Overview of NZWIG research projects

Heather North, NZWIG research sub-committee, 22 September 2007

The research sub-committee of the NZ Walnut Industry Group manages a range of research projects on topics important to New Zealand growers.  The main projects are outlined in this article, and further information can be found on the NZWIG website, the NutsNZ journal Health in a Shell, and the New Zealand Walnut Growers’ Manual.

1. Managing walnut blight

Walnut blight (caused by the bacterium Xanthomonas campestris pv juglandis) is the main disease challenge for New Zealand growers.  Blight causes black lesions on fruit, young twigs and leaves.  When a lesion goes all the way through into a kernel it will look black and rotten and the kernel is spoiled.  Blight is less severe in low humidity climates such as the eastern parts of New Zealand but, even there, can cause high nut losses.  Blight incidence varies by cultivar, so, for example, cultivar Rex has lower levels of blight than Meyric in all measurements carried out in Canterbury.

Aim: The primary method of control for walnut blight is spraying with copper-based products.  This is a significant cost to growers, and there is well-founded concern that, over time, copper could build up in the soil and affect soil organisms.  Thus, effective control of blight using the minimum amount of copper is the aim of NZWIG’s blight project.

Funding and support: NZWIG received three years of funding from MAF’s Sustainable Farming Fund (2001-2004) and then a second period of funding (2004-2007).  NZWIG contributes funding to the project, as well as time commitment by members and use of members’ orchards for experiments.  NZWIG contracted researchers from Lincoln University, HortResearch and Connect Agriculture for different parts of the work, and has also been collaborating with walnut blight researchers in Tasmania.

Main findings:

1. Mankocide is far more effective than Kocide or other copper-based sprays.

2. Many alternative sprays were tested, but the non-copper-containing sprays were all ineffective.

3. It is most important to apply 1–4 sprays at weekly to fortnightly intervals at bud burst to reduce the initial bacterial population and protect the newly emerging tissue.

4. To minimise the number of in-season sprays, the project has developed a climate model so growers can trigger their sprays when they are really needed (in wet, warm conditions) and back off when they are not.  Growers can stop spraying in early January, as the shell has hardened by this time.

5. Biological control using bacteriophages (viruses that are the natural “predators” of bacteria) showed promise in laboratory conditions, but more work is needed to extend this to orchard conditions.

2. Benchmarking

Aim: This project is collecting data on growth, yield and health of trees from a wide range of orchards, along with information about the cultivars and management practices on those orchards.  The reason is two-fold:

1. So that growers can compare the performance of their own orchard against benchmarks to determine whether they are performing well or badly,

2. So we can investigate the huge between-orchard variations in tree growth, yield and health, and try to determine what management factors these are related to.

Progress so far:In 2004, two RSNZ Teacher Fellows collected initial data on tree growth and orchard management from 10 orchards in Canterbury.  A further outcome of this work was a recommendation on a method to measure tree growth.  We have settled on trunk diameter at 600mm height as our standard measure.  Through the blight project, our researchers have developed a standard measure of blight incidence.  This is a January count of the fraction of nuts from a specified sample that have any sign of blight on the husk.  We have measured blight incidence on a range of Canterbury orchards (for Rex and Meyric) for several years.  We have also decided on standard methods for measuring nut yield and pollination rate, though will work further on the yield method, to make it less time consuming.  The benchmarking page on the NZWIG website enables growers to take measurements on their own orchards and send in the data to contribute to the project.

The future:We have just received funding from AgMardt (on the basis of 60:40 AgMardt to Industry contribution) for three years.  This will enable us to collect data over a wider range of orchards around New Zealand, and to derive benchmarks, eg, expected growth and yield for different cultivars at different ages.  We also hope to begin inferring what management practices may be most influential in orchard performance.

3. Cultivar trial

Aim: In the 1980s and 1990s, NZTCA worked with Lincoln University to trial the best New Zealand walnut selections against the best overseas material that was available at the time.  This trial provided the excellent cultivars that our industry is now based on.  NZWIG aims to continue this good work by searching for further top quality cultivars that will broaden the range available to growers.  This project is funded by NZWIG.

Progress so far: Nine candidates were selected using the same criteria as used in the earlier walnut search; these criteria include flavour, colour and crackout percentage of the nuts.  The parent trees were cut to promote shoot growth and this scion was grafted onto Serr rootstock.  The young trees were planted out at four trial sites: one in Wanaka, two in Canterbury and one in Wanganui.  There are 5 replicates of each candidate at each site, as well as Rex and Meyric replicates for comparison.  The trees have been in the ground for two seasons so far, and some initial growth data has been measured.

The future:Tree growth will be measured each year, but yield (plus blight resistance) will be the major factor of interest once the trees get to cropping age.

4. Training young walnut trees

Aim: The best current knowledge about training of young walnut trees is in David Murdoch’s chapter in the New Zealand Walnut Growers’ Manual.  The NZWIG trial aimed to build on this knowledge by comparing hard, medium and light training options, and measuring the effect of these on the growth and form of young walnut trees.  A total of 216 trees on three orchards were included in the trial (2-3 years old at the start).  The treatments were applied and measurements made each year for 3 years.  This project is funded by NZWIG.

Main findings: Trees can be trained hard for the first 2 years without any reduction in growth, and this helps produce a tree of good form.  The training involves removing the branches each winter, and heading back the leader.  This produces vigorous shoots which offer good options for a strong, straight leader.  However after age 3 or 4, the pruning should be less severe, otherwise overall tree growth is reduced, and branch breakages and twisting occur due to excess shoot growth.  It is still very important to select a single leader, and remove any branches that are competing with the leader.  The largest branches should be removed each year, except for those to be kept as permanent scaffold branches.

New experiments:In winter 2007 we concluded the above experiment and began two new, smaller experiments which are looking at (1) how pruning method affects yield in cropping trees, 6+ years old, and (2) what is the best heading cut for trees 3-5 years old.

5. Other initiatives

Lincoln University trial block: We do the pruning while Lincoln University looks after irrigation and spraying (with our assistance on timing).  We have used the trial block for some of the blight research.

Rootstock trial: Paradox is a special cross of Juglans hindsii pollinated by J. regia, and is used as a high vigour rootstock in California and Tasmania.  We are testing Paradox vs J. regia (Serr) rootstock (both have Rex grafted on top) to see whether it is vigorous here.