The vineyard becomes the lab in investigations of Botrytis cinerea. It’s a “Dr. Jekyll and Mr. Hyde” type of fungus because it causes two very different types of infections. It produces sweet wine grapes during noble rot but causes the plant’s demise in grey mold.
Noble rot causes wine grapes to shrivel and dehydrate. The fungus first penetrates the grape. Then, it induces decomposition of the skin making them porous. Water loss by evaporation leads to enzymatic maceration and increased sugar concentration leads to sweet dessert wines. Lastly, the high sugar concentrations kill the fungus. Rieslings, Tokaji, and Sauternes, for example, benefit from noble rot.
In contrast, grey mold kills the host plant cells allowing other yeasts, molds, and bacteria to colonize. Grey mold is not unique to wine grapes. It infects over 200 crop hosts, causing large amounts of damage to tomatoes, berries, and petunias.
Differences between noble rot and grey mold
So when does noble rot or grey mold occur? Noble rot occurs when dry conditions follow wet conditions, while grey mold appears in consistently humid or wet conditions. Not surprisingly, grapes affected by noble rot versus grey mold have differences in chemical compositions and noble rot gives a distinct profile of aroma precursors and volatile thiols.
Symptoms of both noble rot and grey mold can be found in the same vineyard, on the same plant, and even within the same grape. But it’s unclear if different symptoms are caused by genetically differentiated strains of B. cinerea or if the same strains cause different symptoms depending on environmental conditions.
Are noble rot and grey mold caused by the same strains of Botrytis cinerea?
In 2002, researchers sampled B. cinerea from three French vineyards that produce sweet wine grapes: Alsace, Anjour, and Bordelais. They analyzed the samples from grapes with grey mold and noble rot using multilocus microsatellite genotyping, a method that examines the same genes across samples to help categorize them.
They found that all noble rot symptoms were caused by B. cinerea (sensu stricto) and not B. pseudocinerea, a species initially designated B. cinerea, or another cryptic species. Any genetic diversity they found in the populations was also better explained by geography, rather than type of symptom. Thus, it seems that noble rot symptoms aren’t caused by a specific B. cinerea population but depends essentially on microclimatic conditions.
Genes involved in noble rot
Almost 20 years later, we have a more comprehensive look at gene expression changes during noble rot or grey mold infections. Researchers published work in 2019 using an artificially induced botryzation model in two grape varieties. They sampled the infected grapes at different stages of noble rot and examined gene expression. They compared these results with a previously reported grey mold infection gene expression profile and with healthy grapes undergoing the post-harvest withering process.
They identified a few key genes specifically involved in noble rot infection, some which may impart a better quality to the wine grape: genes involved in secondary metabolism, mainly affecting stilbenes and phenylpropanoids, and resveratrol oligomers. Other genes induced include those that are involved in defense and stress response.
The noble rot transcription profile had reduced virulence at the onset of infection and thus a reduced plant defense response. Some key differences include the absence of two phytotoxins during noble rot that were present in grey mold that are required to kill host cells and colonize plant tissue.
TL;DR
The next time you uncork a bottle of sweet wine, thank Botrytis cinerea and dry weather conditions that produced noble rot rather than grey mold.
References
‘Omics’ and Plant Responses to Botrytis cinerea
