Rainfastness retention in orchard fungicide suspension concentrates is governed by the interaction between the deposition-spreading threshold and the concentration at which soluble formulation components begin to re-solubilize the dried deposit. In practice, retention after rainfall is measured by artificial rainfall simulation under protocols such as EPPO PP 1/239(2), using a target rainfall amount of 10 mm to 30 mm applied over 15 min to 60 min, depending on orchard canopy and fungicide mode of action. The threshold concentration for rainfastness is not a single value; it is a formulation-specific boundary defined by the free concentration of dispersant, wetting agent, and water-conditioning adjuvants in the spray tank. For a suspension concentrate containing a crystalline active ingredient with a median particle size of 1.0 µm to 3.0 µm, the dispersant demand on the particle surface is typically in the range of 2 mg/m² to 6 mg/m² for naphthalene sulfonate condensates, while the corresponding soluble free dispersant concentration that remains in the continuous phase after adsorption is a critical rainfastness variable. If the free dispersant concentration exceeds the threshold required to form a continuous residual film, rain events can redisperse the deposit and translocate active ingredient away from the target phylloplane. Commercial airblast sprayers operating at 0.7 MPa to 1.2 MPa and delivering 400 L/ha to 1000 L/ha produce droplet spectra that interact differently with leaf surfaces, and the resulting deposit thickness and distribution determine whether rainfastness failure appears as diffuse wash-off or localized erosion at higher concentration gradients.
The rainfastness boundary can be described as the dispersant concentration at which the product’s suspended particle layer transitions from a hydrophobic, water-insensitive film to a partially rewettable solid matrix. In suspension concentrates stabilized with anionic polyelectrolytes, this threshold is frequently observed at the point where the equilibrium zeta potential declines below −30 mV under spray-dilution conditions. The change in zeta potential is not directly caused by rainfall itself but by the dilution of the continuous phase, which shifts the adsorption equilibrium of dispersant molecules on the particle surface. Surface coverage follows Langmuir or Freundlich isotherms, and when the free dispersant concentration drops below the critical adsorption concentration during rainfall, partial desorption occurs over a period of 5 min to 20 min. This desorption produces bare patches on the crystalline active ingredient surface; the exposed surface has a higher contact angle with water than the dispersant-coated surface, and the capillary forces generated during droplet evaporation and re-wetting cause greater aggregate detachment. Field experience with airblast-treated pome fruit orchards indicates that deposits located on the abaxial leaf surface and at the junction of the petiole and leaf blade are the most vulnerable to early rainfastness loss, because the local spray concentration there is lower and the retention time before the first rainfall event is often less than 4 h. In such scenarios, formulation adjustments that raise the dispersant content beyond the conventional stabilization requirement, from 20 g/kg to 35 g/kg, can extend rainfastness retention but may reduce pourability and increase the risk of Ostwald ripening. The operational boundary is therefore narrow: excess dispersant increases the ionic strength of the continuous phase, compresses the electrical double layer, and triggers depletion flocculation, while insufficient dispersant results in irreversible aggregation during spray dilution.
After the suspension concentrate has been milled to the target particle size, the resulting distribution becomes a primary determinant of rainfastness because particles below 1 µm have higher surface free energy and dissolve faster in rain water, while particles above 5 µm settle more quickly and form loosely adhered deposits. Comminution in a horizontal bead mill with a chamber volume of 5 L and an internal length-to-diameter ratio of 3.2:1 typically uses yttria-stabilized zirconia beads of 0.6 mm to 0.8 mm at a fill fraction of 75% to 85% and a tip speed of 10 m/s to 12 m/s. The resulting particle size distribution, measured by laser diffraction per ISO 13320:2020, should be narrow, with D90 below 6 µm and D10 above 0.4 µm to minimize differential erosion between fine and coarse fractions during simulated rainfall. The milling process also generates heat and mechanical stress that can destabilize temperature-sensitive active ingredients; therefore, a cooling jacket maintaining the product temperature below 35°C is necessary to avoid melting or polymorphic transformation of low-melting fungicide crystals. Batch-to-batch variability in the mill residence time, recirculation flow rate, and bead wear affects the threshold rainfastness performance. Production records from suspension concentrate manufacturing lines show that lot-to-lot D50 variation greater than 0.4 µm can shift rainfastness retention by more than 10% under a 15 mm rain event, even when the wet sieve retention and suspensibility values remain within specification. To maintain reproducible rainfastness, the milling process should be halted when the D50 reaches the target window of 1.5 µm to 2.5 µm, and the bead charge should be checked daily for surface wear because fractured zirconia beads increase the coarse fraction above 10 µm and reduce rainfastness by creating discontinuities in the dried deposit.
