Understanding Fungicide Modes of Action

Dr Susan KnightSyngenta Asia Pacific Pte Ltd

1

2

Refresher: how do fungicides work?

● Fungicides work by inhibiting

an essential activity in the

pathogen, e.g.

– Respiration

– Cell wall development

– Cell division

● The target site is the specific

biochemical process (enzyme)

affected by the fungicides

‒ Fungicides which share a

target site have the same

FRAC code

Diagram: SYNGENTA

3

De-mystifying fungicide terminology......

UPTAKE

IN PLANT

HOW THE

FUNGICIDE

WORKS

SINGLE OR

MULTIPLE

MODES OF

ACTION

MODE OF

ACTIONTARGET SITE

TARGET

SITE

GROUP

NAME

ACTIVE

INGREDIENT

(EXAMPLE)

FRAC

CODE

contact preventativemultiple

sitesmulti-site

multiple

enzymes

dithio-

carbamatesmancozeb M03

contact preventative single site

uncouplers of

oxidative

phosphorylation

uncouplers of

oxidative

phosphorylation

2,6-dinitro-

anilinesfluazinam 29

systemic/

penetrant

preventative

and curativesingle site respiration

ubiquinol

oxidaseQoI azoxystrobin 7

succinate

dehydrogenaseSDHI fluopyram 11

4

What is resistance and how does it develop?

‘An acquired, heritable reduction in sensitivity of a fungus to a fungicide’

1. Rare individuals may be present in the fungal population, which have:

● Point mutations in the target site gene, or duplication of the target site gene, which

reduce effectiveness of the fungicide

● Mutations in other genes that prevent the fungicide from reaching the target site, e.g. by

metabolism, reduced uptake or efflux of the fungicide (non-target site resistance)

2. The resistant population increases if there is overuse & incorrect use of the

fungicide

Example: point

mutation in the cytb

gene gives

resistance to QoI

fungicides

5

Fungicide Resistance Action Committee (FRAC)

● The purpose of FRAC is ‘to provide

fungicide resistance management

guidelines to prolong the effectiveness

of "at risk" fungicides and to limit crop

losses should resistance occur’

‒ Resistance management guidelines

are published for each mode of action

● The "FRAC Poster" and "MoA Code

List” classify active ingredients

according to mode of action (MoA)

‒ Based on scientific evidence

‒ Provides information on cross

resistance pattern between different

fungicides

https://www.frac.info/

6

FRAC mode of action poster 2020 (www.frac.info)

>230 fungicides, 65 MOA groups

7

High (e.g. QoI, phenylamide)

High

Medium

Low

Medium (e.g. DMI)

High

Medium

Low

Low (e.g. multisites)

High

Medium

Low

Low (e.g. Rhizoctonia spp.; soilborne fungi

(e.g. Phytophthora spp.)

Medium (e.g. Alternaria solani)

High (e.g. Mycosphaerella

fijiensis)

Agronomic

risk

Fungicide

risk

FRAC resistance risk matrix

Pathogen

risk

8

How is fungicide resistance managed?

● Avoid over-use of each mode of action

- Do not exceed recommended number of

applications

- Rotate with other effective modes of action

- Mix with other modes of action (premix, tank mix)

● Use diverse disease management practices to

reduce inoculum & slow disease development

- Remove or destroy plant debris & weed hosts

- Crop rotation

- Tolerant varieties

- Adjust sowing date, crop density, irrigation type,

etc.

● Know the mode of action!

● Follow resistance management guidelines for that mode of action!

Photo: APS Net/ Mauricio Guzman

9

The main fungicide mode-of-action groups used in Philippines

10

Fungicides that prevent energy production

Diagram: Michael W. Davidson and The Florida State University

● Enzyme complexes

are bound to the

inner membrane

– Oxidative

phosphorylation

takes place in these

complexes

Diagram: Joseph-Horne et al, Biochimica et Biophysica Acta 1504 (2001) 179^195www.bba-direct.com

● Mitochondria are the

centres of energy

production:

– They have an inner

and outer

membrane

11

Fungicides that prevent energy production

● Respiration inhibitors disrupt the energy cycle and stop ATP production

Mode of

actionTarget site Group name

Examples of active

ingredients

respiration

succinate

dehydrogenase

SDHIboscalid, fluopyram,

fluxapyroxad, isopyrazam

ubiquinol oxidase

QoI azoxystrobin, famoxadone,

fluoxastrobin, picoxystrobin,

pyraclostrobin, trifloxystrobin,

metominostrobin

uncouplers of oxidative

phosphorylation

-

fluazinam

ubiquinone reductase

at Qo site, stigmatellin

binding sub-site

QoSI

ametoctradin

● These fungicides stop energy-demanding processes such as spore germination

‒ Most effective if used preventatively (or at the early stages of fungal development)

