Kiwifruit Journal Articles

First FON for Opotiki

Fraser Rd Orchard

A personal challenge and the opportunity to lift Opotiki’s kiwifruit-growing game – this is why the manager of Opotiki’s first Focus Orchard John Phelps signed up to the Zespri Orchard Productivity Centre’s programme.
John is an orchard manager with OPAC and manages the 10-hectare Fraser Road Orchard on the Paerata Ridge, west of Opotiki township across the ocean towards the distant East Cape.

Why Opotiki?

“We want to lift Opotiki’s game - that’s the aim. It doesn’t matter who you pack with, this region’s got something to offer,” John explained.
With around 10 percent of Zespri’s fruit grown in the Eastern Bay of Plenty, the warm climate with plenty of rain and thick, rich soils make the region one of the top growing areas in the country. Shane Max, Zespri’s orchard productivity manager for global supply, said the Focus Orchard Network was set up to help growers improve their orchards’ productivity and profitability and help them grow the premium-quality fruit that markets demand.
Orchard trials and demonstrations are a key part of the programme, taking new science and grower innovations out to orchardists.
“The local community was very keen to set up a FON, given its success in other areas,” Shane said. “It was a difficult decision choosing between the nominated orchards and all the managers were keen to participate, as they could see the value it could bring to the whole district.”
Zespri’s Beth Kyd has recently joined the OPC team and will be directly responsible for the Opotiki programme. Four field days a year will be held on the orchard and this programme is a great opportunity for local growers and their staff from all three local packhouses to get together and share their experiences and tips.
“We’ve found the FON programme in other districts has really helped bring the local growing communities together,” Beth said.

Why Fraser Road?

Fraser Road Orchard is at an elevation of 90m and has around 6ha of Gold3 grafted in 2011 and 2013, as well as 4ha of Hayward. High taste levels and improving productivity are a key focus for this orchard, which is owned a consortium of shareholders.
John explains his goal is to improve production from 8,500–9,000 te/ha for Green to 12,000 te/ha, as well as reaching 15,000 te/ha of good-taste, good-size 76 Gold3 – and putting himself up in front of his peers is an additional incentive to achieve those goals.
“I’m competitive and I don’t like to be beaten. I’ve got high standards and I beat myself up if I don’t achieve them,” John said, with a hint of a smile.
He describes kiwifruit as “a pretty good life” and moved to Opotiki in 2009 from Tauranga with his family – wife Claire and two young children. “I like the quiet little towns,” the Gisborne-ite, who takes full advantage of the area’s outstanding hunting and fishing, said.

How FON works

An orchard management team will be working with John and Zespri staff to determine their goals for the coming season. As part of the Focus Orchard Network programme, John and the
team will look right across the orchard and review everything from pest management, pollination, soil and leaf testing, Psa monitoring and fruit sampling methodology. There is also a formal reporting process at the end of the year to measure progress towards meeting those goals.
Sustainability is another area of interest for John. “We need to back up what we say. We call ourselves green in the marketplace – are we what we say?”
Being part of the FON is another way for him to tap into expertise in this area, which can then be shared with others.
“I’m looking forward to being closer to the information and meeting new people,” he said.

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Briant family hope to harvest Gissy gold

It’s only when you get inside Gisborne’s newest kiwifruit orchard that the scale becomes apparent.
Geometric lines of vines disappear into the distance, with the perspective made even more impressive by the dance floor-flat contour.

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Sean Carnachan

Katikati

After the Easter weather event, it really emphasised how important it is to keep monitoring and using crop protection programmes. It was an eye-opener to show the growers what was in their orchard and that the bacteria was still around – lots of juvenile wood with wind-rub was quite clearly showing signs of Psa.
We had a great harvest and our Gold3 crops were stunning. We had amazing pack-outs with large, clean fruit and it handled beautifully through picking and the pack-houses.
Gold9 crops were also outstanding – we ranged from between 16,000 and 23,000 te/ha, with better dry matter this year and better size. It also picked well this year and as the vines mature, the stalks seem to be snapping off a lot cleaner.
Dry matters were a little bit down in Hayward crops on previous years, although size and yields were both good.
We were incident-free until two big frosts late in the harvest – the second one got down to -2.8degC, which is the coldest I’ve ever had it in May. All the fruit in frost-prone orchards should’ve been off but there were a few caught out and some of that fruit might be concerning.
The frosts contributed to our longest picking day ever – we had a crew that started at 8am and finished at 1.25am the next morning!
There will be very little Gold3 grafted this winter because most people have already mopped up any licenses still out there. The alarming thing is potentially how well it’s going to yield in years to come and how we’re going to manage the volumes of fruit post-harvest.
In terms of where we have been in the last few years, they are good problems to have though!

