(Wine-Searcher) - An outbreak of fungal disease after a damp spring means trouble for struggling growers.

A warm and wet spring has caused chaos in the vineyards of Andalusia, where mildew is threatening this year's Sherry production.

Winegrowers fear that as much as 80 percent of their crop could be destroyed thanks to the fungus caused by warm temperatures and unusually high rainfall during spring.

Winemaking cooperatives across the Jerez region have reported devastating effects on their crop, according to Spanish English-language news agency Efe.

Among the worst vineyards affected are around Sanlúcar de Barrameda, where the blight is expected to have damaged between 50 and 70 percent of growth, and Trebujena, which has suffered damage of between 30 and 60 percent.

Francisco Guerrero, head of the Asevi growers cooperative in Jerez confirmed that the unusual weather conditions this year had allowed the mildew fungus to spread.

"After a dry and mild winter no-one expected the heavy rainfall during May," Guerrero said.

"Many growers hoped to make savings where they could and hadn't taken preventative measures against such unusual weather."

One winegrower expected losses of an estimated 80 percent of his harvest. "It's been a disaster," Jose Sumariva, who runs a vineyard of 8500 hectares, told El Pais.

The threat comes at a bad time for Sherry producers, who have been seeing a glimmer of hope on the horizon after decades of falling sales. Sherry has experienced a mild renaissance among millennials, with bars in the US and UK singing the praises of a wine once relegated almost exclusively to an aging, shrinking demographic.

The damage continues a run of weather-related damage in Europe that has seen floods and hail impact on vineyards across France, Italy, Germany and Spain. The worst damage occurred at the end of May, when Cognac lost as much as 8 percent of its vineyard area to hail. More than 5000ha were ravaged by hail, with many growers seeing 100-percent losses in affected areas.