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
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.