The problem is becoming a regular occurrence, concerning Central Otago Winegrowers Association president James Dicey.
''It's happening in Central Otago almost every year and we're talking hundred of thousands of dollars lost, if not millions, especially if the crops are affected for two years,'' he said yesterday.
He declined to identify the vineyard affected in the most recent incident but said it was a double blow, as there was similar spray damage on the same vineyard last year.
This time, 25ha was affected and a specialist analyst who assessed the vines said they were hit by a hormone chemical about November 9.
''Grapes are incredibly sensitive to certain types of chemicals. It's very difficult to kill them but certain herbicides do affect the growth, affecting the crop this season and potentially next season as well,'' Mr Dicey said.
Grapes were particularly sensitive to metsulfuron-methyl and hormone-based sprays, he said.
The latter tended to be applied by ground-spraying or by helicopter.
In some conditions, hormone sprays could vaporise and be carried by the wind, drifting up to 30km before settling on crops, making it difficult to identify the source, Mr Dicey said.
''Because it could've been applied up to 30km away, you might not see the helicopter, and it might not affect the immediate neighbours.''
The impact was noticeable within two weeks.
The hormone made the vine grow faster: ''it almost grows itself to death''.
The source of the Cromwell basin vineyard's damage had not been identified, he said.
The potential fruit loss cost was $525,000, based on six tonnes of grapes to the hectare and $3500 per tonne.
There would also be additional operating costs to maintain the spray-damaged vines and the fruiting ability could be affected next year.
Mr Dicey asked people spraying herbicides to think before they sprayed.
''They have a right to farm, of course - we respect that - but we'd ask them to be extremely careful from the time of grape bud-burst in mid-October through to leaf fall in about mid-May.''
The association had worked with aerial applicators to set up industry standards for agricultural chemical applications, seven years ago.
It was also creating a map of the 1750ha of grapes planted in the region, to send to rural property owners.
''If this problem continues, we'll be lobbying the Otago Regional Council, asking for a ban on the use of these chemicals at certain times of the year.''
The cost of taking out insurance against spray damage was prohibitive, he said.
