Phenylamide (PA) [Group 4] resistance management strategy

The phenylamide fungicide metalaxyl-M
is used to control downy mildew of onion
caused by Peronospora destructor

Updated October 2023, R.M. 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.

Product perspective

In the absence of resistance, Phenylamide fungicides are highly active against oomycete plant pathogens e.g. Phytophthora and Pythium diseases and downy mildews (Table 1). They have been in commercial use since 1978. Phenylamides are rapidly absorbed into plant tissue and are translocated acropetally within the plant. They have a site-specific mode of action, inhibiting rRNA synthesis in the target fungi. They show no cross-resistance with other fungicide groups, but there is cross-resistance among all the active ingredients within the phenylamide group. In the mid 1990s it was discovered that one of the two isomer constituents of metalaxyl was more active than the other and the active one was introduced commercially as metalaxyl-M (mefenoxam) in 1996 (Table 2).

**Table 1:** Pathogens and crops targeted by phenylamide fungicides in New Zealand.
Pathogen	Disease	Crops
Peronospora destructor	Downy mildew	Onions
Peronospora parasitica	Downy mildew	Brassicas
Peronospora sparsa	Downy mildew (dry berry)	Brambles
Peronospora viciae¹	Downy mildew	Peas
Phytophthora cinnamomi	Root rot	Avocados
Phytophthora infestans¹	Late blight	Potatoes, tomatoes
Phytophthora megasperma	Spear rot, crown rot	Asparagus
Plasmopara viticola	Downy mildew	Grapes
Pseudoperonospora cubensis	Downy mildew	Cucurbits
Pythium spp. and Phytophthora spp.	Damping off, seedling diseases, root rots	Ornamentals, seedlings, tree nurseries, lucerne

¹Pathogens with resistant strains identified in New Zealand.

**Table 2:** Phenylamides marketed in New Zealand.
Common name	Trade name
metalaxyl	in Max Cl with chlorothalonil Phytospear Speartec
metalaxyl-M (mefenoxam)	Apron XL (seed treatment) Ridomil Gold 2.5G, Ridomil Gold EC in Ridomil Gold MZ WG with mancozeb in Wakil XL with fludioxonil and cymoxanil (seed treatment)

Current status of phenylamide resistance in New Zealand

Resistance has been recorded in New Zealand to metalaxyl (including metalaxyl-M) in Phytophthora infestans in potatoes (Hartill et al. 1983) and in Peronospora viciae in peas (Falloon et al. 2000). Overseas, resistance has also been recorded in Bremia lactucae, Peronospora destructor, Peronospora parasitica, Plasmopara viticola, Phytophthora cactorum and Pseudoperonospora cubensi. It is important to heed resistance management guidelines in New Zealand to prevent resistance developing in any of these pathogens here. Adherence to resistance management guidelines in the UK and Europe for Phytophthora infestans and Plasmopara viticola appears to have limited the build-up of resistant strains, allowing continued effective use of these fungicides in potatoes and grapes. Resistant strains appear to survive less well than sensitive strains in the absence of phenylamide use. The mode of resistance to phenylamides may involve one or two major genes and potentially several minor genes. The site of mutation involved in resistance has not yet been mapped.

General guidelines for phenylamide fungicides

Observe manufacturers' recommendations for application rate and timing for specific products and the maximum number of applications of phenylamide products for specific crops (Table 3).
For foliar application, phenylamide active ingredients must be mixed with an effective dose of a fungicide from a different cross-resistance group. Pre-mixed formulations are available.
Apply phenylamides preventatively when disease level is low, but disease risk is high. Do not apply phenylamides when disease has become obvious.
To optimise systemic activity confine phenylamide use to the period when plants are growing fastest (e.g. in the pre-flowering period for potatoes). At other times, and particularly when foliage is mature, do not use phenylamide fungicides.
Phenylamides should not be used for foliar treatment in glasshouses because development of resistance is favoured in this situation.

**Table 3:** Phenylamide use guidelines for specific crops.
Crop	Disease	Recommendations
Asparagus	Phytophthora spear rot	One application of granule formulation before spear emergence in spring, or a maximum of two soil drench applications in spring, made on the same day before and after cutting.
Avocados	Phytophthora root rot	One spring application of granule formulation.
Boysenberries, brambles	Dry berry (downy mildew)	Maximum of three foliar applications per season. Where crops are grown sequentially, no more than three phenylamide treatments within a 12 month period. Always ensure phenylamide active ingredients are mixed with an effective dose of a fungicide from a different cross-resistance group.
Potatoes, tomatoes	Late blight
Cucurbits, grapes, onions	Downy mildew
Seed treatment for various crops	Pea downy mildew, various damping off diseases	For seed treatment, phenylamide active ingredients should be mixed with an effective dose of a fungicide from a different cross-resistance group. Where a phenylamide seed treatment has been used, foliar treatments with phenylamide-containing products should be avoided.
Soil drench for various crops	Various damping off and root rot diseases	Phenylamides may be used alone as a soil drench in a single application, preferably in spring.

Implementation recommendations

Product labels for products that contain phenylamides should include an appropriate statement about resistance management.

Acknowledgements

This strategy includes information from The Fungicide Resistance Action Committee (FRAC) about mode of action groups, the occurrence of resistance worldwide and resistance management recommendations.

References

Falloon RE, Follas GB, Butler RC, Goulden DS 2000. Resistance in Peronospora viciae to phenylamide fungicides: reduced efficacy of seed treatments of pea (Pisum sativum) and assessment of alternatives. Crop Protection 19: 313-325.

Hartill WFT, Tompkins GR, Kleinsman PJ 1983. Development in New Zealand of resistance to dicarboximide fungicides in Botrytis cinerea, and to acylalanines in Phytophthora infestans, and to guazatine in Penicillium italicum. New Zealand Journal of Agricultural Research 26: 261-269.