Kiwifruit and Psa Kiwifruit Journal March/April 2011 MIKE CURRIE, PETER BLATTMANN, JOEL VANNESTE PLANT & FOOD RESEARCH SHANE MAX & RICHARD PENTREATH - ZESPRI ORCHARD PRODUCTIVITY CENTRE Bacterial canker caused by Pseudomonas syringae pv actinidiae (Psa) has now been confirmed on a number of kiwifruit orchards in New Zealand. The New Zealand kiwifruit industry has acted fast to limit the spread and impact of this potentially serious disease and has agreed to a strategy of aggressive containment. This has implications for every facet of plant management in our orchards including girdling practices. In this article, we outline how growers should respond in terms of application of girdles. compounds and that these provide some protection against successful entry of Psa bacteria into girdling wounds. However, this has not been proven, so a precautionary strategy is still recommended when girdling, particularly where Psa has been confirmed to be present. THE DISEASE Psa is a gram-negative bacterium that does not produce spores (Takikawa et al., 1989). This is good news, because these sorts of bacteria are probably the most susceptible to cleaning and sterilising procedures that can be adopted practically. The life cycle of Psa is not well understood yet. However, observations from Italian orchards suggest that under their climatic conditions the most critical time for production of inoculum by infected plants and/or the infection of healthy plants is in spring and autumn. At these times in New Zealand orchards, weather conditions may also be the most favourable for disease spread because of higher rainfall and wind run. DISEASE ENTRY Unlike fungi, bacteria are unable to enter plant cells pro-actively, but require physical transfer onto wounds or natural openings where they enter the plant. They then multiply between cells and in some cases start colonising the plant. Currently, the best information available suggests that the bacteria can enter kiwifruit plants via open wounds and leaf scars (Vanneste et al., 2010). Currently, there have been no reports of an association between girdling and the spread of Psa. It is possible that the rapid browning of phloem tissues observed on girdling wounds is indicative of high concentrations of polyphenolic Figure 1. Severe leaf symptoms of kiwifruit bacterial canker caused by Pseudomonas syringae pv actinidiae (Psa). HYGIENE OPTIONS Cleaning and sterilising tools is an effective option to prevent the transfer of Psa between plants. Although procedures for sterilising kiwifruit girdling tools have always been recommended, even before the advent of Psa in New Zealand (e.g. Currie, 2004; Currie et al., 2007; Currie et al., 2009), it is a good opportunity to apply these more specifically to what is now a known threat.
Cleaning the tools is the first and most important step (Figure 2). Sterilising agents may be unable to penetrate deeply into infected tissues fast enough to be practically effective, unless tools are cleaned first. In addition, cleaning significantly dilutes any inoculum present, thereby reducing the risk of obtaining an infection (Dvorak, 2008). The best, most practical and cost effective options for sterilising tools are probably alcohol and chlorine (sodium hypochlorite) solution. Gramnegative bacteria are reported to be particularly susceptible to ethyl and isopropyl alcohols diluted to 70 to 90 percent (Dvorak, 2008). Although chlorine-based disinfectants are also effective, they can result in relatively rapid corrosion of tools and can be deactivated in the presence of organic material, so need to be replaced more often. RECOMMENDATIONS 1. Providing two girdling tools for each worker, plus a bucket of soapy water and a container of 80 percent methylated spirits (meths), preferably with a quick release lid to avoid spillage. 2. After girdling each plant, wash the girdling tool in soapy water with a plastic scrubbing brush to remove all plant material (Figure 2). If this is not possible, it could be helpful at least to remove any loose plant material adhering to the tool with a dry brush before sterilising it. If using a chainsaw chain, vigorous shaking of the chain in a bucket of soapy water will remove most of the plant material. 3. Transfer cleaned girdling tools to 70 to 90 percent meths and leave to soak for at least two minutes to sterilise (Figure 2). 4. Shake off excess meths from the second cleaned and sterilised tool and continue to girdle the next plant. Note that these guidelines apply most particularly to orchards where Psa has been confirmed on the orchard or on nearby orchards. On orchards with Psa-like symptoms that have not been confirmed, we also recommend cleaning and sterilising tools more regularly. Orchards not displaying any Psalike symptoms and are located in areas where there are no confirmed outbreaks may be able to relax their hygiene procedures and sterilise tools less often, such as after every row or block of vines. However, as an absolute minimum, any tools entering a new orchard block must be thoroughly cleaned and well sterilised. The effect of sterilising solutions applied to girdling wounds to reduce the risk of Psa infection is unknown. Various products have been trialed in New Zealand and Italy but there is no evidence that these have provided benefit and it is possible that some products may reduce the speed at which wounds heal. Orchards considered to be in higher risk areas may want to consider application of sterilising solutions. The application of pruning pastes and tape to girdle wounds has not increased the speed of callus development. Figure 2. Cleaning girdling tools with soapy water to remove plant residue and reduce any inoculums levels (LHS) is important before sterilizing tools in 70 to 90 percent alcohol (RHS) or chlorine for two minutes.
