Qo inhibitor (QoI) management strategy

Black spot on apple leaves.
Black spot Venturia inaequalis on apple leaves

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

(Revised August 2005)

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.


These guidelines are designed to manage or prevent the problem of resistance to QoI (also known as strobilurin or STAR) fungicides in New Zealand. They have been developed from previously published information (Beresford et al. 1999) in consultation with the New Zealand Association for Animal Health and Crop Protection (AGCARM) and are based on recommendations from the Fungicide Resistance Action Committee (FRAC) in Europe.

QoI Product Perspective

QoI fungicides are synthetic derivatives of naturally occurring strobilurin compounds and are active against a broad spectrum of fungi (Table 1). They have a site-specific mode of action that inhibits fungal respiration at the Quinone "outside" (Qo) binding site of the cytochrome bc1 complex. The site-specific mode of action means they are at risk from the development of resistance in the target plant pathogens. QoIs are not cross-resistant with other fungicide groups, but there is cross-resistance among all the active ingredients within the QoI group. QoIs were first introduced into New Zealand in the late 1990s. Of the active ingredients so far developed (azoxystrobin, dimoxystrobin, famoxadone, fenamidone, fluoxastrobin, kresoxim-methyl, metominostrobin, orysastrobin, picoxystrobin, pyraclostrobin and trifloxystrobin), five are commercially available in New Zealand (Table 2). The range of pathogens against which the various QoI compounds are effective differs with the compound.

Table 1: Pathogens and crops targeted by QoI fungicides in New Zealand.
Horticultural crops
Alternaria solani Early blight Potatoes, tomatoes
Botrytis cinerea Grey mould Grapes
Erysiphe cichoracearum, Sphaerotheca fuliginea Powdery mildew Cucurbits
Peronospora destructor Downy mildew Onions
Phytophthora infestans Late blight Potatoes, tomatoes
Plasmopara viticola Downy mildew Grapes
Podosphaera leucotricha Powdery mildew Apples
Uncinula necator Powdery mildew Grapes
Venturia inaequalis, V. pirina Black spot Apples, pears
Cereal crops
Blumeria graminis Powdery mildew Cereals
Cochliobolus sativus Spot blotch Barley
Didymella phleina Didymella leaf scorch Barley, wheat
Puccinia spp. Rusts Cereals, ryegrass
Pyrenophora teres Net blotch Barley
Species of Alternaria, Cladosporium, Stemphylium and Fusarium Head disease complex Cereals
Ramularia collo-cygni Ramularia leaf and awn spot Barley
Rhynchosporium secalis Scald Barley
Septoria nodorum, S. tritici Glume blotch, speckled leaf blotch Wheat
Table 2: QoI fungicides marketed in New Zealand.
Common nameTrade names
azoxystrobin Amistar, Atlantis Flo, Inspire, Miridor, Salute, Tazer
fenamidone Reason
fluoxastrobin in Fandango with prothioconazole
kresoxim-methyl Ardent, Vista Flo,
in Collis with boscalid
picoxystrobin Acanto


in Pristine with boscalid

trifloxystrobin Flint, Twist, Protiva

Current status of QoI resistance

Instances of QoI resistance have been confirmed in New Zealand

Overseas, several instances of reduced sensitivity or field resistance have been recorded:

For these diseases FRAC considers it mandatory to use QoI fungicides in mixture with a fungicide from a different cross resistance group.

Resistance prevention strategy

Apply a maximum of three QoI fungicide-containing sprays per season or per 12 month period, unless otherwise indicated for specific crops (Table 3). Whenever practical, use in mixture with effective partners from different cross-resistance groups. Apply QoI fungicides according to manufacturers' recommendations for the target disease and crop. Effective disease management is critical in delaying the build-up of resistant pathogen populations. Different QoI active ingredients applied in one season count towards the maximum numbers of applications recommended for a given crop. Use application rates recommended by the manufacturer in order to ensure good disease control and resistance management. The FRAC QoI working group is concerned with the trend towards the application of decreased dose rates. Apply the QoI fungicide preventively or as early as possible in the disease cycle. Do not rely only on the curative potential of QoI fungicides.

Table 3: QoI use guidelines for specific crops.
Grapes Powdery mildew, downy mildew A maximum of three QoI-containing applications per vine crop, in single or block application in alternation with fungicides from a different group. Use early in the season before powdery mildew is well established.
Apples Black spot, powdery mildew Apply QoI fungicides in mixture with an effective dose of a fungicide from a different cross-resistance group.
Use a maximum of three QoI-containing applications per season. Where more than 12 fungicide applications are made in total per season, apply a maximum of four QoI fungicides.
Apply a maximum of two consecutive QoI-containing applications. Where there may be a loss of field performance, apply QoI-containing fungicides in strict alternation with an effective non-QoI fungicide.
Apply QoI fungicides preventatively.
Follow individual product label recommendations regarding shorter application intervals during periods of high disease risk.
Cereals Powdery mildew, speckled leaf blotch, glume blotch, rusts, net blotch, scald, spot blotch, Ramularia leaf and awn spot, Didymella leaf scorch Apply a maximum of two QoI fungicide-containing sprays per cereal crop. Limiting the number of sprays is an important factor in delaying the build-up of resistant pathogen populations.
Apply QoI fungicides in mixtures to control cereal pathogens. At the rate chosen each mixing partner on its own has to provide effective disease control. Refer to manufacturers recommendations for rates.
Repeated applications using "split" or reduced application rates must not be used, as they provide continuous selection for resistance.
Other fruit and vegetable crops Apply a maximum of three QoI fungicide-containing sprays per crop. A maximum of four QoI fungicide applications may be used where 12 or more applications are made per crop.
Apply in mixture with an effective dose of a fungicide from a different group.
A maximum of two consecutive QoI fungicide sprays is preferred.

Implementation recommendations

The rapid increase in the number of label claims for QoI use in New Zealand, together with the known high resistance risk, dictates that QoI resistance management guidelines must be strictly adhered to. Product labels for products that contain QoIs should include an appropriate statement about resistance management.


This resistance management strategy has been compiled with assistance from Jack Richardson (AGCARM), George Follas (Syngenta Crop Protection), Grant Hagerty (BASF New Zealand), Max Moore (Bayer CropScience), Ken Jeffery (Fruitfed Supplies Ltd) and Nick Pyke (Foundation for Arable Research).


Beresford R, Pak H, Brown G, Follas G, Hagerty G. 1999. Strategies to avoid resistance development to stobilurin and related fungicides in New Zealand. Proceedings of the 52nd New Zealand Plant Protection Conference: 179-181.