Benzimidazole management strategy

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

R.M. Beresford
Plant & Food Research, Private Bag 92169, Auckland 1142, New Zealand

(Revised February 2004)

A new fungicide resistance management strategy for apple black spot (Venturia inaequalis) has been put in place and supersedes the individual Mode of Action strategies shown elsewhere on this site for apple black spot in pipfruit.

The new strategy is available here.

Introduction

This fungicide use strategy is designed to manage the problem of resistance to benzimidazole (also known as MBC) fungicides in New Zealand. It has been developed from previously published information (Tate et al. 1996) in consultation with the New Zealand agricultural chemical industry (Agcarm Inc.). It incorporates recommendations from the Fungicide Resistance Action Committee (FRAC) in Europe.

Benzimidazole product perspective

Benzimidazole fungicides include benomyl, carbendazim, fuberidazole, thiabendazole, thiophanate and thiophanate-methyl. They all generate MBC (methyl benzimidazol 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 ascomycetes, fungi imperfecti and basidiomycetes (Table 1).

Benzimidazole 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

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

**Table 2:** Benzimidazoles marketed in New Zealand.
Common name	Trade name
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 benzimidazole resistance in New Zealand

Resistance to benzimidazole fungicides is widespread among target pathogens (Table 1). Lack of disease control and yield loss are common when resistance occurs. Strains resistant to one benzimidazole 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 benzimidazole use is discontinued. Resistance frequencies vary considerably, depending on the cropping history and amount of benzimidazole use. Control failure and publicity about the potential for resistance to develop has reduced the use of benzimidazole fungicides, but they are still effectively used in some situations. Benzimidazole resistance has not been found in New Zealand or elsewhere to some important pathogens, such as Pithomyces chartarum, Sclerotinia sclerotiorum, S. minor and Podosphaeria leucotricha.

Alternative fungicides are available and these have sometimes become predominant where benzimidazoles were used intensively in the past. Where resistance has developed or threatens to develop against some of the alternative products, particularly dicarboximides, benzimidazoles can be reconsidered for tactical use, particularly where benzimidazole resistant strains are not common. Dual resistance in Botrytis cinerea to both benzimidazoles and dicarboximides has been observed where benomyl has been used on kiwifruit against S. sclerotiorum. The use of benzimidazoles during bloom against S. sclerotiorum has reduced effectiveness of dicarboximides against B. cinerea preharvest. Dual resistance also occurs for benzimidazoles and dicarboximides in M. fructicola in stonefruit.

Resistance management strategy

Avoid benzimidazole 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 benzimidazoles 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 benzimidazoles in the same spray programme may lead to development of pathogen strains with dual resistance and should be avoided.

**Table 3:** Benzimidazole use guidelines for specific crops.
Crop	Disease	Recommendations
Stonefruit	Brown rot	Confine use to the blossom period only.
Pipfruit	Fruit rots Botrytis dry eye rot	Confine use to the blossom period and use a fungicide from another group for black spot control. Apply only in mixture with a protectant fungicide e.g. thiram, captan, mancozeb or metiram.
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 benzimidazole fungicides that should be applied.

Acknowledgements

This resistance management strategy has been compiled with assistance from Jack Richardson (Agcarm Inc.) and Matthew Cromey (Crop & Food Research).

Reference

Tate G, Fullerton R, Cornwall M 1996. Benzimidazole resistance management strategy. In: Boudot GW, Suckling DM ed. Pesticide resistance prevention and management. New Zealand Plant Protection Society, Rotorua, New Zealand. Pp. 146-150.