The cheapest form of ethanol available is methylated spirits (meths; 95 percent ethanol) just remember that it is not suitable for drinking! Diluting eight or nine parts meths to one part water will produce approximately 75 percent to 85 percent ethanol. At this rate, some further dilution from any soapy water residues on the tool can occur before it drops below 70 percent. WHEN TO APPLY GIRDLES Ideally, spring girdles should be applied around four weeks after midbloom for the maximum impact on fruit size (Currie et al., 2003). However, in the event of Psa being recorded on the property, the timing of spring girdling may need to be adjusted if there is wet weather or high winds. It is important to avoid conditions that would promote the transfer of bacteria onto fresh wounds. Again this will be more critical in orchards with or near confirmed Psa infection. Spring girdling can be advanced by one week or delayed by around two weeks and still achieve close to the full beneficial girdling response (Currie et al., 2003). This allows a three week window to apply spring girdles. Summer girdling in ruary probably presents a lower risk, as there is no evidence that this is a key time for new Psa infections. However, this does not mean that sensible hygiene procedures can be ignored; using clean/sterilised tools and avoiding wet/windy conditions are still recommended. Summer girdling should be done early enough so that girdles are healed before autumn. For most orchards, this will mean that all girdling should be completed by the end of ruary. Autumn girdling (March/April) is not recommended, particularly on Hort16A scions, which appear to heal more slowly than the standard Actinidia deliciosa Bruno seedling rootstocks or Hayward scions. Girdles on Hort16A scions can take until the following spring to heal if girdled in March or April (Currie and Blattmann, 2009; Currie et al., 2009). UPDATE ON CURRENT RESEARCH TRIALS ROOT STRESSED HORT16A In 2009, we reported that trunk girdles (Figure 3) applied in March should be avoided, as in some of our research trials, these autumn girdles had not healed before the 2008 winter, resulting in a stunted canopy and fruit development in the following spring (Currie and Blattmann, 2009). Although not related in any way to Psa, the vines appeared quite sick, displaying significant symptoms of reduced root function, with leaf nutrient deficiency, small leaves and minimal shoot elongation (Figure 4). Subsequent to this, we have been monitoring the recovery of these plants during 2009 and 2010. Figure 3. Applying a trunk girdle with a girdling chain. These previously autumn-girdled Hort16A vines were not girdled for one season, to allow them to recover in 2008/09 (Table 1). We can report that within one year (by spring 2009), visual symptoms of leaf deficiencies and reduced growth of the canopy had abated (Figure 4, page 18). To see what would happen to their cropping potential after a visual recovery, the ex ruary plus March girdled treatment was given a single ruary trunk girdle, while the ex triple girdled treatment (November plus ruary plus March) was left completed ungirdled (Table 1). The results suggested that fruit development had returned to normal on the previously autumngirdled vines. Fruit weights were similar to those on the ruary-only girdled vines and dry matter was lower on the non-girdled vines (Table 1), as would be expected when fruit were compared with those from a ruary girdling treatment. As in previous years, adding a November girdle to ruary girdled vines increased fruit weight, in this case by around 16g. Although we are unable to conclude whether this will be the case in every orchard, the data and observations suggest that kiwifruit vines displaying symptoms of reduced vigour and vine health as a result of autumn girdling and can recover within one-to-two growing seasons. However, we expect that recovery of vines will be more challenging in
soils where there is a smaller natural root mass or where other factors such as Armillaria disease are also challenging roots. In addition, allowing girdles to remain open during autumn and winter is not recommended because of the increased risk of Psa infection. 2007/08 Nov+ 2008/09 Nov+ Nov+ Flower load (Flrs/m 2 ) 81 78 Crop Load (Frt/m 2 ) 63 54 Fresh Weight (g) 97 113* Percent Dry Matter (%) 19.0 19.0 Flesh Hue Angles ( o ) 103.7 102.3 Soluble Solids Content (%) 10.4 11.7* +March Nov++Mar 50 57 43 47 102 97 18.7 17.8* 103.9 105.0 10.9 8.8* LSD 18.4 n/a 6.7 0.68 1.34 1.19 LSD= least significant difference at P=0.05, flrs=king flowers *means in a column are significantly different from (summer) Table 1. Characteristics of Hort16A fruit from the Te Puke Research Centre trunk girdling trial at 2010 harvest. LATE SPRING 2008 LATE SPRING 2009 Figure 4. Photographs of March-girdled Hort16A vines in December 2008, showing small leaves with nutrient deficiency symptoms common after the previous seasons girdles had not healed (left). In December 2009, after being left ungirdled for one season, the symptoms were not found to be present on these vines and leaves appeared to be developing normally (right).
