How Do Pesticide Formulations and Formulations Affect Rainfastness?

Two pesticides may contain the same active ingredient at the same concentration, but they may perform differently when exposed to rain after application.

One key reason is that they use different formulation systems.

A pesticide’s rainfastness depends not only on the active ingredient but also on the formulation’s solvents, surfactants, adhesion agents, and dispersion systems, as well as how the spray solution behaves on the target surface.

After a pesticide is sprayed onto leaves, it undergoes droplet deposition, spreading, drying, adhesion, and absorption. Rainfall may occur at different stages, affecting final control efficacy.

For example, certain contact fungicides rely on foliar residues for protection, so the formulation must help the active ingredient adhere to the surface and resist rain wash-off. Conversely, for systemic pesticides that must penetrate plant tissues, the formulation’s ability to facilitate timely penetration and absorption can also affect rainfastness.

However, this does not mean that any one formulation type is inherently more rainfast than others.

When assessing rainfastness, evaluate the complete commercial formulation and its actual application conditions, rather than concluding based solely on formulation abbreviations such as SC, EC, or WG.

What is pesticide rainfastness?

Pesticide rainfastness refers to a product’s ability to maintain its expected control efficacy after exposure to rainfall following application.

It does not mean the pesticide is completely waterproof, nor does it imply that rainfall of any intensity will not affect its efficacy after a certain time.

For foliar spray products, rainfastness typically involves the following processes:

• Deposition of spray droplets on leaves;
• Spread and adhesion of the spray solution to the surface;
• Water evaporation and residue formation;
• Penetration of the active ingredient into plant tissues;
• Leaching of leaf residues by rainfall;
• Persistence of the biocontrol effect after rainfall.

Different products rely on different processes.

For example, a protective contact fungicide may rely primarily on an effective residue on the leaf surface, while a systemic herbicide requires sufficient active ingredient to penetrate plant tissues.

Therefore, rainfastness cannot be explained by a single mechanism for all pesticides.

Rainfastness Is Not the Same as Drying Time

This is the most common source of confusion.

When the leaf surface appears dry, it only indicates that some or most of the volatile liquid in the spray droplets has evaporated.

However, the active ingredient may remain on the leaf surface and may not have been fully absorbed.

For certain contact-type products, the solid residue left after drying may not necessarily resist wash-off.

Therefore:

Drying the spray solution does not mean the product has met verified rainfastness criteria.

For information on how long to wait after application and whether a re-spray is needed after rain, please read How Long After Pesticide Application Can It Rain Without Affecting Efficacy.

This article primarily explains why different formulations exhibit varying levels of rainfastness.

How do pesticide formulations affect rainfastness?

Formulation effects can be understood through three main aspects: retention on the leaf surface, adhesion and film formation, and absorption of the active ingredient.

Retention of spray droplets on the leaf surface

Just because the spray solution lands on a leaf does not mean all of the active ingredient will remain there.

Some droplets may bounce off, break apart, or run off.

Particularly on plant leaves with a thick waxy coating—where the surface is not easily wetted by water—droplets may form a large contact angle, making it difficult for the solution to spread evenly.

Surfactants and other adjuvants in the formulation can alter the surface tension and wetting behavior of the spray solution, thereby influencing the actual amount deposited.

However, a larger spread area does not necessarily mean higher retention.

If the spray solution spreads excessively, it may actually run down the leaf surface and be washed away.

Therefore, formulation design must balance wetting, spreading, and droplet retention.

Adhesion and Film Formation of Active Ingredients

Some pesticides act primarily on the plant surface.

For such products, whether the deposit formed after the spray droplets dry can be easily re-wetted and washed away may directly affect rainfastness.

Certain formulations or adjuvants can reduce rain-induced runoff by improving adhesion, forming polymer films, or enhancing the bond between the residue and the leaf surface.

However, not all pesticides rely on film formation to improve rainfastness.

Furthermore, forming a strong surface-adhesion structure is not necessarily beneficial for active ingredients that need to penetrate plant tissues.

Penetration and Absorption of Active Ingredients

For pesticides that can enter plant tissues, foliar absorption is another important process.

The active ingredient must first contact the leaf surface and then, depending on its physicochemical properties and the formulation system, gradually enter plant tissues through the cuticle or other pathways.

Certain solvents, oil-based adjuvants, and surfactants may influence this process.

