Botrytis and Mildew Can Change Your Harvest and Winemaking Plans | | | In our area, the wine country north of San Francisco, the harvest is underway while the weather shifts from sun to showers and back again. Plus, the summer was oddly humid. These conditions call for a review of how mold growth can affect our winegrowing and winemaking plans. | | Botrytis and Mildew Can Change Your Harvest and Winemaking Plans The rain can be a blessing or a curse for the wine grapes. As the grape harvest approaches, rain is more likely to become a curse. The wet conditions, if they last long enough, lead to the spread of molds that will deteriorate the quality of the wine made from the grapes. But not every rain ruins the crop, and not every mold deteriorates wine quality. Let’s consider some common rules of thumb that guide our grape rot management practices, and then delve into the details of grape rot. | No More than an Inch of Rain, Please A light rain, followed by sunny days that evaporate the moisture out of the grape bunches, generally does not lead to breakage of the grape skins and infection of the fruit. Often, winemakers reference 1 inch of rain as a marker of where the trouble begins. Excessive moisture that does not dry out may necessitate that the grapes are picked immediately.
No Spraying after Veraison Commonly, sulfur is sprayed in the vineyard to suppress mold growth. Residual sulfur in the fermentor, however, results in hydrogen sulfide production – a compound that smells like rotten eggs and can be detected in PPB or parts per billion! For this reason, it is considered a good practice to spray early in the season and up to the time of veraison. There is typically 1-2 months from veraison to harvest. And these months are often periods with showers or even storms.
Canopy Management – Natural Air Conditioning Leaf pulling and shoot thinning during the summer is recommended to allow for airflow through the vines and grape bunches. If moisture is trapped inside a jungle-like canopy filled with tight bunches of grape clusters – the molds spread. With space for air to flow through the bunches, the moisture can escape and less mold growth will occur.
| Grape rot has a few different nicknames: bunch rot, gray rot, common rot, noble rot. Additionally, we are concerned about mildew, which is generally discussed separately from “rot”. This group of rot nicknames are all specific references to a mold called Botrytis cinerea, which is usually simply referred to as Botrytis. It develops on grapes in rainy and foggy weather. It is identified by its hair-like, gray appearance. It is sinister because, unlike other molds, it can penetrate the grape skin. Inside the fruit it causes rapid evaporation and sugar concentration. Under excessively moist conditions, it will lead to breakage of the grape skin. If that occurs, all manner of infection of the fruit ensues. But if the skins do not break open, it can lead to a high brix must that can be used for excellent dessert wines – this is the tradition behind the Sauternes of France. If the fruit skins remain intact and the weather remains relatively dry, the Botrytis is called “noble rot”. If the weather is wet and the grapes do not desiccate or the skins actually break open, this is called “bunch rot” or “gray rot”. As the famous French enologist, Emile Peynaud, reports, “between gray rot, which depreciates the wine, and noble rot, which improves it, the only distinctive factor may be the influence of the climate”. | | An example of "bunch rot" from Botrytis. | | | |
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