SBI Fungicides
Membership

The Working Group is comprised of the following members:

Klaus Stenzel (Chairman) Bayer CropScience, Monheim , Germany
Frank Goehlich

Bayer CropScience, Monheim, Germany

Andreas Mehl Bayer CropScience, Monheim, Germany
Martin Semar BASF SE, Limburgerhof, Germany
Gerd Stammler BASF SE, Limburgerhof, Germany
Nilton Picinato DuPont de Nemours, Brazil
Greg Kemmitt Dow AgroSciences, Abingdon, UK
Martin Huttenlocher Makhteshim-Agan, Köln, Germany
Rei Matsunaga Sumitomo Chemical Co., Ltd , Japan
Andy Leadbeater Syngenta Crop Protection, Basel, Switzerland
Elizabeth Shaw Syngenta Crop Protection, Basel, Switzerland
Helge Sierotzki Syngenta Crop Protection, Basel, Switzerland

 
Introduction and General Information about Sterol Biosynthesis Inhibitors (SBI's)

The FRAC-SBI (formerly DMI) working group was set up in 1982. The group meets annually to review monitoring data and to agree recommendations for the use of SBI fungicides.

Definitions - SBI-Fungicides

There are four classes of fungicides that comprise the Sterol Biosynthesis Inhibitor's (abbreviated SBI's) of which three classes (G1 to G3) are used as agricultural fungicides: DMI-fungicides, Amines (before called “Morpholine”- fungicides) and the Keto-Reductase Inhibitors (KRIs). All classes inhibit targets within fungal sterol biosynthesis but differ in regard to the precise targets they inhibit.

The SBI based fungicides represent an important class of agricultural fungicides. They make a major contribution to the world’s agricultural production.

Overview on Sterol Biosynthesis Inhibitors

Class I: DMI-Fungicides

SBI-fungicides that inhibit the C14 demethylation step within fungal sterol biosynthesis are commonly characterized as DeMethylation-Inhibitors (Abbreviation: DMI’s).
Chemically, DMI’s belong to different classes. Beside triazoles, numerous imidazoles, pyridines and pyrimidine have been shown to act as demethylation inhibitors.
Typically, DMI's have a broad spectrum of activity against a range of economically important pathogens on arable crops, top fruit, industrial crops, vines, plantation crops, etc.

Class II: Amines

Like the DMI’s the Amines also belong to different chemical classes. The first representatives of this group were chemically morpholines. Although representatives of two other chemical groups (piperidines and spiroketalamines) have entered the market, the group designation ‘morpholines’ is still partly used for all three chemical classes. Amines inhibit to a variable degree two target sites within the sterol biosynthetic pathway, the Δ8 → Δ7 isomerase and the Δ14 reductase.
Amines have a more narrow spectrum of activity in comparison to the DMI’s. They can be used as a solo treatment, but are often used in mixtures with DMI's to control powdery mildews and rusts.

Class III: Hydroxyanilide - Fungicides

This class is actually represented by two agricultural compounds, belonging to the hydroxyanilides and the amino-pyrazolinones. Both molecules inhibit the C3-keto-reductase step in ergosterol biosynthesis Keto-reductase inhibitors (KRIs). KRIs have a narrower spectrum of activity than the DMI’s and Amines – the KRIs are specific botryticides, which does neither show cross-resistance to other classes of anti-Botrytis fungicides nor to other SBI fungicide classes.

Resistance to fungicides

Resistance to fungicides is a normal phenomenon embodied in the natural process of the evolution of biological systems. By close co-operation within the agrochemical industry and collaboration with researchers, advisors and with growers we can ensure that fungicides are used optimally and continue to offer the benefits they currently confer.

Resistance to SBI fungicides

Resistance to SBI fungicides has been well characterized during the last 20 years. Issues with SBI performance typically became obvious only after numerous years of intensive use with efficacy degrading stepwise. Following reduced selection pressure, a partial recovery in sensitivity is often observed.
The mechanism of resistance is mostly controlled by the accumulation of several independent mutations and is generally referred to as “continuous selection”, “quantitative resistance” or “shifting”.


General scheme of a shifting type resistance


Resistance Type is designated as “continuous selection” or “shifting”.

This is based on the observation that resistance to DMIs or Amines is mostly characterized by a slow, stepwise erosion of efficacy over several years of intensive use rather than by a rapid loss of control.

Genetic Basis of Resistance:

Accumulation of several mutations is needed to lower the sensitivity of pathogens to DMIs or Amines ( → polygenic resistance).

Resistance risk is generally considered to be

•  low to medium (amines) or
•  medium (DMIs)

Resistance development is typically correlated with a fitness penalty for less sensitive isolates. Partial back-shift possible, if selection pressure decreases.


Cross Resistance among SBI - fungicides

Whilst there is positive cross-resistance amongst the DMIs, the Amines and amongst the KRIs, there is no cross-resistance between the DMIs, Amines and the KRIs.

GENERAL RECOMMENDATIONS FOR THE USE OF SBI FUNGICIDES

The SBI fungicides represent one of the most potent classes of fungicides available to the grower for the control of many economically important pathogens. It is in the best interest of all those involved in recommending and using these fungicides that they are utilised in such a way that their effectiveness is maintained

The summaries and recommendations included in this report are based upon data generated by members of the FRAC-SBI Working Group and upon the work of non-industry collaborators. The working group concentrates its resources on the major crop/pathogen targets from the point of view of resistance risk. Inevitably many, still important, pathogens are omitted. To help in making recommendations for crops and pathogens not directly covered above, the following general recommendations can be made:

Repeated application of SBI fungicides alone should not be used on the same crop in one season against a high-risk pathogen in areas of high disease pressure for that particular pathogen.

For crop/pathogen situations where repeated spray applications (e.g. orchard crops/powdery mildew) are made during the season, alternation (block sprays or in sequence) or mixtures with an effective non cross-resistant fungicide are recommended (see FRAC fungicide group list on the FRAC website).

Where alternation or the use of mixtures is not feasible because of lack of effective or compatible non cross-resistant partner fungicides, then input of SBI's should be reserved for critical parts of the season or crop growth stage.

If SBI fungicides performance should decline and sensitivity testing has confirmed the presence of less sensitive forms, SBI's should only be used in mixture or alternation with effective non cross-resistant partner fungicides.

The introduction of new classes of chemistry offers opportunities for more effective resistance management. The use of different modes of action should be maximised for the most effective resistance management strategies.

Users must adhere to the manufacturers’ recommendations. In many cases, reports of “resistance" have, on investigation, been attributed to cutting recommended rates of use, or to poor or miss-timed application.

Fungicide input is only one aspect of crop management. Fungicide use does not replace the need for resistant crop varieties, good agronomic practice, plant hygiene/sanitation, etc.

Exclusive frequency measurements of single cyp51 mutations are not sufficient to describe the sensitivity situation towards DMIs but can help to better understand the background of sensitivity shifts.




Monitoring Methods
 
Contact



Dr. Klaus Stenzel
Chairman

Bayer CropScience AG
Alfred-Nobel-Str. 50, Building 6240
D-40789 Monheim
Tel: +49-2173-383652
Fax: +49-2173-384869

Email: Dr. Klaus Stenzel

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