Winter is the time that I look at some scholarly research articles and currently I am reading one entitled An Overview on Botrytized Wines.¹ I'm interested in learning more about the role of Bptrytis cinerea in the production of a late harvest wine.
Noble rot infection is caused by the fungus Botrytis cinerea. Surprisingly, this same fungus, Botrytis cinerea, with the same genotype and possibly the same strains also causes gray/bunch rot in healthy grapes. The question is, how does Botrytis cinerea accomplish this. The answer is related to specific environmental conditions critical for triggering the different metabolic pathways leading to either noble or gray rot infection.
Conditions promoting gray/bunch rot

• Extended humidity and heavy precipitation promotes bunch rot occurrence
• Intensive ruptures produced by birds, insects, worms, hail, light detachment of the berry from the pedicel or berry’s skin bursting due to heavy rainfalls, leads to bunch rot
• Extreme ruptures make berries very susceptible to gray rot and lead to fungus growth at the exterior part of the berry, where oxidation of sugars leads to gluconic acid accumulation

Conditions promoting noble rot

• The climate conditions three to five weeks after berry ripeness are decisive for the quality of the final product
• For noble rot wine production weather conditions are the key factor responsible for the quality of botrytization; sunny and dry days with humid nights and foggy mornings
• Grapes should reach maturity healthy without skin ruptures to avoid bunch rot
• Micro-fissures in the grape berry skin allows B. cinerea to enter and germinate inside, leading to fungus growth in the interior of berry’s skin where glycerol production in the anaerobic environment is supported
• During noble infection, different transcription factors in the berry lead to different enzymatic production and metabolic paths
• The host (the grape) decreases its defenses and produces a lower number of phytoalexins responsible for the creation of hostile environment for B. cinerea
• The up- or down-regulation of specific grape genes decreases the power of the virulence factors secreted during noble rot colonization by B. cinerea, resulting in a ‘‘milder’’ infection

The above photo shows the progression from healthy to noble rot infected grapes. During the pourri plein stage of infection, B. cinerea starts its development towards the external part of grape skin. The oxidation phenomenon causes the berries to gain a characteristic brown color. At the pourri rôti stage the withered grapes can finally be picked.
We bag our grapes with organza wedding bags to facilitate this process in our vineyard.

References:
1. Kallitsounakis, Georgios, Catarino, Sofia, An Overview on Botrytized Wines REVISÃO: VINHOS BOTRITIZADOS, Ciência e Técnica Vitivinícola, 2020, 35, 76, 10.1051/ctv/20203502076.