Farmers in the Breedekloof Wine Valley in South Africa were still assessing the damage on Thursday following a night of widespread black frost, described as “the most devastating frost the valley has seen in 21 years”, a week before.

Black frost occurs when the temperature falls below zero but the air is too dry for hoarfrost, the visible frost, to be produced. Without the protective formation of the needle-like ice crystals a ‘dry freeze’ results in internal freezing and death of vegetation, which develop a blackened appearance.

A week of unexpectedly cold spring weather in the Western Cape resulted in black frost descending and affecting several pockets of vineyards in the valley, notably areas around the town of Rawsonville, on Wednesday night last week.

A week later the full effect was becoming clear on Thursday as whole blocks of vines had started to turn black and shrivel up.

It is feared that the effect of the black frost will spread across South Africa’s wine industry since the Breedekloof Wine Valley supplies an estimated 26% of its annual harvest to wineries elsewhere in the country.

“There are producers in the valley that are estimating a 50% decline in production on their 2017 harvest. Some vineyards are completely written off for the 2017 harvest,” said Nicholas Bruyns, viticulturist at UniWines Vineyards, which manages several farms in the area.

Pieter Cronje, also from UniWines Vineyards, said: “Facing possible short supplies, our main concern is maintaining our loyal domestic and export customers, and getting through a challenging financial year.”

The managing director and cellar master at Bergsig, De Wet Lategan, added: “For the first time in the history of Breedekloof, we might not have enough grapes to serve buyers demand.”

Willie Burger, cellar master at Badsberg Cellar, said he believed the damage would be worse than that inflicted by drought on the 2016 harvest. He added that the frost would also have a long-term effect.

“Many farmers are cancelling their orders for new plantings as they are preparing for the financial burden of half-filled production cellars running at full capacity,” he said.