GROUP 7 FUNGICIDE

GROUP 11 FUNGICIDE

GROUP 29 FUNGICIDE

GROUP 45 FUNGICIDE

12

● QoI fungicides bind to the ‘quinone outer’ (Qo) binding site of the cytochrome bc1 complex (complex III)

‒ They are known as ‘Quinone outer Inhibitor fungicides’ (QoI)

QoI fungicides

GROUP 11 FUNGICIDE

Diagram: Joseph-Horne et al,

Biochimica et Biophysica Acta

1504 (2001) 179^195www.bba-

direct.com

13

High (e.g. QoI, phenylamide)

High Medium

Low

Medium (e.g. DMI)

High Medium

Low

Low (e.g. multisites)

High

Medium Low

Low (e.g. Rhizoctonia

spp.; soilborne fungi (e.g.

Phytophthora spp.)

Medium (e.g. Alternaria

solani)

High (e.g. Mycosphaerella

fijiensis)

● Resistance has been reported from several pathogens, and three target site mutations have been identified (G143A, G137R, F129L)

QoI fungicides

FRAC fungicide resistance risk classification is ‘high’

14

● SDHI fungicides bind to complex II

– The target enzyme is succinate dehydrogenase ( ‘Succinate DeHydrogenase Inhibitors’ (SDHI))

● Resistance has been reported in several pathogens

‒ A number of target site mutations have been identified

SDHI fungicides

GROUP 7 FUNGICIDE

Diagram: Joseph-Horne et al,

Biochimica et Biophysica Acta

1504 (2001) 179^195www.bba-

direct.com

15

Fungicides that target fungal membranes: SBI

● Ergosterol (the main sterol in

fungal membranes) is a target

for many fungicides

‒ SBI fungicides do not inhibit

spore germination (because

fungal spores contain

reserves of ergosterol)

‒ Oomycetes (not true fungi)

do not have ergosterol in

their membranes – SBI

fungicides have no effect!Diagram: Wikimedia

16

SBI Fungicides

● DMIs and amines inhibit

different steps in sterol

biosynthesis

DMI

Amines

Diagram: Chiatogu Onyewu et al. Antimicrob. Agents Chemother. 2003; doi:10.1128/AAC.47.3.956-964.2003

17

SBI Fungicides

Mode of

actionTarget site Group name

Chemical

classExamples of active ingredients

Sterol

biosynthesis

C14

demethylase

SBI Class I: DMI-

fungicides

piperazine triforine

imidazole imazalil, prochloraz

triazole

difenoconazole, epoxiconazole,

fenbuconazole, flutriafol,

hexaconazole, propiconazole,

tebuconazole, triadimenol

D14-

reductase

and D8→D7-

isomerase

SBI Class II: Amines

morpholines fenpropimorph, tridemorph

GROUP 3 FUNGICIDE

GROUP 5 FUNGICIDE

● Resistance to DMI fungicides has been reported in several pathogens

‒ Target site mutations (point mutations or overexpression)

‒ Non-target site mutations (e.g. overexpression of ABC transporter genes, causing increased fungicide efflux)

‒ Efficacy reductions are gradual and stepwise (multiple resistance mechanisms); there may be stabilization or recovery of sensitivity if selection pressure is reduced

FRAC resistance risk ‘medium to high’

FRAC resistance risk ‘low to medium’

18

Fungicides that target nucleic acid synthesis: Phenylamides

● Phenylamide fungicides inhibit ribosomal RNA

(rRNA) biosynthesis

– The target site is RNA polymerase I

● Active ingredients: mefenoxam, metalaxyl

● FRAC resistance risk classification is ‘high’

– Foliar sprays must only be made in mixture

● Resistance has been confirmed in several

pathogens, including late blight of tomato and

potato

‒ There is limited data on the mechanism of

resistance (therefore bioassay is used to detect

resistance)Diagram: Wikimedia Commons

GROUP 4 FUNGICIDE

19

Fungicides that target cell wall biosynthesis:

CAA fungicides

● All CAA-fungicides are assumed to be cross resistant

● FRAC resistance risk classification is “low to medium”

‒ Resistance has never been reported in Phytophthora

infestans

‒ Resistance has been reported in downy mildews (e.g.

