Crop Protection Modes of Action Explained
Crop protection products control weeds, diseases and insect pests in different ways. The biological process a product affects is known as its mode of action.
Understanding mode of action helps growers and agronomists select appropriate products, build effective programmes and manage the risk of resistance developing over time.
It is particularly important where several products may appear to offer similar control but act on the target organism through the same biological pathway.
What does mode of action mean in crop protection?
Mode of action describes the biological process or target site through which a crop protection active ingredient affects a weed, pathogen or insect pest.
A herbicide, for example, may interfere with an enzyme required for plant growth, disrupt photosynthesis or affect cell division. A fungicide may inhibit fungal respiration or cell membrane production, while an insecticide may act on the insect nervous system, growth or energy production.
Products with similar modes of action are grouped together within recognised classification systems. These groups provide a practical way to understand which active ingredients work through similar biological processes.
Mode of action is only one consideration when choosing a crop protection product. Target species, crop safety, growth stage, application timing, resistance status and field conditions also need to be considered.
Herbicide modes of action
Herbicides act on specific processes within susceptible plants.
These processes can include photosynthesis, amino acid production, lipid synthesis, cell division and plant hormone regulation.
The symptoms seen after treatment vary depending on the mode of action. Some herbicides produce rapid visible damage, while others work more slowly as growth processes are disrupted.
Herbicide mode of action groups
Herbicide active ingredients are classified into mode of action groups according to the biological site or process they affect.
Understanding these groups is particularly important in weed resistance management, as repeated use of herbicides acting on the same target site can increase selection pressure for resistant weed populations.
HPPD inhibitors
HPPD inhibitors block the enzyme 4-hydroxyphenylpyruvate dioxygenase.
This disrupts production of compounds needed to protect chlorophyll from breakdown. Susceptible plants typically develop characteristic bleaching symptoms before growth is severely affected.
PPO inhibitors
PPO inhibitors affect the enzyme protoporphyrinogen oxidase, which is involved in chlorophyll and haem production.
Inhibition leads to the accumulation of reactive compounds that damage cell membranes, often producing rapid tissue injury where the herbicide contacts susceptible plants.
Synthetic auxins
Synthetic auxin herbicides mimic the activity of natural plant growth hormones.
In susceptible broad-leaved weeds they disrupt normal growth regulation, leading to abnormal development and eventual plant death.
Other important herbicide modes of action
Other groups used in UK agriculture include herbicides that inhibit acetolactate synthase (ALS), acetyl-CoA carboxylase (ACCase), photosystem II activity, very-long-chain fatty acid production and microtubule formation.
Which modes of action are appropriate depends on the crop, target weed, resistance risk and the approvals available for that use.
Contact and systemic herbicides
Contact and systemic describe how a herbicide behaves within the target plant rather than its mode of action.
Contact herbicides act mainly on the plant tissue reached by the spray, making good coverage particularly important.
Systemic herbicides are absorbed and move within the plant to varying degrees, which can help control growing points or underground structures depending on the active ingredient.
A product's movement within the plant should therefore be considered alongside its mode of action when planning an application.
Fungicide modes of action
Fungicides control plant pathogens by interfering with specific biological processes within the pathogen.
Important fungicide modes of action include inhibition of respiration, disruption of sterol production, interference with cell division and effects on other essential metabolic pathways.
Fungicide mode of action is particularly important in resistance management because some crop pathogens can develop reduced sensitivity to individual fungicide groups.
SDHI fungicides
Succinate dehydrogenase inhibitor (SDHI) fungicides affect fungal respiration by inhibiting succinate dehydrogenase, an enzyme involved in mitochondrial energy production.
This limits the pathogen's ability to generate the energy required for processes such as spore germination, growth and infection.
QoI fungicides
QoI fungicides, often referred to as strobilurins, also interfere with mitochondrial respiration, but at a different site from SDHIs.