Once enough of the active ingredient has entered plant tissue, the risk of rainfall washing that portion of the pesticide away is typically reduced.

However, systemic activity does not mean the pesticide is immediately rainfast after application.

Absorption rates vary by active ingredient, and the same active ingredient may also absorb differently across commercial formulations.

How do rainfastness differences vary among pesticide formulations?

SC, EC, EW, WP, WG, and SL have different physical structures and form different spray systems when mixed with water.

These differences may affect spray deposit, leaf spread, active ingredient release, and rain wash-off behavior.

However, the formulation abbreviation itself does not determine the final rainfastness.

SC: Particle size does not equal rainfastness

The active ingredients in SC formulations typically exist as solid granules.

When the product is mixed with water and sprayed, these granules may settle on the leaf surface along with the formulation adjuvants.

Particle size, particle size distribution, and surface properties may influence their deposition behavior and interaction with the leaf surface.

However, it cannot be simply assumed that:

The finer the SC particles, the better the rainfastness.

Particle size may affect dispersion, coverage, and the release behavior of certain active ingredients, but whether rainfall washes away foliar residues also depends on adjuvants, the deposited structure, the leaf surface, and rainfall conditions.

The particle size of solid particles in SC formulations differs from the size of spray droplets produced by a spray nozzle.

Both can affect application performance, but they work through different mechanisms.

EC: Solvents and Emulsification Systems May Affect Penetration

EC formulations typically contain active ingredients, solvents, and emulsifiers.

When mixed with water, the product forms a spray-ready emulsion.

The solvents and surfactants in the formulation may affect spray spread on the leaf surface, interactions with the cuticle, and active-ingredient absorption.

For certain active ingredients that require plant uptake, these factors may influence how quickly they achieve rainfast conditions.

However, it cannot be concluded that:

EC formulations are necessarily more rainfast than SC formulations.

Solvent systems and adjuvant formulations can vary significantly between different EC products.

Furthermore, stronger penetration may also increase the risk of phytotoxicity in some crops.

Therefore, only specific product validation can determine whether a formulation truly improves rainfastness.

EW: Emulsion Structure and Foliar Deposition

EW uses water as the continuous phase; the oil droplets and emulsification system within it may affect how the spray solution spreads and behaves on foliage.

However, the emulsion structure after dilution with water and the deposition state after application are also jointly influenced by the active ingredient, the oil-phase composition, and the adjuvant system.

The fact that EW is a water-based emulsion formulation does not mean its rainfastness is stronger or weaker than that of EC.

WP and WG: Dispersibility Does Not Equate to Wash-Off Resistance

When mixed with water, WP and WG typically form a suspension of solid particles.

WP requires wetting and dispersion, while WG must first disintegrate to release particles that can form a suspension.

These properties are critical for spray uniformity.

However, a product’s good dispersibility and suspension efficiency in the spray tank do not guarantee that the foliar deposit will resist rainfall.

For example, a WG formulation may disintegrate rapidly in water to form a uniform suspension.

Yet, whether the particles are sufficiently retained on the leaf surface after application depends on surface adhesion, the adjuvant system, and rainfall conditions.

Therefore:

Dispersibility, suspension rate, and rainfastness are distinct performance metrics.

SL: Dissolved does not mean absorbed

The active ingredients in SL products exist in a dissolved state; however, simply adding the product to spray water does not mean the active ingredients have already entered plant tissues.

For SL products used for foliar application, the spray solution still needs to deposit on the target surface and be absorbed appropriately.

If rainfall occurs too soon, rain may dilute or wash away the active ingredient before it is absorbed.

Ultimately, rainfastness also depends on the active ingredient’s water solubility, its ionic state, the adjuvant system, and the conditions for foliar absorption.

Therefore, you cannot determine a product’s rainfast duration solely because it is a solution-type formulation.

Do different formulations of the same active ingredient have different rainfastness?

It is possible.

Even if two commercial products have the same active ingredient and the same concentration, their complete formulations may still differ.

For example, two products may differ in the following aspects:

• The salt form or other chemical form of the active ingredient;
• The solvent system;
• surfactants;
• adhesives;
• penetration enhancers;
• particle size and distribution;
• actual application concentration after dilution with water.

These differences may affect how spray droplets deposit, dry, adhere, and are absorbed on the leaf surface.

