Expert warns growers of pruning dangers

Sauvignon blanc grape growers in Marlborough might be pruning their vines the wrong way, South African vine physiologist Dr Kobus Hunter suggested at the Romeo Bragato wine industry conference in Blenheim last week.

Most sauvignon blanc vines were generally pruned to four canes layered on two levels of wire, but this was "a high-risk operation" that could cause crowding, Dr Hunter said.

The four-cane system created massive problems in South Africa, he said. Top quality management was needed to get away with this type of pruning.

Marlborough vineyard consultant Jeremy Hyland said the lyre-type two-cane training promoted by Dr Hunter would result in overly vigorous growth in Marlborough conditions. The only place he knew of where this type of vine training was being used in New Zealand was at Kumeu, near Auckland, where the climate was warm and wet.

Managing four-cane vines might be difficult "but we are very good at it", Mr Hyland said.

Dr Hunter said raised carbon dioxide, higher temperatures and drier conditions caused by climate change would inevitably affect grape and wine quality. The breakdown of the ozone layer would lead to complex chemical changes which would inevitably feed through to wine flavours.

A shift from 30 degrees Celsius to to 37C in a vineyard area would significantly reduce the window for optimal photosynthesis, he said.

But climate change was not the only culprit for stressed vines, with wind, water-logging, over fertilisation, nutrient deficiency and root to leaf imbalances among other negative influences.

"Sometimes we do things very wrong in the vineyard, then we blame climate change," he said.

With water scarcity a problem in all winegrowing regions, drought-tolerant rootstocks would become increasingly important, Dr Hunter said.

"Vines will need large, deep-penetrating root systems to reduce the effect of stress."

Uniform vines was a recurring theme in Dr Hunter's talk, which featured slides of neatly tended vines in one shade of green sprawling across hillsides.

Using a range of site-suitable scionwood, rootstocks and cultivars could help achieve uniformity in vineyards over a range of sites. Other options included adapting vine spacing and trellis size, changing row orientation and ridging.

He showed photos of heavy machinery breaking up soil pans to encourage healthy root development, which was foreign to the New Zealand audience used to defined topsoils and subsoils.

Poor planting, the wrong rootstocks, bad pruning and incorrect canopy management could all increase variation and compromise grape and wine quality, he said.