224March/April 2014

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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