Assume both products use the same active ingredient.

Product A uses an adjuvant system designed to prioritize retention on the leaf surface.

Product B uses a different adjuvant system that emphasizes active-ingredient penetration.

Under appropriate conditions, they may exhibit different rainfastness behaviors.

However, this illustrates a technical scenario where formulation differences may affect rainfastness; it does not imply that Product A or B is necessarily superior.

A true comparison requires validation under specific rainfall conditions and using consistent evaluation methods.

Identical active ingredients do not guarantee identical rainfastness, and identical formulations do not guarantee identical rainfastness.

How do adjuvants affect pesticide rainfastness?

Adjuvants play an important role in evaluating rainfastness.

Adjuvants may be included in commercial formulations or added separately to the spray tank, if label instructions permit.

Different adjuvants function differently.

Spreaders: Improve leaf coverage

Spreaders typically work by altering the spray solution’s surface tension, making it easier for droplets to spread across the target surface.

Proper spreading may improve coverage.

However, a larger spread area does not necessarily mean better rainfastness.

If droplets spread too far across the leaf surface and run off, this may reduce the effective deposit.

Adhesives: Reduce the Leaching of Surface Residues

Certain adhesives can improve the retention of spray deposits on leaf surfaces.

This may be of practical value for contact pesticides that rely primarily on foliar residues to exert their effect.

For example, adjuvants with appropriate film-forming properties may make some pesticide residues less susceptible to rain wash-off.

However, there is an easily overlooked issue here:

Improved surface adhesion does not necessarily increase plant uptake.

Some adhesion systems may cause the active ingredient to adhere more firmly to the leaf surface, yet slow its entry into plant tissues.

Therefore, determine whether an adhesion adjuvant suits a particular product based on its mode of action.

Penetration enhancers: Affect the rate of entry into plant tissues

Certain oil-based adjuvants and surfactants may alter how the leaf cuticle interacts with the active ingredient, affecting absorption.

For active ingredients that rely on plant absorption, an appropriate penetration system may reduce losses caused by early rainfall.

However, this should not be interpreted to mean that:

adding a penetration adjuvant will necessarily shorten the rainfast time.

Different crops and active ingredients respond differently to adjuvants.

An inappropriate penetration system may also increase the risk of crop phytotoxicity or compromise the product’s safety.

Pesticide application technical materials from Oregon State University also emphasize that adhesion agents and penetration-enhancing adjuvants do not necessarily function in the same way and may sometimes interfere with one another.

Therefore, do not add so-called “rainfastness enhancers” on your own initiative unless the product label permits it.

Why does the leaf surface also affect rainfastness?

Rainfastness depends not only on the pesticide product but also on the plant surface being sprayed.

Leaf structure can vary significantly across crops.

For example:

• The composition and thickness of the cuticular wax vary;
• The roughness of the leaf surface varies;
• The number of trichomes varies;
• The angle of leaf inclination varies;
• The leaf surface’s hydrophilicity and hydrophobicity vary.

These differences affect how spray droplets behave after they contact the leaf.

On leaves with a thicker waxy layer, some droplets may struggle to adhere or spread evenly.

On leaves with a higher density of trichomes, droplets may also initially rest on the trichomes rather than coming into direct contact with the cuticle.

However, it cannot be simply assumed that a thicker waxy layer necessarily results in lower rainfastness.

This is because the leaf wax composition, surface structure, and formulation system collectively determine deposition and rain-off behavior.

Studies on contact fungicides such as Mancozeb show that differences in wax content and leaf surface roughness can affect active-ingredient retention and rainfastness.

This suggests that the same product’s rainfastness may vary across crops.

Therefore, when assessing a product’s rainfastness duration, you cannot ignore the target crop and leaf surface conditions.

Why are rainfall timing, amount, and intensity equally important?

Even with the same product formulation, different rainfall conditions can lead to different outcomes.

The three most important variables are:

the interval between application and rainfall, the total rainfall amount, and the rainfall intensity.

Timing of Rainfall

If rainfall occurs before the spray droplets have fully settled, dried, or been absorbed, it may wash away a significant amount of the active ingredient.

Over time, surface adhesion or plant absorption of some products may gradually increase.

However, this change does not apply equally to all pesticides.

You also cannot assume rainfastness will increase linearly over time.

Total Rainfall

Total rainfall may affect how much foliar residue is washed away.

