To Rot or Not to Rot: That is the Question

Our common sense would tell us that we don’t want to let fungus or rot into our crops, but what if that wasn’t always the case. Winemakers are constantly trying to figure out new ways to make the flavour and experience of wine better or more complex, and eventually that led to the discovery of Botrytis cinerea in its “Noble Rot” form, as opposed to its “Grey Rot” form (Figure 1). Botrytis is a fungal pathogen that is responsible for grey mold diseases (Cheung et al. 2020). Botrytis first invades plants through natural openings or injuries, and from there acts as a biotroph for a short amount of time, before entering a necrotrophic phase due to an increase in volatile organic compounds, sugar and nitrogen contents as the host ripens. At this point, the fungus secretes virulence factors that disrupt host metabolism, immune system and cellular structure, leading to the flesh softening and the skin becoming brown and leathery, while the fungus itself grows as grey masses on the surface.

Figure 1: (A) Regular grapes, (B) grapes with Grey Rot, and (C) grapes with Noble Rot.

However, if the conditions of the grapes can be carefully controlled, Noble Rot can instead lead to a sweet and rich wine that has an aroma reminiscent of citrus, dried fruits, honey, and spices, while retaining a freshness through aging (Darriet and Stamatopoulos 2022). The shift from Grey Rot to Noble Rot relies on many environmental conditions, including the characteristics of the soil, as well as alternating weather conditions that cycle between moist nights, foggy mornings, and dry/warm days (Negri et al. 2017). Noble Rot tends to favour nutrient poor and well drained limestone soils, which reduce the chance of mechanical damage which would allow the B. cinerera to penetrate the grapes (Vannini and Chilosi 2013). During certain stages of berry growth where the cuticle is not that thick, microfissures appear through which the Botrytis infects the grape, leading to the development of an intercellular mycelium on the outermost layers of epidermis. Following the infection, it is important for there to be a 10-15 day climate cycle that includes high night time humidity and frequent morning mists to induce development, followed by warm, sunny and windy afternoons to facilitate water evaporation and limit fungal growth.  

Once the fungus has inoculated in the grape, the penetration and microfissures leave the skin more permeable, encouraging more water loss (Negri et al. 2017). As the grapes lose more water, the sugars concentrate, and eventually the grape undergoes enzymatic maceration.  Eventually, the sugar concentration will reach a point where the fungal development can no longer continue. At the end of this process, this leads to a wine with a much higher sugar content, and a lower alcohol content, resulting in a smoother and more flavorful wine. In addition to the evaporation, Noble Rot tends to work side by side with Brewer’s Yeast (Saccharomyces cerevisiae) (Hegyi et al. 2023). This yeast drives the fermentation, and leads to enzymatic conversion that leads to further concentration of sugars, as well as acids, glycerols, minerals. This process also leads to the development and concentration of aroma compounds such as d or c-lactones, which give Noble Rot wines their characteristic aroma.

Overall, the field of viticulture is a fast and fascinating one. When something as simple as a couple days, or a specific weather condition is enough to turn something that would normally destroy a grape harvest into the key part in elevating the experience of wine, it’s a miracle that things like this were even discovered in the first place. The complexity that goes into even the slightest change in wine flavour or aroma is an incredible thing, and organisms like Botrytis only exemplify that. 

References

Cheung, Nicholas, Lei Tian, Xueru Liu, and Xin Li. 2020. “The Destructive Fungal Pathogen Botrytis Cinerea—Insights From Genes Studied With Mutant Analysis.” Pathogens 9 (11): 923. https://doi.org/10.3390/pathogens9110923.

Darriet, Philippe, and Panagiotis Stamatopoulos. 2022. “Botrytized Wines.” Managing Wine Quality, 669–715. https://doi.org/10.1016/b978-0-08-102065-4.00003-1.

EverydayHealth.Com. n.d. “What Are Grapes? Benefits, Risks, Types, How to Grow, More.” Accessed September 26, 2026. https://www.everydayhealth.com/diet-nutrition/diet/grapes-benefits-risks-types-how-grow-more/.

Hegyi, Ádám István, Margot Otto, József Geml, et al. 2023. “The Origin of the Particular Aroma of Noble Rot Wines: Various Fungi Contribute to the Development of the Aroma Profile of Botrytised Grape Berries.” OENO One 57 (3): 165–76. https://doi.org/10.20870/oeno-one.2023.57.3.7314.

Negri, Stefano, Arianna Lovato, Filippo Boscaini, et al. 2017. “The Induction of Noble Rot (Botrytis Cinerea) Infection During Postharvest Withering Changes the Metabolome of Grapevine Berries (Vitis Vinifera L., Cv. Garganega).” Frontiers in Plant Science 8 (June). https://doi.org/10.3389/fpls.2017.01002.

Radovanovic, Milan. 2020. “Liquid Gold: The Magic of Noble Rot.” The Artisan Food Company, January 2. https://theartisanfoodcompany.com/liquid-gold-the-magic-of-noble-rot/.

Sciencephoto.Com. 2026. “Checking for Robot Traffic – Science Photo Library.” https://www.sciencephoto.com/media/14864/view/botrytis-cinerea-mould-on-grapes.

Vannini, Andrea, and Gabriele Chilosi. 2013. “BotrytisInfection: Grey Mould and Noble Rot.” Sweet, Reinforced and Fortified Wines, April 14, 159–69. https://doi.org/10.1002/9781118569184.ch11.