Definitions - SBI-Fungicides

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

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

Overview on Sterol Biosynthesis Inhibitors

Class I: DMI-Fungicides

SBI-fungicides that inhibit the C14 demethylation step within fungal sterol biosynthesis are now commonly characterised as DeMethylation-Inhibitors (Abbreviation: DMI's).
Chemically, DMI's belong to different classes. Beside triazoles, numerous imidazoles, pyridines and pyrimidine all 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, 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 partly still used for all three chemical classes. Amines inhibit to a variable degree two target sites within the sterol biosynthetic pathway, the ? 8 ? ? 7 isomerase and the ? 14 reductase.

Amines have a narrower spectrum of activity than the DMI's. They can be used alone but are often used in mixtures with DMI's to control powdery mildews and rusts.

Class III: Hydroxyanilide - Fungicides

Currently this class is represented by fenhexamid. Hydroxyanilides inhibit the C3-keto-reductase step in ergosterol biosynthesis. Hydroxyanilides have a narrower spectrum of activity than the DMI's and Amines – fenhexamid is a specific botryticide, which does not show cross-resistance to other classes of anti-Botrytis fungicides.

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. Problems with SBI performance typically became obvious only after several 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 ( à 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