Mark Hudson

Whakatane

Whakatane growers finished harvesting by the start of June, which is the earliest for some years. Good teamwork between growers, contractors and pack-houses has enabled this, along with the most favourable weather. The region also had an abundance of harvesting capability, to get the crop off at the optimum time.
The most challenging aspect of the season has been the picking and grading of hail-damaged fruit, stemming from a storm in mid-January.
The extent of the damage was often worse than expected and the undamaged fruit did not size up, due to the poor canopy which was also damaged by the hail. Many affected growers did not thin, hoping to maximise Class 2 returns, but this meant some lines had 60-70 percent non-Class 1 fruit that was very slow to grade and pack.
Hayward crops were generally much better than last year with less flower drop.
Green14 crops were, on the whole, disappointing with small fruit and possible pollination issues. To top it off, blocks suffered from fruit drop of hard and softer fruit just prior to harvesting.
Those growers who still have Gold9 had excellent crops of large fruit. Some growers had issues of out-of-line maturity, with a portion of fruit remaining unripe.
Gold3 crops came off well, at better than estimate and with high taste. For most growers, this was their first crop. Growers are looking forward to possibly doubling their crop next year with the quantity of cane they have to tie down.

Paul Jones

Te Puke

Growers and packhouses are celebrating the completion of a very successful harvest season.
Te Puke picking conditions were unusually favourable and, coupled with excellent quality crops, meant that the picking teams and post-harvest were able to process crops in record time. Everybody should have made it to the Fieldays this year!
Crop quality was outstanding overall, with the Hayward size ideal for Zespri’s requirements. Te Puke Hayward orchards at lower elevations generally saw a heartening recovery from the Psa- affected volumes of 2013. The new variety crops were also very rewarding, with many Gold3 and Gold9 orchards exceeding pre- harvest estimates. However, Green14 continues to be problematic this year due to on-orchard fruit drop and overripe fruit.
Winter arrived rapidly in the last week of May with two severe frosts, around -3degC in exposed areas. This has done a great job of removing the leaves and gaining some worthwhile winter chill units.
Te Puke, the first area to crash when Psa arrived, has now become the epicentre for orchard value recovery, which has seen a dramatic lift to pre-Psa values.
We now look forward to a Psa-kind winter and a continuation of increasing grower confidence. Best of all, we have a hungry marketplace for our 2014 crops!
Best regards to all growers for the coming season.

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Carol Franklin hastens to point out she’s nothing special; just a hard-working mum flying solo in her little patch of Opotiki paradise.

But that doesn't diminish the incredible story of how tragedy thrust Carol into an industry she knew little about - and how she's come up trumps.

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Running a trial: How hard can it be?

