While natural fermentation with indigenous yeasts is feasible in all situations, it is not without risks (stuck fermentation, volatile acidity production, reduction, ethyl acetate, etc.). In the case of grapes affected by botrytis, it is better to control the fermentation environment. A laccase test can be done on the must to evaluate any risks.
In all cases, during a natural fermentation, it is possible to make a fermentation starter vat by collecting a small quantity of grapes prior to harvest's start, so as to be able to add this juice at the appropriate moment to start the fermentation. This enables a strong start with minimal risks.
When the harvest is well advanced, it is possible to use must from another vat that is already fermenting. Beware of cross-contamination caused byBrettanomyces bruxellensis.