Pseudoperonospora cubensis); target site mutation)

Target siteTarget

siteGroup name Chemical class

Examples of

active

ingredients

cell wall

biosynthesis

cellulose

synthase

CAA-fungicides (Carboxylic

Acid Amides)cinnamic acid amides dimethomorph

mandelic acid amides mandipropamidGROUP 40 FUNGICIDE

20

Fungicides that target cell division

● Spindles help to align the chromosomes

during cell division

– They are composed of microtubules

containing the protein tubulin

● The MBC & thiazole carboxamide

fungicides bind to the ß-tubulin molecule &

inhibit microtubule assemblyDiagram: Dept. Biol Penn State

Mode of

actionTarget site Group name

Examples of active

ingredients

FRAC

Risk

Cytoskeleton

and motor

protein

ß-tubulin

assembly in

mitosis

methyl benzimidazole carbamate (MBC)

carbendazim High

ß-tubulin

assembly in

mitosis

thiazole carboxamide

ethaboxamLow to

medium

GROUP 1 FUNGICIDE

GROUP 22 FUNGICIDE

Resistance to MBC fungicides has been reported in many pathogens (based on

several target site mutations)

21

Fungicides that target signal transduction

● Resistance to the dicarboximides is common in Botrytis and some other pathogens

‒ Several target site mutations have been identified

Mode of

actionTarget site Group name

Examples of

active

ingredients

FRAC Risk

Signal

transduction

MAP/ HistidineKinase in

osmotic signal transduction

dicarboximides

iprodione Medium to high

MAP/ HistidineKinase in

osmotic signal transduction

phenylpyrroles

fludioxonil Low to medium

GROUP 2 FUNGICIDE

GROUP 12 FUNGICIDE

22

Mode of

actionTarget site Group name Active ingredients FRAC Risk

amino acid

and protein

synthesis

methionine

biosynthesis

(proposed)

anilinopyrimidines pyrimethanil

Medium

Unknown Unknown

cyanoacetamide-oxime cymoxanil

Low to

medium

Lipid synthesis

or transport /

membrane

integrity or

function

cell membrane

permeability,

fatty acids

(proposed)

carbamates propamocarb

hydrochlorideLow to

medium

Unknown

cell membrane

disruption

(proposed)

guanidines dodine

Low to

medium

Other fungicide groups

GROUP 27 FUNGICIDE

GROUP 28 FUNGICIDE

GROUP 9 FUNGICIDE

GROUP U12 FUNGICIDE

23

Multisite fungicides

● Inhibit sulphur-containing enzymes, disrupting fungal energy production

● FRAC resistance risk classification is “low” (no reports of resistance in fungi)

● Potentially useful mixture or alternation partners for managing resistance

– Must be effective against the target pathogen!

– Must be used correctly (preventatively; protect new growth)

– No limits on application for resistance management (but labels MUST be followed)

Target site Group nameActive

ingredient

Multi site

inorganic (electrophiles) copper

inorganic (electrophiles) sulphur

dithiocarbamates and relatives

(electrophiles)mancozeb

Phthalimides (electrophiles) captan

chloronitriles (phthalonitriles)

(unspecified mechanism)chlorothalonil

GROUP M01 FUNGICIDE

GROUP M03 FUNGICIDE

GROUP M04 FUNGICIDE

GROUP M05 FUNGICIDE

GROUP M02 FUNGICIDE

24

Mode of action Target site Group nameActive

ingredientsFRAC Risk

Plant defence

inductionphosphonate

Phosphonate

fosetyl-Al Low

Biologicals with

multiple modes of

action

multiple

microbial (living microbes or extract,

metabolites) Bacillus spp. Resistance

not known

Lipid synthesis or

transport / membrane

integrity or function

cell membrane

disruption

plant extract Melaleuca

alternifolia (tea

tree)

Resistance

not known

Plant defence inducers, biocontrol agents and plant extracts

GROUP P06 FUNGICIDE

● Potentially useful alternation partners for managing resistance

– Must be effective against the target pathogen!

– Must be used correctly (follow the label)

– No limits on application for resistance management (but labels MUST be followed)

GROUP 44 FUNGICIDE

GROUP 46 FUNGICIDE

25

Maraming salamat/ Thank you for your attention

Ingat!

Stay safe and well!