Bactericide resistance management strategy for kiwifruit bacterial canker (Pseudomonas syringae pv. actinidiae; Psa) in New Zealand

J. L. Vanneste; updated 2023, R. Beresford

Disclaimer:

The fungicide and bactericide resistance information in the mode of action group table, the mode of action-based strategies and the crop-based strategies is intended to help manage the effects of pesticide resistance on plant disease control by agrochemicals in New Zealand. The New Zealand Committee on Pesticide Resistance (NZCPR) is a subcommittee of the New Zealand Plant Protection Society (NZPPS). The following parties: NZCPR, NZPPS, any people named in the strategies and any organisation the people may be affiliated with, do not guarantee that the resistance information is complete, consistent, accurate or up-to-date, or that resistance will not develop, or that the degree of resistance will decrease, as a result of the resistance information being used, and the parties accept no liability for loss or damage arising from the use of the resistance information contained in these strategies.

Introduction

In 2010 Psa-V was first reported in New Zealand, and the following year the bacterium caused severe damage to both the green and gold kiwifruit industries.

Pseudomonas syringae pv. actinidiae (Psa-V) carries no identified risks associated with human or animal health and does not affect plants other than kiwifruit vines. Psa is believed to be spread by weather events, namely wind and rain, and plant material. It is also believed to be spread by footwear, vehicles, and orchard tools. In an orchard it can exist as:

 Growth of the bacteria outside/inside the vines can result in leaf spotting, cane/leader dieback and, in extreme cases, vine death accompanied by the production of exudates.

Since July 2015, resistance or reduced sensitivity to streptomycin and copper has been found on several orchards. These finds were detected in an ongoing monitoring programme. Product resistance can occur naturally and therefore these results are not unexpected. Repeated use of a given group of bactericides may also cause a gradual shift in the Psa-V population from predominantly sensitive strains, through an increasing proportion of less sensitive strains to an eventual predominance of strains highly resistant to that group. The time period for this shift depends on the mode of action of the bactericide group, the genetics of resistance in the bacteria and the frequency with which the bactericide group is used. “Practical resistance” is the term used when the predominant strains are so resistant that they cause a measurable loss of disease control when a product containing the at-risk bactericide group is used on its own. The purpose of paper is to outline strategies which will reduce the likelihood of “practical resistance” occurring.

This is important as few products are currently known to have efficacy against Psa-V it is essential all growers manage resistance, as the loss of an effective product due to resistance will make Psa-V more difficult to control.

Good Practice Strategies

It is essential all kiwifruit growers incorporate best-practice strategies to reduce the risk of resistance of Psa to control products developing; and to minimise the build-up and spread of resistant populations when present.

  1. Monitor orchards regularly for Psa-V symptoms. Regular monitoring will identify Psa symptoms early and enable swift action.
  2. Remove and dispose of diseased material from the orchard. Removal and disposal of infected material reduces the level of bacteria and limits potential for spread.
  3. Maintain a year-round comprehensive Psa-V protective spray programme. Use a combination of and /or alternate effective products from the Zespri Crop Protection Programme.
(See Table 1 below)Products with different modes of action should be utilised during the growing season – either alone or in combination, e.g. KasuminÔ  then copper. The repeated use of the same crop protection compound or class of compound can result in resistance developing.
  4. Always use label rates. Applications of crop protection products at less than optimal rates can allow resistance to develop.
  5. Essential that good spray coverage is achieved. Maintain a flat, open canopy structure to support even distribution of the fungicide/bactericide throughout the canopy. Consider the use of super spreaders to improve spray coverage. Calibrate sprayers correctly and alternate sprayer direction.
  6. Contact Zespri if resistance or a new strain is suspected. Growers who believe spraying is not providing Psa-V control and suspect resistance should contact Zespri on  0800 155 355. It is important suspected resistance is reported early. Zespri may arrange testing to be carried out. If resistance is confirmed, affected growers will be notified and advised of next steps.
  7. Ensure strict orchard hygiene measures are in place. Good hygiene measures are essential to reduce the risk of transferring resistant or tolerant Psa-V within and between orchards. This includes sanitising tools between vines, washing down orchard equipment between orchards and ensuring all visitors are aware of orchard hygiene procedures.
Table 1:  Mode of Action Groups – Products registered for the control/management of Psa-V in New Zealand. 
Mode of Action Group Code and Activity Group Active ingredient Trade name
24. Hexopyranosyl antibiotic kasugamycin Kasumin™
25. Glucopyranosyl antibiotic streptomycin Keystrepto™
M01. Inorganic 1 Copper Tri-Base Blue®
Copper chelated by TEPA Coptyzin
Copper 1 oxide Nordox™ 75WG, AG Copp 75
Copper hydroxide Champ DP, ChampION++®, Kocide® Opti™, Hortcare® Copper Hydoxide 300
NC. Diverse Aureobasidium pullulans Aureo Gold
Ulocladium oudemansii U3 BOTRY-Zen®
Trichoderma spp Kiwivax®
Elicitors (not active directly on Psa-V pathogen) Acibenzolar-s-methyl Actigard®
Plant growth regulator (not directly active against Psa-V pathogen) Forchlorfenuron Ambitious, Gro-Tech