They inhibit electron transfer within the respiratory chain, restricting energy production within susceptible pathogens.
DMI fungicides
Demethylation inhibitor (DMI) fungicides include the triazole group.
They interfere with sterol biosynthesis, reducing production of ergosterol and other sterols required for normal fungal cell membrane development.
Multi-site fungicides
Some fungicides affect several biological processes rather than acting at one highly specific target site.
These products can play an important role within resistance-management programmes where they are approved and provide useful activity against the target disease.
Protectant, curative and systemic activity
Terms such as protectant, curative, systemic and translaminar describe how a fungicide behaves in relation to the plant and infection process. They are not themselves modes of action.
Protectant activity generally refers to control before infection becomes established.
Some products offer activity during the early stages of infection, often described as curative activity, although the length and strength of this activity varies considerably between products and pathogens.
Systemic fungicides move within plant tissue to varying degrees, while translaminar activity refers to movement through the treated leaf from one surface to the other.
These characteristics influence product positioning and timing and should be considered alongside mode of action.
Insecticide modes of action
Insecticides also work through a range of biological processes.
Many act on the insect nervous or muscular system, while others interfere with growth and development, feeding, respiration or other essential processes.
Different insecticide active ingredients are grouped according to their mode of action to help support effective resistance management.
The most appropriate option depends on the crop, pest species, life stage, resistance risk and approved use.
Why mode of action matters for resistance management
Resistance develops when individuals within a weed, pathogen or pest population survive treatment and pass resistance traits to subsequent generations.
Repeated use of the same mode of action can increase selection pressure, allowing resistant individuals to make up a greater proportion of the population over time.
Using different modes of action can therefore form an important part of resistance management, but simply alternating group numbers is not enough.
Effective resistance management combines crop protection with wider agronomic measures that reduce the population or disease pressure being treated.
Use integrated crop management
Resistance management starts before a crop protection product is applied.
Rotation, variety choice, cultivation, drilling date, competitive crops, crop hygiene, monitoring and other integrated measures can all reduce reliance on individual active ingredients depending on the target.
Use effective modes of action
Where different modes of action are available, programmes should make effective use of them according to the target and resistance situation.
This may involve mixtures, sequences or alternation, depending on the crop, product approvals and resistance-management requirements.
Using an additional active ingredient with little or no useful activity against the target does not provide meaningful resistance-management benefit.
Use products at the right timing and dose
Crop protection products should be applied at timings and rates appropriate to the target, crop and level of pressure.
Allowing weeds, diseases or insect populations to become well established can reduce control and increase the pressure placed on the remaining effective chemistry.
All applications must follow the product label and current technical guidance.
Mode of action classification groups
International classification systems are used to group herbicides, fungicides and insecticides according to mode of action.
These classifications are commonly shown as group numbers or codes on technical information and product literature.
They provide a useful reference when comparing active ingredients, but group number alone should not determine product choice. Agronomic effectiveness, resistance status, target susceptibility and the wider programme must also be considered.
Using mode of action in crop protection planning
Mode of action information is most useful when it supports practical decisions in the field.
When building a crop protection programme, consider:
- the weed, disease or insect being targeted;
- the modes of action already used within the crop and rotation;
- known or suspected resistance;
- the effectiveness of each active ingredient against the target;
- application timing and crop growth stage;
- other agronomic measures that can reduce target pressure; and
- product label and statutory requirements.
This helps ensure active ingredients are used where they are most likely to provide useful control while protecting their effectiveness for future seasons.
How agronomy advice supports mode of action decisions
Mode of action is only one part of product selection.
Your Agrii agronomist can help assess the target, resistance risk, crop condition and available chemistry before building a programme around the individual field and farming system.
This includes identifying where different modes of action can be used effectively, where non-chemical measures can reduce pressure and how product timing and application can be optimised.
The objective is effective control today while reducing unnecessary selection pressure on the crop protection tools that remain available.