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Guide to the main fungicide modes of action

Ornamentals Advisory Blog
25.09.2020

Here we outline the main elements of pathogen biology that different fungicides are designed to target. Within each broad mode-of-action category, fungicides are classified by a FRAC code according to their own chemistry, the specific biochemistry in the pathogen they affect, and their likely risk of pathogen resistance, which is why several fungicides with a shared chemistry may have the same FRAC code. All of those listed here include one or more products authorised in the UK but not all have products authorised for ornamentals.

Mode of action: Nucleic acids metabolism

FRAC codes 4, 8, 32

Several fungicides interfere with the way the molecules that genes are made of, the nucleic acids DNA and RNA, are created or copied in cells. As a result growth, reproduction and the pathogen’s ability to control key aspects of its biology are disrupted.

The target for these fungicides are four key enzymes involved in nucleic acid metabolism.

Most are at a medium to high risk for resistance, and for cross-resistance between those acting on the same enzyme.

Mode of action: Cytoskeleton and motor protein

FRAC codes 1, 20, 22, 43, 50

Fungal cells contain a microscopic network of protein filaments, the cytoskeleton. Its functions include control of the shape of cells and support for the chromosomes during cell division.

Cell division involves motor proteins as well, which are also needed for the transport of other biochemicals around and between cells.

The roles these systems play in cell division, and hence growth, make them good candidates as fungicide targets. Several cases of resistance to some of these actives are known, in some powdery and downy mildews for example, and all are at medium to high risk of resistance says FRAC.

Mode of action: Respiration

FRAC codes 7, 11, 21, 29, 38, 39, 45

A range of fungicides target different aspects of the biochemistry involved in converting nutrients into the energy needed for a pathogen to grow. They are grouped according to their target sites on various enzymes (some of which are highly complex with more than one target site) involved in these processes.

They include some actives you may be familiar with including SDHI fungicides, such as boscalid, and strobilurins, such as azoxystrobin in Amistar.

Some resistances and cross-resistances are already known and most of the target sites carry a medium to high risk of resistance developing.

Mode of action: Amino acids and protein synthesis

FRAC code 9

This grouping includes fungicides that disrupt a pathogen’s ability to make enzymes and other proteins, leading to any of a number of key physiological processes failing.

Only one, however, of the five target sites, code 9, includes crop protection fungicides approved in the UK but it’s of particular interest to Syngenta because it includes cyprodinil, one of the active substances in Switch (the other is fludioxonil, which has a completely different mode of action, against spore germination). Cyprodinil inhibits synthesis of methionine, a key component of the enzymes secreted by fungal pathogens to digest plant tissues.

FRAC says there is a risk of resistance developing to fungicides in this group. For that reason, for example, we advise using Switch as a protectant or in the early stages of disease development. A maximum of three treatments is permitted per year on ornamentals and these should be alternated with products from different mode-of-action groupings.

Mode of action: Signal transduction

FRAC codes 12, 13

Fungicides in this group interfere with the mechanisms by which fungal cells respond to environmental changes such as temperature and, particularly, moisture. They are classified according to which of three different targets they work on (one of them includes no UK-authorised products).

The fludioxonil component of Switch is one of these (code 12). It inhibits spore germination and growth of the germ tube that penetrates the plant tissue – a process normally initiated when the spore responds to a specific combination of moisture and temperature.

Mode of action: Cell membrane targets

FRAC codes 28, 46, 49

Largely composed of lipid and sterol molecules, the cell membrane protects the cells’ contents and all the complex biochemistry within. Treatment with fungicides that can disrupt it results in cells drying out and dying.

The targets are either the biochemistry involved with synthesis of the lipid components of membranes or the integrity of existing membranes. The group includes carbamate fungicides and some biofungicides based on defensive compounds extracted from plants.

Mode of action: Sterol biosynthesis in membranes

FRAC codes 3, 5, 17

These fungicides work on the sterol components of membranes. These include the widely-used DMI fungicides, such as the triazoles propiconazole and tebuconazole, for example, (code 3). They have a medium risk of resistance and FRAC considers it wise to accept that cross-resistance will also be present. Codes 5 and 7 include fungicides with a low to medium risk of resistance.

Mode of action: Cell wall

FRAC codes 40

The protective outer layer of a fungal pathogen’s cells, the cell wall, is made of chitin, the same material that insect exoskeletons are formed from, whereas the cell wall of an oomycete pathogen, such as Pythium or Phytophthora, is constructed with cellulose, the same as in plants. Fungicides with the FRAC code 40 (such as mandipropamid in Syngenta’s Revus) target the cellulose-producing enzyme in oomycetes. Others, coded 19, target the fungal enzyme involved in chitin synthesis but none of these are currently approved for use in the UK.

Some fungal cell walls contain the pigment melanin which not only helps protect from stresses such as exposure to UV light but in some pathogens, such as black spot in roses, plays a role in strengthening cell walls of the fungal structures that penetrate the host’s tissues; taking out the melanin can reduce the virulence of these pathogens. No fungicides with this mode of action are approved for use in the UK, however.

Mode of action: Host plant defence induction

FRAC codes P04, P06, P07

Likely to play an increasingly important role in disease control and resistance management, these active substances don’t target the pathogen itself but stimulate the plant’s disease defence mechanisms.

They include plant extracts and microbe-based products, to which resistance is low risk or not known.

Mode of action: Multi-site activity

FRAC codes M01-04

Because fungicides in this group each act on several target sites they’re considered to be at low risk of triggering resistance in pathogens. FRAC says no signs of resistance have been found as yet even though some, such as copper and sulphur, are among the oldest materials used against plant diseases. The group includes other familiar fungicides such as the dithiocarbamates (eg mancozeb) and triazines.

Mode of action: Biologicals with multiple modes of action

FRAC code BM02

There’s an increasing range of biological products with fungicidal activity based on beneficial fungi, bacteria and plant extracts. FRAC says no resistances are known for any in this group.

They include those based on bacteria such as Bacillus subtilis and B. amyloliquefaciens and fungi such as Trichoderma asperellum and Gliocladium catenulatum, which have more than one way of working. Effects can include parasitism, membrane disruption or simply out-competing the pathogen.

Mode of action: Unknown

FRAC codes 27, U06, U12

These are active substances for which the exact target and mode of action hasn’t been confirmed. In some cases the resistance risks, too, remain unknown. For others, resistances have been reported, or FRAC considers them to be at some risk.

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