The main insect pest of grapes is phylloxera (Daktulosphaira vitifoliae), a tiny aphid that sucks sap from vine roots. Within a few years of infection, the vine declines in vigour, losing leaves and producing small crops. There are no economic ways to rid a region of phylloxera once it has arrived. It spreads from vineyard to vineyard on soil carried by the wind, people or machinery, or through contaminated plants. Although most vineyards are planted on phylloxera-resistant rootstocks, in 2007 a quarter of plantings in the Wairarapa and nearly 40% in Central Otago were of ungrafted vines, which are not resistant to the pest.
After phylloxera, mealybugs are the main insect problem in New Zealand’s vineyards. They too are sap-suckers, and decrease the vigour of vines. They also transmit leafroll viruses from vine to vine. Most growers use insecticide sprays to control mealybugs.
Birds and rabbits
Starlings, blackbirds and silvereyes can ruin an entire grape crop over the two-month period from when the berries swell until harvest. Growers protect their crop with fine netting or use guns to frighten birds.
Rabbits can debark young vines, so growers enclose their stems in plastic sleeves.
Falcons for Grapes
The Falcons for Grapes project is an attempt to boost the numbers of the rare New Zealand falcon in the Marlborough region, while reducing damage to grapes from other birds. Fledgling falcons are hand-raised in artificial nests and fed daily. They fly around the vineyards, scaring away small birds and rodents, and catching some as supplementary food.
Powdery mildew first devastated Northland vineyards in the 1870s and has been a problem in New Zealand ever since. Unlike most fungal diseases, it flourishes in dry weather. It affects all green parts of a vine and begins as powdery blotches on the upper surface of leaves. Fungicides are sprayed to restrict its spread.
Downy mildew is another mould that affects all green parts of the vine. It appears in moist conditions and requires fungal sprays for its control.
Botrytis is a grey mould that is prevalent in warm, damp conditions, and rots bunches of grapes. Growers reduce the risk of botrytis by plucking leaves around the berries, allowing air to circulate. Very occasionally a condition known as noble rot develops in botrytis-infected grapes. The berries shrivel, then dry out and do not rot. They produce a sweet, full-flavoured wine.
A large number of viruses infect vines and can significantly affect crop yield and quality. They arrived in the country with infected propagating material before New Zealand was able to test for their presence. Much virus-infected grape stock was unwittingly distributed through the country by the government viticultural research station at Te Kauwhata.
Grape leafroll viruses are of the most concern. There are at least nine different associated viruses. One of them (GLRaV-3) is widespread in North Island vineyards and by 2004 was becoming common in Marlborough. The virus was transmitted to vines and rootstocks during grafting. In the vineyard it is spread by mealybugs. There is no treatment for infected vines – they must be removed.