NZ Walnut Blight Research Update
NZ research is helping growers make better decisions about copper spray products, application rates and methods, and spray timing.
Dr Tim Jenkins, Connect Agriculture has recently reported on the results of experiments conducted in the 2004/2005 growing season. This was a year of high blight pressure, largely related to the high rainfall and humidity levels in December 2004.
The full report is available as a pdf file here. The following is a summary of the key findings.
Spray Product, Application Rate and Timing
Best practice in spraying for blight control will be based on a trade-off between the level of blight control and the amount of copper applied. This requires a choice about the chemical to be used, when to apply it and at what rate.
The best timing appears to lie between spraying at bud burst only (three sprays at weekly intervals) and a full spray programme. It is therefore suggested that orchard spraying include spraying at bud-burst, plus one, two or, in some seasons, more, subsequent sprays, related to environmental conditions (leaf wetness - and/or relative humidity - and temperature). There is probably some value in simply spraying just prior to an expected prolonged rainfall event).
Future research will focus on pinning down the environmental triggers and climate indicators for determining spray timing.
Spraying does not appear to be required after mid January for a current season's harvest, given the good level of control achieved when ceasing spraying on 6 January.
Indications based on this research are to strike the balance between control and total amount of copper by using Mankocide, or Liquicop plus Mancozeb, at the reduced rate tested in the experiment.
The chemical for best control remains as Mankocide. At this stage there is no clear evidence of benefit of additives.
From previous research in Tasmania and France it would appear that Bordeaux mixture gives best results of organically allowable sprays.
The rates used in the walnut blight experiments to achieve leaf coverage were comparatively low due to the efficiency of the mist blower sprayer. The spray rate for full foliage was around 2.4 litres per tree (depending on tree size) which translates to roughly 840 L per hectare equivalent (depending on density of planting). Rates of around double this amount per canopy hectare may be required to achieve adequate leaf coverage using tractor based spraying.
Early January is a key time to assess the level of walnut blight present and will be useful in making comparisons between orchards as part of the walnut benchmarking project.
The method recommended is to assess five nuts closest to an imaginary line drawn at each north, south, east and west point of the tree (detailed instructions are available from annual WIG workshops). The proportion of nuts with obvious blight symptoms at this stage is roughly double the eventual loss of saleable walnuts from blight (this will vary markedly but the January measurement remains the easiest and most reliable comparison between orchards).
The primary environmental aim of the walnut blight research is to find methods of applying lower amounts of copper (atomic symbol Cu) per hectare on walnut orchards while maintaining a reasonable level of blight control.
The amount of copper applied per hectare-equivalent for each treatment is given in the full report (Appendix section A).
The definition of sustainable copper application differs according to different authorities, including 2 kg/ha for conservative assessments, 3 kg/ha for some organic certification authorities and sometimes 4 kg/ha.
The amounts of copper applied on a per hectare-equivalent in this experiment were sustainably low for four of the treatments regimes, giving poor levels of control. Copper rates were marginally sustainable for two treatment regimes, giving good control. The full spray programme represented an unsustainable application of 13.3 kg Cu/ ha.
It is important to note that the spray rates used are roughly half those expected to be required by tractor based application per hectare to achieve sufficient leaf coverage. So there is still a research requirement to seek to significantly lower spray rates while achieving adequate control.
On-going Research Commitment
This research is part of a wider research programme being undertaken by the NZ Walnut Industry Group. It is being fund with a grant from the MAF Sustainable Farming Fund (SFF. The other major project being funded by the SFF examines the possibility of using a virus (bacteriophage) to destroy the walnut blight bacteria. NZWIG is also conducting research into possible new cultivars and relating a variety of environmental and management practices to orchard outcomes (the Benchmarking Project).
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