by Bill Snelgar - Plant & Food Research and Shane Max - Zespri

Your trial is valuable

Research trials are about learning so you need to believe your data, even if the trial does not show what you hoped. Carrying out a ‘rough’ comparison then discarding the results you don’t like - “the rows must have been different!” - but keeping those that fit your theory is a waste of time. You are only confirming your prejudices - you are not learning anything new.
Orchardists and scientists generally carry out trials to see if they can improve the productivity and, therefore, the profit of an orchard. This means the outcomes can be of huge value, and any errors can be costly.
Reaching a wrong conclusion can:
•    Add needless cost, by encouraging you to use a product or technique that does not work;
•    Lose money by not using a product or technique that does work.
With the payment structure used in the kiwifruit industry, even small increases in fruit size or fruit dry matter can give large increases in orchard gate return (OGR) (see Photo 1). For a Hayward orchard carrying 10,000 trays of fruit, we estimate that:
•    1 percent dry matter (0.15 TZG) is worth about $3,700/ha;
•    3g fresh weight (1 count size) is worth about $2,500/ha.
It is not easy to carry out trials that will always give you the correct answer when differences are this small. To avoid costly mistakes, a trial usually needs:
•    A control – where no treatment is applied so that you can see exactly what your treatment has changed;
•    Replication – to give some idea of the inherent variation among vines/fruit. Replication makes calculation of error possible;
Photo 1. If this spray increases the size of these (Green14) fruit by 3g, it’ll be worth about $3,000/ha, but it costs about $1200/ha to apply it. You really need to know if it works or not.
•    Randomisation – to avoid treatment bias and ensure that effects unknown to the experimenter are averaged out for each treatment.

Control

In a trial, you will want to see what changes are caused by your treatment so you need the control vines to be very similar to your treated vines to start with. Control vines should not be:
•    Your best row or block;
•    The shelter row;
•    The odd-shaped block in the corner;
•    Your neighbour’s orchard;
•    Last year’s crop.
Using good controls can actually help you a great deal when assessing the value of a treatment. For instance, Figure 1 shows the results of a trial where two alternative chemicals were compared with Hi-Cane®. The results look very promising, with the alternatives producing more than 11,000 trays/ha. Most orchardists would be very
happy with these new products. However, this trial also had a control where vines were not sprayed with anything. These vines yielded over 10,000 trays/ha. With this additional information, you’d probably decide that although all chemicals did increase yield, basic orchard management was the dominant factor producing the high yields in this orchard.

Replication and pseudo-replication

This is one of the hardest things to grasp - if you spray 10 vines in an orchard row, why don’t you have 10 replicates? The answer lies in the number of decisions you make about the trial layout. If you spray one row and leave another unsprayed,
you have made one decision and have one replicate of each treatment (Figure 2-A). This design is not statistically analysable and may well be misleading, since the two rows may be slightly different anyway - especially if one row is a shelter row. You are probably better off not doing this trial. In version B (Figure 2-B), at least the Figure 1. Results of a spray trial showing the yield of Hayward after vines had been sprayed with Hi-Cane®, or with alternative chemicals.

Randomise

The aim is to make sure you spread the treatments around the block randomly but without giving yourself the chance to bias the layout by choosing where treatments go. It is a good idea to select the vines for your trial carefully and reject those in poor condition or in badly-performing areas of the orchard.
For instance, if you are worried that the north end of the block may crop better than the southern end, then make sure you tag equal numbers of vines in each end but then randomly assign treatments within each half of the block. You can see in Figure 2-C, we have made sure there are three replicates of each treatment in the north end of the block and three in the southern end.
It is tempting simply to treat every second vine but this can lead to bias if there are gradients in productivity along the block. Randomising is the safest way to lay out a trial, plus it is easy. If you have only two treatments, flipping a coin is the quickest way. With more treatments, you may want to roll a dice.
treatments are spread across both rows, so bias should be reduced, but there are only two replicates and any difference is unlikely to be statistically detectable. Layout C is the one we’d use in a scientific trial. From the pattern of colours, it is obvious that treatments have been allocated to each vine one by one. The unit of replication is the item to which the treatment is applied individually – so individual vines are much better than rows or part-rows here. One glance at layouts A and B tells you they have poor replication and you should be very sceptical of any findings from such a trial.
Pseudo-replication is the term used when someone analyses the layout in
Figure 2-A and claims that they have 10 replicates. Analyses of this type are entirely unacceptable and are likely to be highly misleading. But it happens. Be very worried when someone says the trial was not properly randomised but we analysed it anyway. Statistics: making your analyses objective
Orchards and vines are not all identical so any time we measure attributes like fruit size and dry matter, we will usually see that some vines, or blocks, are better than others.
In our trials, we have found that the average dry matter of fruit typically varies by about 1 percent between vines. This variation can easily hide a good result, while a poor trial layout may also throw up large differences that are simply due to the between- vine variation we expect in any block. Statistics are the only way of deciding if the difference you see is due to the treatment you applied - or merely from chance variation. Even statistical analyses are not infallible. It is conventional to accept a difference as ‘significant’ if the probability of the difference occurring by chance is 1 in 20 (often referred to as P=0.05). That means that if you test 20 treatment comparisons, you are likely to obtain one ‘significant’ difference just by chance.
If run correctly, simple ‘on orchard’ trials have the ability to increase our knowledge rapidly, and cheaply. There are a number of resources to help growers and technical staff to undertake and analyse trials, including a series of KiwiTech Bulletins developed by Plant & Food Research on this topic. If you are unfamiliar with setting up trials, you are encouraged to discuss your idea with one of the team from Zespri’s Orchard Productivity Centre or with a friendly scientist. This should not only include how to set up the trial up but also how to measure and analyse the effects.

