Grape crops hit by mildew

An example of powdery mildew on grapes.

An example of powdery mildew on grapes.

A white powdery disease that grows on grapes and leaves has forced some Marlborough grapegrowers to walk away from their crops this season.

Powdery mildew, which affects the quality of grapes, has been spotted in vineyards across Marlborough, and contractors have their workers fighting to keep the disease at bay.

Some are spraying their vineyards, while others are simply cutting off the affected fruit.

Alapa Viticultural Services owner Alan Wilkinson said at least three vineyards had been written off as a result of powdery mildew.

"It's quite significant. It's definitely worse than last year," he said. "It gets into the bunches and taints the fruit and the fruit gets sooty white spores.

"It's not very pretty."

Grapeworx Marlborough owner Mack Pouwhare said there was a lot of powdery mildew in Marlborough vineyards this year.

"I've heard some wineries aren't going to take the fruit at all - they are just walking away.

"It completely condemns the fruit."

His workers had also spent a lot of time in vineyards spraying for growers who had powdery mildew, he said.

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Thornhill Horticultural Contracting South Island manager John Bibby said there was more disease on the leaves and fruit this year, but it was "nothing major".

"I've seen it on the second set, not on the main fruit."

The second set is a new bunch of grapes that form after the main fruit set. The second set is not needed and is usually cut off prior to harvest.

"I think we've probably taken off most of it for our clients."

Another Blenheim contractor said he had been busy cutting affected fruit to the ground.

"It's definitely worse than last year."

Plant & Food Research liaison scientist Rob Agnew acknowledged there was more powdery mildew in the vineyards this year.

"It's more of a concern than it has been in the past, but vineyard managers are aware of the issue and are cutting it off."

Weather in Marlborough had been favourable for powdery mildew to grow, he said.

"Powdery mildew likes warm temperature, above 21 degrees [Celsius] and it doesn't require rainfall to grow."

While the mean temperature for December to February was just 17.8C, Marlborough had 36 days where the daily maximum temperature topped 25C for the summer - an extra 14 days compared to the previous summer.

"So, yes, there has been more of it this year than last year but the industry is researching how to minimise it.

"The biggest problem is with vine taint . . . [so] if people have a lot of powdery mildew they drop it and leave it on the ground.

"That's how they get rid of the issue."

Sustainable Winegrowing New Zealand general manager Philip Manson said while he did not have any "quantitative data" about how widespread the disease was in Marlborough, he had received quite a few reports of powdery mildew.

The long hot dry summer had been pretty favourable for powdery mildew, Manson said.

"If it gets to really high levels, the fruit gets tainted, which has an undesirable affect on winemaking because its main impact is fruit quality."

If the disease was bad enough, it would also affect the vines and fruit the following year, he said.

 - The Marlborough Express

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