NZPPS pesticide resistance management strategy

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Streptomycin management strategy

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

J.L. Vanneste
Plant & Food Research, Private Bag 92169, Auckland 1142, New Zealand

(Revised October 2011)

Introduction

The goals of these guidelines are to preserve the efficacy of streptomycin, where it has not been compromised by the development of resistance, and to limit the development and spread of streptomycin resistance in plant pathogenic and commensal bacteria in New Zealand. Streptomycin is an aminoglycoside that kills bacteria by interfering with protein synthesis. It is the only antibiotic used in New Zealand for the control of plant pathogenic bacteria. There are only a limited number of alternatives to the use of this antibiotic for bacterial disease control, the most frequently used being copper-based compounds. The use of these can lead to copper resistance and to plant toxicity.

Streptomycin product perspective

Only one product containing streptomycin, Keystrepto™, is on the market in New Zealand. It is formulated as streptomycin sulphate and is registered for the control of a relatively narrow range of plant bacterial diseases (Table 1). Resistance to streptomycin has been reported both overseas and in New Zealand. 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. Non-antibiotic products, such as biological control agents, that can help to control or limit the incidence of bacterial diseases are also becoming available overseas and in New Zealand.

¹A limited approval for the use of streptomycin on kiwifruit vines was granted in August 2011 by The Agricultural Compounds and Veterinary Medicines Group (ACVM) of the Ministry of Agriculture and Forestry (MAF). This approval is subject to the following conditions: streptomycin is to be applied in the Priority Zones and the High Risk Areas as determined by Kiwifruit Vine Health (KVH). Those are the areas where a highly virulent strain of the pathogen has been found. Streptomycin is to be applied no more than three times between harvest and the pre-flowering period. These conditions of use may change as new information about the pathogen and control of this disease becomes available.

Current status of streptomycin resistance

Development of resistance to streptomycin in plant pathogens and in other plant-associated bacteria seems to be relatively common. Such a resistance has been found in New Zealand. Resistance to streptomycin results either from an enzymatic modification of streptomycin (preventing it from binding to the bacterial ribosome) or from the modification of the ribosomal protein S12, which is the target molecule in the bacterial ribosome. Modification of the target molecule results from a point mutation in the gene rpsL which codes for the protein S12. Such a mutation makes bacteria resistant to extremely high levels of streptomycin (over ten times the recommended amounts), but the resistance cannot be easily transferred to other bacteria. It is usually transferred only during bacterial division. Bacteria that are able to inactivate streptomycin enzymatically have usually acquired this capability through the acquisition of genes, which code for the enzyme(s) necessary to inactivate streptomycin. These genes are carried by genetic elements, such as plasmids or transposons, that can be transferred directly from cell to cell, and can confer resistance to other bacteria, including bacteria from other species or other genera. Those bacteria are resistant to lower levels of streptomycin than bacteria that have a mutation in the rpsL gene.

Strains of Erwinia amylovora, the fire blight pathogen, that are resistant to streptomycin have been isolated from Hawke's Bay (Thomson et al. 1993). All strains of E. amylovora that have been isolated from New Zealand were found to carry a mutation in the rpsL gene (Vanneste & Voyle 2001). Several years of monitoring failed to find resistance to streptomycin outside Hawke's Bay. However, no survey has been conducted since 2000.

Some strains of Pseudomonas syringae pv. syringae isolated from apple orchards in Hawke's Bay and stone fruit orchards in Otago have been found to be resistant to streptomycin (Vanneste & Voyle 2001). These strains have acquired the genes strA and strB, which are carried on a plasmid or on a transposon (Vanneste & Voyle 2001). In some cases this resistance seems to be linked to copper resistance (Vanneste & Voyle 2003). Since no formal survey has been carried out, the current status of streptomycin resistance in P. syringae in New Zealand is unknown. No information is available on bacterial pathogens of tomato. At the time of this update, none of the isolates of P. syringae pv. actinidiae tested since this pathogen was first detected in New Zealand in November 2010 has been found to be resistant to streptomycin.

Overseas, streptomycin resistance has been detected for most if not all the pathogens targeted by this antibiotic, and in particular for E. amylovora, Erwinia carotorovora, P. syringae pv. syringae, P. syringae pv. papulans, Pseudomonas cichorii, Pseudomonas lachrymans, Xanthomonas campestris pv. vesicatoria, Xanthomonas diffenbachiae (McManus et al. 2002) and P. syringae pv. actinidiae (Nakajima 2002). Streptomycin resistant strains of E. amylovora have been detected in the USA (California, Idaho, Michigan, Missouri, Oregon and Washington) and in Israel (Psallidas & Tsiantos 2000). In contrast to the situation found in New Zealand, strains of E. amylovora carrying the genes strA, strB, have been isolated on several occasions in the USA (Jones & Schnabel, 2000). Streptomycin-resistant strains of X. campestris pv. vesicatoria, which causes bacterial spot on tomato, have been isolated in the USA (California, Florida Georgia and Pennsylvania) since the early 1960s (Stall & Thayer 1962). It has also been found in Argentina, Brazil and Taiwan (McManus et al. 2002). The presence of streptomycin-resistant strains of P. syringae in Canada has been documented since the late 1970s (de Boer 1980). This was the time when the isolation of streptomycin-resistant strains of the same pathogen was being recorded in New Zealand (Young 1977). Streptomycin-resistant strains of P. syringae have also been isolated in the USA (Michigan, New York, Oklahoma and Oregon) (McManus et al. 2002). Strains of P. syringae pv. actinidiae resistant to streptomycin have been detected in Japan since 1987. These strains carried the genes strA and strB on a plasmid that also codes for copper tolerance (Nakajima 2002).

Resistance management strategy

Recommendations for use of streptomycin for specific crops are presented in Table 2. In general, streptomycin should only be applied when climatic conditions are favourable for disease development, and the target pathogen is present. Do not use for emergency disease management in cases of a severe disease outbreak. When other compounds are available, they should be used in preference to streptomycin. At the first sign of lack of efficacy, the use of streptomycin should be suspended until the presence or absence of resistant strains of the pathogen can be confirmed. This will require the isolation and identification of the pathogen and determination of the streptomycin resistance status by a microbiology laboratory. Since streptomycin could kill epiphytic bacteria, which otherwise would have competed with the pathogen for space and nutrients (and therefore may have reduced disease severity), the use of streptomycin on crops infected by streptomycin- resistant strains of the pathogen can increase development of the disease.

¹ A limited approval for the use of streptomycin on kiwifruit vines was granted in August 2011 by The Agricultural Compounds and Veterinary Medicines Group (ACVM) of the Ministry of Agriculture and Forestry (MAF). This approval is subject to the following conditions: streptomycin is to be applied in the Priority Zones and the High Risk Areas as determined by Kiwifruit Vine Health (KVH). Those are the areas where a high virulence strain of the pathogen has been isolated. Streptomycin is to be applied no more than three times between harvest and the pre-flowering period. These conditions of use may change as new information about the pathogen and control of this disease becomes available.

Recommendations on implementation of this strategy

Label directions for the use of streptomycin should include an appropriate statement about resistance management and recommendations about the maximum number of treatments that should be applied for each the targeted pathogens. The only streptomycin product available in New Zealand for the treatment of plant bacterial pathogens does not currently have a statement about resistance management.

Acknowledgements

This resistance management strategy was prepared by J.L. Vanneste of The New Zealand Institute for Plant & Food Research Limited.

Reference

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