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Dennis Robinson

Te Puke

The beautiful fine weather in late March and early April enabled the industry to get off a fantastic start in the 2014 harvest season.
With the markets generally in short supply of fruit, they await the first shipments with eagerness.
Good cooperation between growers and post-harvest operators enabled the first shipments to Japan and Europe laden with great quality, high dry matter kiwifruit.
There has also been some consolidation in exporters supplying Australia with Class 2 fruit, which will hopefully flow through to better returns for growers in this important market.
Another Queensland fruit fly find reminds us that we are only as strong as our weakest link and that we must remain vigilant. Biosecurity must remain a focus.
Hot summer temperatures may have reduced the Psa risk but growers need to stay focused on their best practice post-harvest management techniques.

Evan Heyward

Nelson

Nelson has experienced an exceptionally dry autumn, with only one rain event recorded through most of February and March. That was the sub-tropical low – formerly Cyclone Lusi - that moved down the country, dropping a much-needed 70-100mm of rain across the Tasman region.
All kiwifruit varieties are tracking well in both size and dry matter, with the Gold varieties looking especially good.
The apple season continues to run at least seven days ahead of normal and the big question growers have down here had was will the kiwifruit be similarly early?
On behalf of all Nelson growers, I wish everyone a successful season ahead.

Dave Mackie

Hawke’s Bay

Since my last roundup, Hawke’s Bay has had two more orchards with positive Psa results and these growers are cutting and spraying to contain any further spread.
The Hawke’s Bay weather has been favorable to growers with hot, dry conditions. The mercury hit 36degC in my orchard at 3pm one Saturday - apple pickers melt at this point!
Harvest started in week 12 and growers are working in with the apple harvest to get the crop off.
Labour is OK at the moment, as there are backpackers around to fill any gaps created by competition between crops. All the varieties are looking good for size and dry matter.
Hort16A conversion to Gold3 is continuing and other Gold3 crops are now getting to full canopy, with very good results. Green crops are above average for size this year and looking clean. KVH has been holding meetings in Hawke’s Bay and these have been well attended and helpful reminders to growers not to let our guard down.
Water hasn’t been as much of an issue this year as it was last year. Twyford growers have used a global consent system this year and have had no restrictions, only using less than a third of the consented volume.
Wishing the industry a happy harvest and long may the good weather last.

Chris Anstis

Opotiki

Picking has been underway for a couple of weeks now and it has been a good start to the season.
The dry period we have been through does appear to be slowing the sizing on the Green, where good-looking crops in January didn’t quite have the same look in late March.
They were still OK but not what we thought they would be. Young blocks of Gold3 are just starting to reach maximum dry matter accumulation and good brix levels to harvest but there is a bit of trepidation on handling lines of fruit with average sizes of 150g.
With the second dry, warm summer behind us, Psa has been low-key for the last couple of months. It does, however, still need focus through harvest and protecting vines after picking is paramount for a good start in spring.
Let’s also hope the second Queensland fruit fly incursion turns out to be just a lone male fly and that we don’t have another new pest or disease to manage in both the orchard and supply chain.

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It started out as a wild goose(berry) chase but a trip to Whanganui in pursuit of New Zealand’s oldest kiwifruit vines may have borne unexpected fruit.