However, the same total rainfall does not necessarily produce the same results when it occurs over different durations or at different intensities.

Rainfall Intensity

Short, intense rainfall may produce larger droplet impact and surface runoff.

Slow, steady rainfall, on the other hand, may affect deposits through prolonged re-wetting and leaching.

Therefore, even if total rainfall is the same, you cannot assume two rainfall events will affect the pesticide the same way.

It should be noted that this table describes potential mechanisms and does not imply that any single rainfall condition will cause all products to fail.

How should pesticide rainfastness be evaluated through testing?

Rainfastness cannot be confirmed solely by the formulation’s appearance, the time it takes for the leaf surface to dry, or a specific adjuvant’s promotional claims.

A reasonable evaluation should consider whether the product can still maintain the expected biological control efficacy under specified rainfall conditions.

The technical guidance document *Rainfastness of Foliar-Applied Plant Protection Products*, published by the UK HSE, specifically discusses how to use testing to support product claims of rainfastness for periods shorter than 12 hours.

The 12-hour period mentioned here refers to the scope of claims discussed in this guidance; it is not a uniform rainfastness duration for all pesticides.

The interval between application and rainfall

During evaluation, you must clarify when rainfall occurs after application.

For example, you can set different rainfall intervals to compare the product’s performance at different stages.

Only then can you determine whether the product meets the claimed rainfastness conditions.

However, you must determine the specific trial design based on applicable standards and the target product’s intended use.

Rainfall Conditions

Simulated rainfall tests require control of key variables that affect the results.

These may include:

• rainfall amount;
• rainfall intensity;
• duration of rainfall;
• environmental conditions during rainfall.

If two products are tested under different rainfall conditions, a direct comparison of their rainfastness times may be misleading.

Foliar Residue

One evaluation method involves analyzing how much active ingredient remains on the leaf surface before and after rainfall.

This method helps understand changes in residue levels caused by rainwash.

However, it also has limitations.

For active ingredients that can penetrate plant tissues, a reduction in residue on the leaf surface does not necessarily mean that the entire amount was washed away by rain; a portion may have already been absorbed.

Therefore, measuring active-ingredient residue on the leaf surface may not be sufficient to explain final control efficacy.

Biocontrol Efficacy

A true evaluation of rainfastness must also consider whether the product maintains its expected control efficacy after rainfall.

For example:

For herbicides, the control performance against target weeds can be evaluated under an appropriate experimental design.

For fungicides, protective efficacy can be assessed through relevant disease control trials.

For insecticides, determine appropriate evaluation methods based on the target pests and mode of action.

A reduction in pesticide residue observed in experiments does not necessarily equate to a proportional decrease in control efficacy.

Similarly, a high residue level after rainfall does not automatically prove that the product has better biological control efficacy.

Claims regarding rainfastness must be based on specific products and appropriate trial results.

What is the difference between rainfastness, residual efficacy, and shelf life?

These three concepts relate to time, but they describe different properties.

For example, a protective fungicide may have a long residual effect under appropriate conditions. However, if heavy rain occurs immediately after application, some of the foliar residue may still be lost.

Conversely, even if a product quickly meets its verified rainfastness criteria, this does not mean it will remain effective on the crop surface for several weeks.

Shelf life is a separate concept.

It assesses whether the pesticide continues to meet the relevant physical and chemical quality specifications while stored in its original packaging.

BigPesticides’ “How Pesticide Formulations Affect Product Shelf Life” discusses the different stability issues that may arise during storage for formulations such as SC, EC, WP, and WG.

Therefore, storage stability, rainfastness, and residual efficacy in the field should not be conflated into a single performance metric.

Why Do Contact- and Systemic-Type Products Differ in Rainfastness?

The distinction between contact and systemic pesticides lies in where the active ingredient primarily exerts its effect and whether it needs to penetrate plant tissues.

For contact fungicides that rely primarily on protective residues on the leaf surface, rainfall may reduce the effective deposit on the target surface.

For example, Copper Oxychloride is a contact-protective fungicide whose efficacy depends on the proper distribution of the active ingredient on the plant surface and the release of copper ions.

Therefore, foliar coverage and residue retention are critical factors in understanding the protective performance of such products.

You can learn about the formulation types of this class of protective fungicides through the Copper Oxychloride 50% WP product data sheet.