Rheological behavior at low shear rates governs the thickness and uniformity of the liquid film that remains after droplet impact on the leaf surface. In orchard fungicide suspension concentrates, a low-shear viscosity between 300 mPa·s and 1200 mPa·s at 20°C, measured per ISO 3219, correlates with improved deposit adhesion after drying because the formulation resists immediate runoff from hydrophobic leaf cuticles. However, excessively high low-shear viscosity above 2000 mPa·s can reduce rainfastness indirectly by causing poor atomization in airblast sprayers, producing a smaller fraction of droplets in the 50 µm to 200 µm range that are required for penetration into the inner canopy. Airblast nozzle performance is sensitive to viscosity; formulations with high yield stress may require an operating pressure above 1.0 MPa to achieve acceptable fan development, which increases drift risk and reduces the amount of active ingredient deposited on the fructification zone. The yield stress, typically controlled by xanthan gum or modified smectite clay at 0.5 g/kg to 3.0 g/kg, should be sufficient to prevent sedimentation but low enough to allow complete redispersion after 24 h of storage. If the yield stress is above 2.0 Pa, the formulation may gel in the tank when diluted with hard water, and the resulting spray liquid can produce uneven deposits with weak rainfastness. A thixotropic recovery time of 30 s to 120 s after shear removal is preferable for orchard applications; slower recovery permits the spray film to level on the leaf surface and form a continuous deposit, while faster recovery can trap air bubbles at the deposit-leaf interface and create pathways for rain water ingress. Published data for the exact correlation between low-shear viscosity and rainfastness retention in orchard fungicide SCs is limited; the available information suggests that the optimum viscosity range depends on the leaf surface energy, spray volume, and fungicide formulation’s ionic strength.
In hard-water regions, bicarbonate alkalinity above 200 mg/L as CaCO₃ can reduce the effectiveness of anionic dispersants by calcium bridging, causing flocculation in the tank and uneven deposition on the crop. Hardness ions, particularly Ca²⁺ and Mg²⁺ at combined concentrations above 250 mg/L, compress the electrical double layer around suspension concentrate particles and lower the magnitude of zeta potential below 20 mV. Under such conditions, the rainfastness of the dried deposit declines because the flocculated particles are coarser and form a discontinuous film that is more easily eroded by raindrop impact. The use of ammonium sulfate or citric acid as water conditioners has practical limits; ammonium sulfate addition at 1.0 kg/100 L to 2.0 kg/100 L softens hard water but increases the ionic strength of the spray solution and may reduce rainfastness if the formulation already contains a high level of inorganic salts. Spray water pH outside the range 5.5 to 7.5 can hydrolyze ester or carbamate fungicide active ingredients and alter the surface charge of suspension particles. In alkaline water above 8.5, the degradation rate of some multisite contact fungicides increases rapidly, and the resulting reaction products can act as ionic contaminants that accelerate rain-induced detachment. Therefore, field operators should test water pH and hardness before adding the suspension concentrate to the induction hopper using a calibrated conductivity meter and pH electrode compliant with ISO 10523:2008 for pH measurement.