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Carol Craig

Auckland/Franklin

The warm winter and spring have resulted in some good Hayward crops in the Auckland region. There was a lot of early Psa spotting in Hayward but this seems to have caused very few problems. A couple of orchards in the Franklin area that had Psa last season have completely removed their Hort16A crops and grafted to Gold3, while others are still managing to produce a Hort16A crop but are cutting out Psa-affected areas each week. Most Hort16A in the region was notch-grafted to Gold3 last year and these are growing well. Gold3 and Gold9 crops are looking really good, with some of the second and third year grafts producing reasonable crops. Growers are quite optimistic about things now.
Kiwifruit growers in northwest Auckland were grateful for the swift action when a 1ha block of recently notch-grafted Hort16A returned a positive test for Psa in late November. A team of monitors, led by KVH, checked most other orchards in the 10km controlled area around the positive orchard and found no other signs of Psa. The owner and leasee of the affected orchard then agreed to cut out the Hort16A, in an effort to protect the other orchards in the region.
There are only three other growers with Hort16A in the region and they all notch-grafted their crops in winter 2012. Following the find, two growers cut out their Hort16A males and we can expect to hear the sound of chainsaws after harvest. Fortunately, the weather has been favourable and mandatory monitoring has, so far, not found any further signs of Psa in the region.
It has been a relief, too, that many of the abandoned orchards in the region have now been removed. The growers would like to acknowledge the support of both the Auckland Council and KVH for making this possible.

Paul Jones

Te Puke

Te Puke growers have been enjoying an exceptionally favourable summer, in terms of both growth conditions and Psa virulence. The Gold3 crops are looking terrific, although flower numbers arising from 2012 grafts were patchy. Psa is at very minimal levels, a happy contrast to the previous season. Most Gold9 crops are also tracking very well.
Gold14 growers are having an excellent year, with minimal flower drop experienced in the spring. Many growers used artificial pollination and this seems to be giving a better size result.
Green crops are more variable, with many growers observing blocky fruit, suggesting suboptimal pollination. There is certainly a Psa effect on some orchards, particularly at the highest and lowest elevations.
Overall, however, Te Puke growers are in a positive frame of mind, enjoying excellent fruit prices and, at the time of going to print, ideal weather.
We are also enjoying a significant rebuild of orchard values. The Zespri five-year plan is being viewed positively and we’re looking forward to a successful harvest and a rebuild of industry volumes.
Finally, I urge all growers to take an active part in the on-going KISP reviews.

Andrew Hill

Coromandel

Growers on the Coromandel Peninsula are resigned to the fact we’re exposed to Psa but optimistic about the efforts to minimise the impact and produce viable crops.
Before Christmas, it all looked pretty quiet. Post-Christmas, however, there has been more Psa symptoms showing up on some of the Gold orchards. At last count, around 50 percent of the KPINs in the area were Psa positive.
There have been some highlights though - there was a find over at Pauanui late last year in a 5ha Gold orchard but after quick reaction and continuation of the spray programme, they haven’t found any new symptoms since.
It all started for us in three Gold9 orchards and a lot of it was subsequently removed but the remaining Gold9 is going to produce some good crops.
Hayward has had typical leaf-spotting impact of Psa but it hasn’t really translated into an economic impact. Crops are still there and looking good, because the symptoms didn’t really hit at the key time of pollination.
There’s not a lot of Gold3 production on the Peninsula this season but there’s a lot of conversion going on, so it will ramp up next year. Most of the conversions are going really well and we’re pretty optimistic about Gold3.
One orchard in the Whenuakite Valley has all four varieties - the Hort16A on the way out, Gold3 coming in and Hayward and Gold9 producing. The only struggle was in the Hort16A, which is already notch-grafted in Gold3.
Harvest preparations are going well and the estimates from the local packhouse are looking higher than last year, probably on less hectares because of conversions.

Mark Gardiner

Waikato

The Waikato region enjoyed a warmer than usual spring and crops got away to a strong start. Gold3 is proving to be an exceptional crop thus far and has grown well, in spite of Psa.
Our Gold9, and even the Gold14 crops, are looking quite respectable after some pretty variable results last year. The Gold14 in particular did not respond well to last year’s spray programme but this year, with a reduction in copper spraying, it is looking healthy again.
Hayward crops continue to be challenging, with pollination results varying greatly. They ranged from excellent to poor, with a lot of fruit thinning being done in a number of cases.
While the region welcomes the comprehensive KISP review of our industry that is now underway, Waikato growers strongly support the underlying premise, that our industry must remain grower- based and market-led, to ensure continued profitability and stability.