However, just because a product is a contact fungicide does not mean it is necessarily rain-sensitive, and you cannot specify a uniform rainfastness duration.

For products with systemic or translaminar movement, subsequent rainfall may reduce direct wash-off of the portion that has already entered plant tissue.

However, not all systemic products have the same absorption rate, and systemic activity should not be equated with a longer residual period.

Contact and systemic action can help explain rainfastness mechanisms, but they cannot replace rainfastness testing for commercial products.

Why is rainfastness a greater concern for pesticide products in regions with heavy rainfall?

In agricultural markets with distinct rainy seasons, rainfastness is not merely a technical term; it can also affect a product’s actual performance in the field.

For example, in tropical fruit and vegetable growing regions, foliar fungicides may need to withstand frequent rainfall.

If a product relies primarily on surface residues for disease protection, it warrants special attention to how these residues are retained after rainfall.

For systemic foliar herbicides, whether sufficient absorption can be achieved before rainfall may be even more critical.

However, products cannot simply be categorized as:

“SC formulations suitable for the rainy season”

or:

“WP formulations unsuitable for the rainy season.”

Market suitability still requires consideration of:

• specific active ingredients;
• complete commercial formulations;
• target crops and pests, diseases, and weeds;
• verified rainfastness;
• local registration labels;
• actual application and rainfall conditions.

For pesticide importers, distributors, and brand owners, rainfastness data can explain technical differences between products and support more accurate customer usage instructions.

Especially when two suppliers offer products with the same active ingredient and concentration, you cannot determine which is more suitable for a rainy market based solely on the formulation name or adjuvant claims.

Priority should be given to performance evidence specific to the individual product.

Frequently Asked Questions

Does a pesticide become rainfast once it dries?

Not necessarily. Surface drying simply results from water evaporating from droplets and does not prove the active ingredient has fully adhered or been absorbed. Assess actual rainfastness based on the product’s technical data and label.

Is SC always more rainfast than EC?

No. While SC and EC have different formulation structures, rainfastness also depends on factors such as the active ingredient, solvent, surfactants, adhesion system, and target leaf surface. One cannot determine which product is more rainfast based solely on the formulation abbreviation.

Will two products with the same active ingredient have different rainfastness durations?

Possibly. The salt form, solvents, and adjuvant systems in different commercial formulations may affect leaf deposition, penetration, and resistance to wash-off. Therefore, different products containing the same active ingredient may have different rainfastness claims.

Does adding an adjuvant always improve a pesticide’s rainfastness?

Not necessarily. While some adhesion agents can reduce rain wash-off of contact pesticides, they may also alter the penetration behavior of active ingredients that require plant uptake. Always follow the specific product label.

Are systemic pesticides immune to rain?

No. Systemic products require specific conditions and time to be fully absorbed. If rainfall occurs before the active ingredient has fully entered the plant tissues, it may still affect control efficacy.

Does good rainfastness mean longer residual efficacy?

No. Rainfastness measures a pesticide’s ability to maintain control efficacy after rainfall, while residual efficacy measures how long it remains effective after application. The two are related but are not the same metric.

Can a pesticide’s rainfastness be determined solely through laboratory rain wash-off tests?

It should not be judged based solely on a single physical wash-off result. Residue testing can provide valuable information, but claims about a product’s rainfastness should also be supported by evidence of biological efficacy and consider the intended use and applicable evaluation requirements.

Why must pesticide rainfastness be evaluated based on the complete formulation?

Rainfastness is not an inherent, fixed property of any single formulation type.

SC, EC, EW, WP, WG, and SL each have different physical structures. Still, their rainfastness ultimately depends on the active ingredient, the complete adjuvant system, the target surface, and application conditions.

For contact-action products, leaf adhesion and residue retention may matter more.

For products that need to penetrate plant tissues, penetration and absorption rates may play a greater role.

Rainfall timing, amount, and intensity can further alter these processes.

Therefore, rainfastness evaluation cannot be determined solely from formulation type, droplet drying time, or adjuvant type; it should be based on test data from specific commercial products.

Importers, distributors, and agricultural brands that need to select products for different agricultural markets can use POMAIS’s Pesticide Product Center to learn about different active ingredients and formulation types, and evaluate suitable product solutions by considering the registration requirements of target markets, product technical specifications, and local rainfall conditions.