Which weather condition is most detrimental to grape quality at harvest?

Heavy rainfall is indeed the most detrimental weather condition to grape quality at harvest. When grapes experience substantial rainfall close to harvest time, it can lead to several negative outcomes. Firstly, excessive moisture can promote the risk of diseases such as botrytis (noble rot) or downy mildew, which thrive in wet conditions and can damage grapes or alter their characteristics. Secondly, heavy rainfall can cause grapes to swell, leading to dilution of flavors and sugars, ultimately resulting in lower quality wines. Finally, rain can also delay the harvest, as vineyard conditions become difficult for picking and processing grapes in a timely manner, which can further impact the fruit's quality and the resulting wine.

In contrast, freezing temperatures can cause frost damage, but this is more likely to affect the grapes early in the growing season rather than at harvest. Excessive sunshine can lead to sunburn on grapes, affecting their quality, but it’s typically less critical than the immediate effects of heavy rainfall at harvest time. High humidity can contribute to disease risks too, but it usually does not have the same direct negative impact on grape quality as does heavy rainfall.