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“My goal has always been to have a totally self-integrated kiwifruit business,” explains Noel Cooper, Wanganui’s largest grower and owner of Cooper Coolpac Ltd. The Cooper’s family-owned business is run by Noel and Sue Cooper and their son Andrew and includes 48 hectares of orchard, as well as a packhouse and coolstores which pack and store all their own fruit.

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Avoiding resistance development to copper and
antibiotics while controlling Psa

by Joel L. Vanneste - Plant & Food Research

One of the questions raised during last year’s Psa 2013 conference was: How can we ensure Psa will not become resistant to the copper based products and antibiotics we used for its control? In this article, we look at the precautions and strategies that can reduce the risk of selecting strains of the pathogen resistant to those products. Those strategies and precautions are relatively
simple; they consist of using antibiotics and copper in combination and in alternation, and to keep the populations of the pathogen as low as possible using other products and maintaining strict orchard hygiene. A coordinated control strategy applied on a large scale will also help in reducing the probability of selecting strains of the pathogen that are antibiotic resistant.

Whether bacterial pathogens attack plants, animals or humans, few products are available for their control. Most of these products are antibiotics or heavy metals. One of the most common concerns about using these products is the risk of selecting strains of the pathogens which are resistant to one or more of them. In the worst case scenario, resistance to antibiotics could spread from bacterium to bacterium, until it prevents us from controlling human pathogens that are today easily controlled, turning them into lethal pathogens.
In New Zealand, two antibiotics - streptomycin (KeyStreptoTM) and kasugamycin (KasuminTM) - have been allowed for the control of Pseudomonas syringae pv. actinidiae (Psa). Resistance to these two antibiotics as well as to copper, a heavy metal also used for the control of Psa, has been documented in a number of plant pathogenic bacteria.
A bacterium can become resistant to an antibiotic for different reasons. The antibiotic is being inactivated, its target has been modified, or the bacterium prevents it from entering the cell or exports it outside the cell. These different mechanisms can lead to either complete or partial resistance. For example, streptomycin kills bacteria by binding to the ribosome, a very complex structure necessary for protein synthesis. If the ribosome is modified
such that streptomycin cannot bind to it, then no amount of this antibiotic will kill the bacterium. The resistance is complete. On the other hand, if streptomycin is being modified by an enzyme, by increasing the concentration of streptomycin we can overload the enzyme and kill the bacterium. This partial resistance is also called reduced susceptibility or tolerance.
Resistance to antibiotics can result from a spontaneous mutation, or from the acquisition of a gene or a set of genes from other bacteria. Streptomycin resistance can be caused by either mechanism. A simple mutation (the change of an adenine for
a guanine in codon 43 of the rpsL gene) prevents streptomycin from binding to the ribosome, making the cell resistant to this antibiotic. A number of factors can influence the frequency of such mutations, including the concentration of antibiotic used. For example, mutation leading to kasugamycin resistance in the plant pathogenic bacterium Erwinia amylovora occurred in the laboratory only when using less than 100 ppm of the antibiotic.
Bacteria can acquire genes by several mechanisms which are collectively called horizontal gene transfer. One of those mechanisms is the transfer between bacterial cells of mobile genetic elements called plasmids or transposons. Horizontal gene transfer can also occur by a process known as transduction, in which DNA is introduced to the cell by a bacteriophage, which is a virus which infects and replicates within bacteria. A third mechanism known as transformation occurs when free DNA is picked up by bacterial cells. Resistance to streptomycin in plant pathogenic bacteria has been associated with the acquisition of the genes strA and strB. These genes are already naturally present in plasmids and transposons of bacteria isolated in New Zealand. Genes that confer tolerance to copper have been also been found to be carried by similar mobile genetic elements.
Unlike antibiotics, copper is necessary for bacteria to survive; bacteria can never become resistant to copper, only tolerant. Tolerance to copper is usually the result of several enzymes exporting excess copper to the outside of the cell.
