Antibiotic bactericides [Group 24 and Group 25] resistance management strategy

Fire blight Erwinia amylovora on pear shoots
(Photo: J.L. Vanneste)

Updated 2023, R. Beresford and M. Christie

Introduction

The purpose of this guideline is to help maximise the useful life of fungicides in the Fungicide Resistance Action Committee (FRAC) Group 9 for controlling plant diseases in New Zealand by providing relevant information about the nature of pathogen resistance to this group and guidance on practices that may prevent or reduce the development of resistance.

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.

Antibiotic product perspective

Two antibiotics, often referred to as aminoglycoside antibiotics (AGAs), are used against bacterial plant pathogens in New Zealand. These are: 1) streptomycin, classified by FRAC (the Fungicide Resistance Action Committee) as a Group 25 glucopyranosyl antibiotic, and 2) kasugamycin, classified by FRAC as a Group 24 hexopyranosyl antibiotic. Group 24 and 25 antibiotics both inhibit amino acid and protein synthesis in bacterial plant pathogens. One streptomycin product (Keystrepto™) and one kasugamycin product (Kasumin™) are registered for use in New Zealand on plants by The Agricultural Compounds and Veterinary Medicines Group (ACVM) of the Ministry for Primary Industries (MPI) (Table 1).

**Table 1:** *Bacterial plant pathogens and crops targeted in New Zealand by antibiotics.*
Pathogen	Disease	Crop
1. Streptomycin
Erwinia amylovora	fire blight	Apple, pear and nashi
Pseudomonas syringae pv. syringae	bacterial blast	Stonefruit
Xanthomonas arboricola pv. pruni	bacterial spot	Stonefruit
Pseudomonas syringae pv. tomato	tomato speck	Seedling tomatoes
Pseudomonas syringae pv. syringae		Seedling tomatoes
Xanthomonas campestris pv. vesicatoria	bacterial spot	Seedling tomatoes
Clavibacter michiganensis pv. michiganensis	bacterial canker and wilt	Seedling tomatoes
Pseudomonas syringae pv. actinidiae	PSA-V (bacterial canker)	Kiwifruit
2. Kasugamycin
Pseudomonas syringae pv. actinidiae	PSA-V (bacterial canker)	Kiwifruit

Current status of antibiotic resistance

The FRAC classification for streptomycin resistance risk is ‘high’ and for kasugamycin the risk is ‘medium’, and resistance management is required for both these antibiotics. There is no cross-resistance between streptomycin and kasugamycin.

Streptomycin binds to the bacterial ribosome, interfering with peptide synthesis. Resistance can result from either a modification of the ribosome or a modification of the antibiotic itself. Streptomycin is used in veterinary medicine as well as for plant protection. Resistance to streptomycin appears to be relatively common and has been reported in plant pathogens and other plant-associated bacteria both in New Zealand and overseas. It seems the presence of streptomycin-resistant strains in bacterial plant pathogen populations does not necessarily compromise disease control.

Kasugamycin is used only in plants, not in human nor veterinary medicine(Verhaegen et al. 2023). Resistance to kasugamycin in plant pathogens is known internationally but does not appear to be associated with major disease control problems. Kasugamycin resistance has not been studied in New Zealand.

Resistance management strategy

See Table 2 for resistance management recommendations on antibiotic bactericide use in specific crops. In general, antibiotic bactericides should only be applied when weather conditions are favourable for disease development, the target pathogen is present and disease has not yet developed. Do not use for emergency disease management during a severe disease outbreak. When other compounds are available, they should be used in preference to antibiotic bactericides.

At the first sign of lack of efficacy in one of these antibiotics, or if resistance has been previously detected onsite, their use should be suspended until the presence or absence of resistant strains of the pathogen can be confirmed. (Streptomycin-resistant strains of bacterial plant pathogens can survive a long time.) Since antibiotics could kill epiphytic bacteria that would compete with the pathogen for space and nutrients (and therefore may reduce disease severity), the use of antibiotics on crops infected by antibiotic-resistant strains of a bacterial pathogen could actually increase disease development.

**Table 2:** *Antibiotic bactericide (streptomycin and kasugamycin) use guidelines for specific crops.*
Crop	Disease	Recommendations
1. Streptomycin
Apples, pears	Fire blight	Use no more than 3 times over the bloom period. Use only when climatic risk and level of inoculum are high. Periods of high disease risk should be determined with the help of disease risk models such as Maryblyt or Cougar blight.
Stonefruit	Bacterial blast and bacterial spot	Use no more than 3 times over the period from green tip to mid-December. Use in autumn, but only if severe symptoms were detected during the growing season.
Tomato seedlings	Bacterial speck, spot, canker and wilt	Each lot of plants should be sprayed not more than 3 times with streptomycin. Disinfect the glasshouse (including substrate and equipment) between each lot of plants to reduce the risk of carryover of streptomycin resistant strains. Use disease free seeds or treated seeds whenever possible.
Kiwifruit	Psa-V (bacterial canker)	Make a maximum of 2 antibiotic applications per season (streptomycin or kasugamycin) at full label rate. Also refer to the Zespri user guide protocols for Keystrepto™ KeyStrepto™User Guide Use only as a surface/foliar spray with ground-based methods up to 7 days before any flowers are open. Apply prior to infection events identified by the Zespri Psa-V Risk Model accessed on Canopy \| Weather & disease portal KVH PSA-V risk model, or as soon as possible afterwards.
2. Kasugamycin
Kiwifruit	Psa-V (bacterial canker)	Make a maximum of 2 antibiotic applications per season (streptomycin or kasugamycin) at full label rate. Also refer to the Zespri user guide protocols for Kasumin™ Kasumin™ User Guide Use only as a surface/foliar spray with ground-based methods up to 7 days before any flowers are open. Apply prior to infection events identified by the Zepsri PSA-V risk model, found on Canopy \| Weather & disease portal or as soon as possible afterwards.

Recommendations on implementation of this strategy

Label directions for the use of antibiotic bactericides should include an appropriate statement about resistance management and recommendations about the maximum number of applications that should be applied each year.

Acknowledgements

This resistance management strategy has been updated from the original streptomycin strategy prepared by J.L. Vanneste in 2011.

References

Verhaegen M, Bergot T, Liebana E, Stancanelli G, Streissl F, Mingeot-Leclercq MP, Mahillon J, Bragard C 2023. On the use of antibiotics to control plant pathogenic bacteria: a genetic and genomic perspective. Front Microbiol. 2023 Jun 27;14:1221478. doi: 10.3389/fmicb.2023.1221478. PMID: 37440885; PMCID: PMC10333595.