Methyl Benzimidazole Carbamate (MBC) [Group 1] management strategy

Resistance to benzimidazole fungicides is common
in many Botrytis cinerea populations affecting
outdoor and greenhouse crops.
(Photo: Dion Mundy)."

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 1 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.

Also see the separate crop-based strategy for use of MBCs against blossom blight in Summerfruit (here).

MBC product perspective

This fungicide use strategy is designed to manage the problem of resistance to Methyl Benzimidazole Carbamate (MBC) fungicides in New Zealand.

MBC fungicides include benomyl, carbendazim, fuberidazole, thiabendazole, thiophanate and thiophanate-methyl. They all generate methyl benzimidazole carbamate, either as the principal active ingredient, or as a breakdown compound formed on mixing with water. They have a common mode of action, interfering with cell division and hyphal growth of sensitive fungi. They are upwardly systemic with a broad range of activity against sensitive populations of ascomycetes, fungi imperfecti and basidiomycetes (Table 1).

MBC fungicides were first introduced into New Zealand in the late 1960s and they offered systemic, curative fungicidal activity on a number of problem diseases. Those currently available are shown in Table 2.

**Table 1:** Pathogens and crops targeted by benzimidazole fungicides in New Zealand.
Pathogen	Disease	Crops
Botrytis cinerea¹	Grey mould	Beans, berryfruit, grapes
Botrytis elliptica¹	Blossom rot	Tomatoes, onions
Botrytis spp.	Various	Ornamentals
Sphaerotheca fuligena¹	Powdery mildew	Cucurbits
Erysiphe cichoracearum¹	Powdery mildew	Cucurbits
Monilinia fructicola¹	Brown rot	Stonefruit
Monilinia laxa	Brown spot	Stonefruit
Mycosphaerella brassicicola¹	Ring spot	Brassicas
Mycosphaerella fragariae¹	Leaf spot	Strawberries
Penicillium digitatum¹	Green mould	Citrus
Penicillium italicum	Blue mould	Citrus
Pithomyces chartarum	Facial eczema	Pasture
Podosphaera leucotricha	Powdery mildew	Apples
Pseudocercosporella herpotrichoides¹	Eyespot	Cereals
Rhynchosporium secalis	Scald	Barley
Sclerotinia spp.	Sclerotinia	Beans, lettuce, tobacco, tomatoes, kiwifruit
Septoria tritici	Speckled leaf blotch	Wheat
Venturia inaequalis¹, Venturia pirina	Black spot	Apples, pears
Botrysphaeria parva	Fruit rot	Avocadoes
Microdochium nivale¹	Fusarium patch	Turf
Phomopsis sp.	Stem end rot	Avocadoes
Rhizoctonia spp.	Brown patch	Turf
Septoria apiicola¹	Celery leaf spot	Celery
Fusarium avenacerum¹	Carnation stub dieback	Carnations
Penicillium verrucosum¹	Garlic clove rot	Garlic

¹Pathogens for which resistant strains have been identified in New Zealand.

**Table 2:** MBCs marketed in New Zealand.
Common name	Trade name (partly out-dated information)
carbendazim	Bavistin DF in Chlorocarb with chlorothalonil Headland Addstem MBC 50 MBC 800 WDG Prolific Protek Spectrum
fuberidazole	in Baytan Universal with triadimenol and imazalil
thiophanate-methyl	in Combo with chlorothalonil in Greenguard with chlorothalonil in Guardall Liquid with chlorothalonil in Taratek with chlorothalonil Topsin M-4 A
thiabendazole	in Aliette Super with fosetyl aluminium and thiram Tecto SC

Current status of MBC resistance in New Zealand

Resistance to MBC fungicides is widespread among target pathogens (Table 1). Lack of disease control and yield loss are common when resistance occurs. Strains resistant to one MBC fungicide are cross-resistant to other fungicides in this group. Resistance occurs as a result of a single gene mutation. Resistant strains are genetically stable and usually of equal fitness to sensitive strains and they persist even after MBC use is discontinued. Resistance frequencies vary considerably, depending on the cropping history and amount of MBC use. Control failure and publicity about the potential for resistance to develop has reduced the use of MBC fungicides, but they are still effectively used in some situations. MBC resistance has not been found in New Zealand to some important pathogens, such as Pithomyces chartarum, Sclerotinia sclerotiorum, S. minor and Podosphaeria leucotricha.

Alternative fungicides are available for B. cinerea control but resistance has often also developed to these, particularly to dicarboximides (DCAs; Group 2). In Botrytis cinerea, dual resistance (separate resistance genes in individual isolates) to both MBCs and dicarboximides has been found in S. sclerotiorum affecting kiwifruit and M. fructicola affecting stone fruit. MBCs are no longer effective in controlling Venturia inaequalis in New Zealand because resistance is widespread.

Resistance management strategy

Avoid MBC use where resistant strains of the pathogen are known or are likely to occur. If resistance frequency is nil or low, use a maximum of two applications per crop per 12 month period (see Table 3 for exceptions), including crops where MBCs may be indicated for the control of two or more pathogens, e.g. Sclerotinia spp. and Botrytis spp. in vegetables. Confine use to periods when disease risk is high, but disease level is low. Where practical apply in a mixture with a protectant fungicide. Comply with the label application rates at all times. Practise good crop hygiene and cultural controls. Using dicarboximides and MBCs in the same spray programme may lead to development of pathogen strains with dual resistance and should be avoided.

**Table 3:** MBC use guidelines for specific crops where it is known resistance is not present.
Crop	Disease	Recommendations
Stonefruit	Brown rot	Maximum of one application per season. Confine use to the blossom period only.
Pipfruit	Fruit rots Botrytis dry eye rot	Maximum of one application per season. Confine use to the blossom period for fruit rot control and use a fungicide from another group for black spot control.
Grapes, strawberries, raspberries, brambles, tomatoes, and beans	Grey mould (botrytis)	Avoid use if resistance is known to be present, otherwise do not exceed one application per season.
Cucurbits	Powdery mildew	Use in mixture or alternation with alternative products.
Tomatoes, beans and lettuce	Sclerotinia	Limit use to early season (blossom to 14 days post-blossom on flowering crops).
Wheat	Eyespot	Apply only in second year crop. If control was below expectation use an alternative in the third year.
Wheat	Speckled leaf blotch	Comply strictly with recommended timing (last week of August).
Barley	Scald	Comply with label rates and timing.

Implementation recommendations

Product labels should include a statement about resistance risk and a recommendation about the maximum numbers of MBC fungicides that should be applied.

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.