The problem of resistance is compounded by the possibility of cross resistance or multiple resistance. Cross resistance occurs when one event (a mutation or acquisition of new genes) leads to resistance to several antibiotics. This is often observed for antibiotics that share the same target. However, this is not always the case. For example, streptomycin and kasugamycin affect the ribosome but they affect different parts of it. Up until now no mutation has been found that confers resistance to both antibiotics. Similarly, no gene or set of genes has been identified that would confer resistance to both. However, we cannot rule out that a bacterium could become resistant to both of these antibiotics by a mechanism which has not yet been found. Likewise, no gene or mutation which in Pseudomonas would confer resistance to both kasugamycin and copper, or to streptomycin and copper, has been identified today.
Even if the risk of cross resistance between streptomycin, kasugamycin and copper seems very low, there is still the risk that bacteria become resistant to these products by a process of multiple resistance. Multiple resistance is the accumulation of mutations or of genes or set of genes which individually confer resistance to only one compound. Those genes can jump from one piece of DNA to another, resulting in DNA molecules, such as genetic mobile elements, carrying resistance to several compounds. This was the case for strains of Psa isolated in Japan in 1987, which were resistant to streptomycin and copper.
There are only a few strategies available to reduce the probability of selecting strains of Psa that are resistant to the two antibiotics allowed in New Zealand and/or copper. These strategies involve combining and alternating compounds.
By combining compounds, only bacteria which have accumulated either two mutations, or two sets of genes each conferring resistance to one of the compounds used in the combination, will be able to survive. The probability of accumulating two independent mutations each conferring resistance to one compound is extremely low. We do not know the exact rate of mutation of Psa that would lead to antibiotic resistance. But if we estimate it to be around 10-8, then one cell in every 100 million cells could carry a mutation leading to resistance to one antibiotic. Under those conditions, the probability of occurrence of a
strain with two mutations is 10-16, or one cell for every 10,000 trillion cells!
The probability of a strain accumulating two sets of genes conferring antibiotic resistance by horizontal gene transfer is more difficult to estimate. However, by alternating the combinations of products, cells that are resistant to two products will be killed by the next treatment, unless they have also accumulated genes leading to resistance to the new compound used in the combination. Of course the probability of accumulating genes which confer resistance to three compounds is much lower than that of accumulating resistance to two of them.
In addition to those strategies some simple precautions can dramatically reduce the probability of selecting strains of the pathogen resistant or tolerant to one or more antibiotic(s) and/ or copper based products. Independently of whether resistance is total or partial and whether it is due to a mutation or to horizontal
gene transfer, the probability of selecting antibiotic-resistant strains of the pathogen increases with the size of the bacterial population being exposed to the antibiotic, with the duration of the exposure and with the concentration to which the bacteria are being exposed. Therefore, it is extremely important to keep the pathogen population as low as possible using basic orchard hygiene (removing all diseased material from the orchard as soon as practically feasible) and using other compounds which have been shown to reduce infection (elicitors for example). Also, it is not advisable to use less than the recommended rate of antibiotic. Low rates of antibiotic can increase the rate of mutation leading to antibiotic resistance as demonstrated for resistance to kasugamycin in E. amylovora. In addition, the same compound or same combination of compounds should not be applied twice in a row and the number of applications should be limited. Alternating compounds or mixtures of compounds prevents the pathogen from being exposed for too long to the same compound(s). However, bacteria can easily move from one orchard block to another, and spraying two adjacent blocks with the same antibiotic a few days apart can result in some bacteria being subjected to this antibiotic for a much longer period of time than anticipated. This increases the likelihood of selecting antibiotic-resistant bacteria. Therefore, a coordinated control strategy applied on a large scale is more effective than one applied independently on a small scale.
We need to use antibiotics and copper-based products wisely so that we can prevent or delay the selection of Psa strains resistant to these antibacterial compounds, and thus so New Zealand can remain a country where the use of antibiotic for control of plant pathogenic bacteria is authorised and effective.

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