SUMMARY AND RECOMMENDATIONS The advent of Psa in New Zealand means that growers need to have an increased awareness of applying hygienic practices for orchard management operations, including trunk girdling. Girdles should be applied in calm, dry conditions if possible, to reduce the risk that Psa bacteria will be splashed or washed onto fresh girdling wounds. tools should be regularly cleaned with soapy water and soaked in 70 to 90 percent ethanol for at least two minutes, between rows or blocks, to reduce the risk of spreading any Psa bacteria directly onto fresh girdling wounds. Tools entering an orchard must be cleaned and sterilised as an absolute minimum precaution. Where there are Psa-like symptoms that have not been confirmed, we would recommend more frequent cleaning/sterilising of tools. Growers are advised to look at the ZESPRI Canopy website for the most up to date recommendations. Growers with confirmed Psa bacterial canker infections should clean and sterilise girdling tools between plants. In a research trial, autumn trunk-girdled Hort16A vines that had produced stunted canopies and weak fruit development in the previous season, produced canopies and fruit with normal characteristics in 2010 when autumn girdling was avoided, indicating that the plants may have recovered. kiwifruit vines after ruary is not recommended because of the risks of girdles remaining unhealed until the following spring. This applied particularly to girdles on Hort16A scions, which can heal more slowly than those on A. Deliciosa rootstocks or Hayward. Currie, M.B., Manning, M., Kingston, C. and Chamberlain, J. 2009. Vine health - A matter of common sense. New Zealand Kiwfruit Sep/Oct:24-29. Currie, M.B., Patterson, K.J., Barnett, A.M., Blatmann, P. and Nichols, P. 2007. Trunk girdling, what are the risks? New Zealand Kiwfruit Jan/:7-12. Currie, M.B., Martin, P., Blattmann, P., Thorp, G., Meier, X., Manning, M., Barnett, A. and de Silva, N. 2003. Optimising the timing of girdling on Hort16A and Hayward kiwifruit. HortResearch Client Report No.5551. Final report to ZESPRI Group Ltd on Project 498.: 185 pp. Dvorak, G. 2008. Disinfection 101. The centre for Food security and public health, Iowa state university: 22 pp. Takikawa, Y., Serizawa, S., Ichikawa, T., Tsuyumu, S. and M., G. 1989. Pseudomonas syringae pv. actinidiae pv. nov.: The causal bacterium of canker of kiwifruit in Japan. Annals of the phytopathology society of Japan 55(437-444). Vanneste, J., Brun, S., Spinelli, R. and Max, S. 2010. Kiwifruit bacterial canker Pseudomonas syringae pv actinidiae. KiwiTech Bulletin No. N68: 6 pp. REFERENCES SITED Currie, M. 2004. Trunk girdling - Hayward and Hort16A. Kiwitech Bulletin No. 36, September 2004. ZESPRI International Tech Transfer.: 5 pp. Currie, M.B. and Blattmann, P. 2009. Warning - do not trunk girdle late in autumn or more than twice. New Zealand Kiwfruit Mar/ Apr:8-12.