1 If practical resistance to copper is identified all copper based mode of action (M1) products will have cross resistance. 

Copper based fungicides/bactericides 

Due to their mode of action, multisite bactericides, such as copper, are considered as a low resistance risk group. Therefore, they offer the possibility for use as mixing partners or alternating with single site and other medium to high resistance risk bactericides.

The most widely used Psa-V protectant sprays are copper-based. Copper products are proven to be active on plant leaf and cane surfaces and knock back high Psa-V populations associated with the establishment of infection. The persistence of copper on the vine depends on the product formulation and can be affected by rainfall.

Copper kills bacteria on contact. The copper ions travel through the cell walls of the bacteria and disrupts the cellular enzyme activity (note: Copper is non-systemic). Copper is a protectant and needs to be applied evenly to the plant surface before the disease develops. Water on plant surfaces (from rain, dew or irrigation) reacts with leaf excretions to form weak acids (pH<6.0). This low pH increases the solubility of copper products, slowly dissolving them to release a small and constant supply of copper ions.

How does resistance to copper develop? 

Copper is an essential trace element that is required by bacteria, hence, will contain a number of functioning copper genes that will be involved in protecting the bacteria from excess, or toxic, levels of copper. Psa has previously been found to contain the genes CopA and CopB, which by themselves will not lead to copper tolerance in Psa.

Copper resistance arises when Psa can survive a low level (or sub lethal concentration) of exposure to copper. Development of copper resistance tends to be incremental, with the bacteria surviving gradual increases in copper levels as they gradually become able to process higher levels of copper. Application of full label rates is thus strongly recommended.

When presence of copper resistant Psa-V has been confirmed:

Bactericide (antibiotic) resistance management strategy

Group 24 (kasugamycin) and 25 (streptomycin) bactericides are classified by the Fungicide Resistance Action Committee (FRAC) as medium to high risk for selecting resistance development and resistance management is required. Prolonged use of these products is likely to result in resistance developing and hence they must be used in a programme with alternative product groups that are active on Psa-V.

The NC (Non Classified) bio bactericide AUREO GOLD is presumed by the manufacturer to have a low risk for selecting resistance development. Guidelines as per Good Practise Strategies as above should be followed to ensure resistance does not occur.

Adherance to kiwifruit industry recommendations: 

Zespri has published charts detailing application windows relating to application rates and kiwifruit development stages. It is strongly recommended that these charts be viewed prior to each growing season to ensure correct timing of registered products to minimise resistance development and optimise Psa-V disease control.

Acknowledgements

The assistance of Kiwifruit Vine Health, Zespri Orchard Quality Services and Global Extension Teams is gratefully acknowledged.

Further information

Contact Zespri – 0800 155 355 or via psa@zespri.com.

References

Kiwifruit Vine Health (KVH) website  https://kvh.org.nz/

FRAC guidelines regroup 24, 25 and M1 mode of action products

Zespri website ‘Spraying’ page <https://canopy.zespri.com/full/dashboard/producing-kiwifruit-/protecting-your-fruit/spraying>

Zespri website ‘Psa’ page <https://canopy.zespri.com/full/dashboard/producing-kiwifruit-/protecting-your-fruit/identify-and-manage-diseases/psa>

Zespri Canopy ‘Crop protection standard’ <https://canopy.zespri.com/content/dam/new-canopy/nz/en/documents/basic/manuals/crop-protection/OO-CPS-NZ-EN-Crop-Protection-Standard.pdf>