Walnuts are good for you. They contain plenty of those polyunsaturated omega-3 fatty acids, and last year American research showed a notable lowering of 'bad' cholesterol levels in regular consumers of walnuts. They grow well in many New Zealand locations provided they get good summer temperatures and cool winters, but they can be damaged by severe late frosts. They prefer a relatively low humidity and moderate rainfall and spring rains can increase susceptibility to the bacterial disease walnut blight.
Perceived as a long term investment, today grafted trees can crop after 6-7 years with useful production after 12 years with an expected 60-80kg per tree yield. New Zealand's largest reported walnut cracking operation 'Cracker of a Nut' in Christchurch, claim that their 200 suppliers from around the country have been receiving $4/kg in the shell, around $10.50 for top grade shelled and $6.50 for baking grade.
According to www.walnut.org on a global scale it is the Californians that lead the way by providing 99% of the commercial US supply, and two-thirds of the world trade. In 1998 an estimated 193,000 acres in California yielded 2300-2800 lbs/acre, and Sun Diamond a Californian co-op is said to control 50% of production. However other sources put the world production at 1,144,500 tonnes, with USA and China near equal at approximately 250,000 tonnes each, with Iran and Turkey each supplying around 130,000 tonnes, and other major suppliers include Ukraine, Romania, India, France, Greece and Pakistan.
New Zealand grows an estimated 50-100 tonnes of walnuts a year, predominantly in the Canterbury region, and we rely on imported nuts to meet our own demand. Interest in the crop has been growing resulting in many nurseries struggling to supply the present demand for grafted trees. In May 2002 an inaugural AGM was held for the NZ Walnut Industry Group, and as of October 2002 it had 84 subscribed members. There are currently 44 properties from Mid to North Canterbury, eight South Canterbury and Central Otago properties, three in the Nelson Marlborough region with one in the North Island.
Besides a lack of young trees for planting, the expansion of walnut production into new areas of New Zealand is limited by the lack of more effective control of their dominant disease problem, walnut blight (Xanthomonas campestris pv juglandis). In China where trees are normally untreated for the bacterial disease the blight results in crop losses, lower quality and lower nut prices, and it is present in all walnut growing areas of the world.
The most widespread control method is copper based bactericides with high application rates, for example American recommendations are for 8lbs/acre of Kocide as often as eight times per season. These kinds of rates have led to copper resistant strains in France and USA as well as unacceptable high levels of copper in the environment. Walnuts are the ninth highest users of pesticides in California.
A project has been undertaken, funded by the Sustainable Farming Fund and the New Zealand Walnut Action Group, which combines and utilises new chemical research results from America, a unique research capability at Lincoln University, and an active growers group to create a much cleaner, safer and profitable growing environment. The objective is to develop an improved, more environmentally friendly and responsive strategy specific for the New Zealand situation, and with the added opportunity of expanding organic walnut production.
The work plan is to closely monitor a selection of trees on five different sites, with Lincoln University providing blight count profiles and analysis. They hope to determine the most appropriate application schedule for copper-based sprays, assess effectiveness of new spray types, and document the ecology of naturally occurring bacteriophages to ascertain their potential as control agents.
Bacteriophages are naturally occurring viruses first identified in 1917 with hundreds of trials carried out for human disease control, until the advent of antibiotics diminished interest in them as control agents. Walnut blight bacteriophages are very host specific and they have been found in great numbers late in the season in the canopy of walnut orchards, and considerable literature exists on them. They are associated with a decline in bacterial numbers.
Selections among the natural strains, and investigation into the availability of methods to promote the activity of these phages, could result in a fully biologically based method of blight control.
This is an internationally unique programme, representing the cutting edge of development in biological control methods for walnut blight worldwide, although it still has many obstacles to overcome. While Lincoln University is believed to be the only group in the world to isolate phages against walnut blight, the isolates for field use will need to be highly active and sturdy. Their usage will require skill due to their fragile nature, and there are considerations regarding relative sensitivity to heat, chemicals and UV light.
Lincoln University Plant Sciences Group research technician, Janaki Kandula explained that it's too early to make any conclusions as to its overall effectiveness. This is the second year of the three year project, and this season had not been a good one for bacteria from a climatic point of view. "At the end of three years of assessment we will know which chemicals are most effective, and when the bacteria is in high numbers and so when to target the spray. And then also which can be mixed with the phages, and to what extent we can use these phages."
