A fungal infection in grapes known as 'bunch rot' improves the aroma of wine, providing it increased levels of fruity and vanilla notes, scientists say.

On the other hand, another fungal infection known as powdery mildew can negatively affect the taste and aroma of wine, according to the study.

Researchers, including those from Fraunhofer Institute for Process Engineering and Packaging in Germany, studied the effects of fungal infection on wine produced from the grape varieties White Riesling, Red Riesling and Gewurztraminer, and the effect of powdery mildew on wine made from the unsprayed hybrid grape type Gm 8622-3.

They found that while bunch rot actually led to more positive aroma ratings, mildew infected wine was rated as 'less interesting' by a test panel.

The team performed a series of tests, including comparative Aroma Extract Dilution Analysis (AEDA) as well as sensory evaluation by a panel of 10 participants who had received extensive training in identifying different wine odourants.

They found that both bunch rot and powdery mildew caused nuanced changes in the composition of aroma substances, changing the odour of the wines to the point where the wines' aroma quality was significantly affected.

However, these changes were not linked to changes in any key chemical compound, but rather "a result of an interplay of subtler changes in multiple aroma active substances in each wine", said Andrea Buettner, from Fraunhofer Institute for Process Engineering and Packaging.

The study, published in the journal Frontiers in Chemistry, found that bunch rot infection increased the fruity, floral and toasty aroma notes in all of the affected wine samples, leading the panel to rate its odour as more pleasant than wine made from healthy grapes.

The floral notes increased much more in the White Riesling sample than in the other wines.

Buettner said that "the observed differences between bunch rot-affected and healthy samples could be partially related to the higher sugar content reported in the infected grapes." 

Powdery mildew, on the contrary, resulted in a decrease of vanilla-like notes, due to a variety of quite subtle chemical changes.

Wine samples affected by powdery mildew received consistently lower aroma ratings from the panel than the healthy control samples.

"This negative rating was, however, not related to any specific off-note but was rather due to a lack of positive aromatic notes; in fact, the wine was described as being rather flat," said Buettner.

These findings are of importance to wine lovers and wine producers alike.

"Fungal infections pose a real economic threat for viticulture," said Buettner.

"Botrytis cinerea and Erysiphe necator are among the most relevant fungi in viticulture and have a world-wide impact on the wine industrial production," he said.

"Infections can lead to both yield losses and reduced grape quality. Recent climate change may have created more favourable conditions for fungi to grow, and at the same time the usage of fungicides is becoming increasingly restricted," he added.