Walnut Blight Research

In the very early 2000’s the Walnut Industry Group was part of a very substantial blight research project.

It was conducted in both New Zealand and Australia and the annual budget was aprox. $150,000 per year for three years. A full description of the project is available in the Growers Manual, Section 5, Chapter 1.2.

Summary of results and recommendations as they were in 2005. Copied from the manual. 

So far we have completed three seasons of experimental work and produced these observations.

Mankocide provides better control than Kocide and Kocide is better than all the other alternatives and better than nothing.  The experimental results have clearly shown that Mankocide is most effective both on a total control perspective and control per kg of copper applied basis.  Mankocide is a combination of Kocide and Mancozeb.  We have tried Mancozeb alone but it does not give good control.  We suspect that the copper and dithiocarbamate act together to provide the improvement through a ‘multi-pronged’ attack on the pathogen. 

Timing of the first copper spray in spring is critical.  Spray at bud burst.  The blight bacteria over-winter primarily inside the buds and catkins, and it is from there that they invade new growing stems, leaves, pistillate flowers and pollen in the spring.  It is therefore important to kill off as many of the over wintering bacteria as possible before warm, damp spring conditions allow a population explosion.  However, copper spray cannot penetrate closed buds, so wait until a good proportion of the buds begin to open. See From Bud to Leaf in the coloured pages for the stage at which the copper can penetrate the bud in spring – just as the outside bracts begin to pull back.  This stage occurs around mid to late September in Canterbury.  It is worth keeping a close eye on your trees from the beginning of September onward, to ensure you get the timing right for the first spray. 

A second (and even third) early season spray may be a good policy.  Current thinking is that the early stages of growth, from bud burst through until when leaflets start to form on (say) 100mm long shoots, provide a good opportunity to reduce bacteria numbers to very low levels and reduce the disease pressure later in the spring.  If the bud-burst spray has been highly effective then spraying at this time is not necessary, but since buds burst at different times, the follow-up spray may be beneficial. It is much easier to achieve full coverage of new season’s growth early in spring than once the canopy is fully formed.

High temperature and humidity are the danger factors for blight growth.  Moisture on plant parts mobilises the bacteria, allowing them to invade new tissues.  Dampness also protects blight bacteria from desiccation.  The optimum temperature for blight population growth is 28 to 32°C, but it will grow, and black lesions will form, at temperatures above 12°C.  The weather in a Canterbury spring typically varies between warm-dry nor’westers and cold wet southerlies with some cool, dry nor’east sea-breezes in-between when anti-cyclones dominate.  All of these weather patterns are generally either too dry or too cold for highly accelerated blight development.  However, we do get periods of warm humid weather (and hence highly accelerated blight growth) usually brought on by a ‘low’ to the east of the North Island bringing warm, wet tropical air to Canterbury (and not over the Alps and dried out as in a nor’wester).  We suggest you look out for such weather patterns and time your sprays to coincide with these patterns.  We will be experimenting further with spraying before and after such events.

Blight bacteria can enter easily into wounds caused by frost, pruning cuts or broken limbs. It is advisable to spray immediately after an event like this unless you are quite sure the weather conditions are too cool or dry for blight development.

It’s safe to stop spraying in late January.  By then the shells are hardened and blight won’t penetrate the shells – though you may notice some black lesions on the husks.

An autumn “clean-up” spray is probably not helpful.  Spraying just prior to leaf fall, or even during the winter, can indeed reduce blight numbers on the outside of the buds.  However, the spray cannot penetrate the tightly-closed buds that will shoot next year.  It is better to get your bud-burst spray right – to kill bacteria on the outside of the buds and penetrate buds that are beginning to open.

Blight is unlikely to survive in dead, dried-off prunings if they are left on the orchard floor. Even if a few bacteria did survive there is almost no way for them to get back into the tree canopies.  Bacteria are mobile in water (e.g., dripping down through a tree canopy in rain drops) and on flying pollen grains, but they do not survive exposure to dehydration or UV light.

The cultivar Rex has consistently lower blight numbers than Meyric (and lower nut loss). We noticed a significant difference throughout several years and in many Canterbury orchards, and expect this would also be the case elsewhere in NZ, at least in similar eastern (dry) climates.  Meyric, in turn, seems less susceptible than other cultivars such as Dublin’s Glory, Vina and Tehama.

Our researchers have isolated several virulent bacteriophages – viruses that attack and kill walnut blight.  Each bacteriophage is capable of killing several strains of walnut blight bacteria in laboratory conditions, but has no effect on any other bacteria or other organisms.  Bacteriophages occur both in the canopy and the soil, but only canopy bacteriophages appear able to survive the dry and high UV conditions in the canopy. The most effective canopy bacteriophages are under further investigation.  Means of propagating, storing and deploying them in the canopy are being developed but there are still many challenges to overcome.

 

Notes on using a spreadsheet to calculate when to spray for blight can be found here. 

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New Zealand Walnut Industry Group