Nonionic wetting agents, typically alcohol ethoxylates or tristyrylphenol ethoxylates, are included in suspension concentrates at 10 g/kg to 40 g/kg to promote spreading on waxy leaves. Their effect on rainfastness is bimodal. At concentrations below the critical micelle concentration, the surfactant reduces contact angle and increases the wetted area, which can improve deposit uniformity and resistance to droplet impact. At concentrations above the critical micelle concentration, the excess surfactant remains in the dried deposit as a hygroscopic residue that rehydrates after exposure to rain and plasticizes the film, increasing the rate of active ingredient release. The critical concentration is not a fixed value; for many tristyrylphenol ethoxylates, the critical micelle concentration in pure water is between 10 mg/L and 100 mg/L, but the presence of calcium ions and formulation dispersants shifts this value downward by 30% to 50%. The practical threshold in the spray tank is therefore often reached at a lower surfactant concentration than expected from pure water measurements. When the nonionic wetting agent concentration in the final spray liquid exceeds 0.1% v/v to 0.3% v/v, rainfastness can deteriorate because the deposit becomes increasingly water-sensitive. This behavior is particularly pronounced with contact fungicides applied to apple and pear orchards under cool, humid conditions where the first rainfall occurs within 6 h of application. The operational incompatibility with tank-mix spreaders is important: adding a high-HLB nonionic adjuvant to a suspension concentrate that already contains a wetting agent can push the total surfactant concentration past the rewetting threshold and result in unacceptable wash-off. The same is true when organosilicone surfactants are used to increase rainfastness; organosilicone levels above 0.05% v/v can cause excessive spreading and run-off on smooth leaves, and their hydrolysis in water over 24 h can generate silanol species that alter the rainfastness properties. Surface tension measurements with a du Noüy ring tensiometer per ASTM D1331-20 provide a direct in-process test for whether the combined surfactant level crosses the rewetting threshold.
During storage at 54°C for 14 days, suspension concentrates can undergo particle growth via Ostwald ripening if the concentration of free surfactant in the continuous phase exceeds the solubility threshold of the active ingredient. This ageing process narrows the particle size distribution and shifts the D50 from the target 2.0 µm to a coarser mode above 3.5 µm. Rainfastness retention after accelerated storage is therefore a critical quality indicator. In a 500 L commercial batch, the transfer from a cooled holding tank to the let-down vessel can introduce air entrainment; if the air content exceeds 2% v/v, the resulting foam interferes with accurate filling and can alter the density of the formulation, which in turn modifies the sprayed volume per hectare. A vacuum deaeration step at 50 mbar to 100 mbar for 15 min is used to lower air content below 0.5% v/v. The onset of rainfastness loss after storage is frequently accompanied by an increase in low-shear viscosity of more than 30% and a drop in suspensibility below 80%. These changes are measurable with ISO 3219 and CIPAC MT 184, respectively. A robust suspension concentrate specification links the aged-particle-size D50, the low-shear viscosity, and the suspensibility to the rainfastness retention under controlled simulated rainfall, so that correction of the dispersant concentration in the next batch cannot be made without reference to the rainfastness data.
Under airblast application conditions, deposition on orchard canopies is measured using water-sensitive papers or fluorescent dye tracers. A typical evaluation of rainfastness requires leaf washing after simulated rainfall, with the amount of fungicide remaining expressed as a percentage of the initial deposit. Rainfastness retention values for well-stabilized suspension concentrates on apple leaves often fall between 50% and 85% after 10 mm of simulated rain applied at 35 mm/h to 60 mm/h under EPPO PP 1/239(2). The threshold concentration for failure is observed when retained active ingredient falls below the minimum biological efficacy threshold for the target pathogen. For many contact fungicides against Venturia inaequalis, the minimum deposit for adequate spore germination inhibition is in the range of 5 µg/cm² to 15 µg/cm² depending on the active ingredient and disease pressure. If rainfastness retention drops below 40% of the initial deposit, the remaining dose may be below this biological threshold, and re-application is required. Rainfall intensity, droplet size, canopy density, and leaf angle interact with deposit structure; the threshold concentration of rainfastness is therefore not solely a formulation parameter but a system property that includes the crop architecture and the timing of the first rain event. The spatial distribution within the canopy is uneven, and deposits on outer-row leaves often exceed the threshold concentration by a factor of 2 to 3 compared with inner-canopy leaves, where lower initial deposit and higher moisture retention increase the probability of early rainfastness failure.
| Parameter | Test Method or Standard | Typical Acceptance Window or Operational Limit |
|---|---|---|
| Particle size distribution | ISO 13320:2020 | D50 1.0 µm–3.0 µm; D90 ≤ 6.0 µm |
| Low-shear viscosity | ISO 3219 | 300 mPa·s–1200 mPa·s at 20°C |
| Suspensibility | CIPAC MT 184 | ≥ 80% after 30 min |
| Wet sieve retention | CIPAC MT 15.1 | ≤ 0.1% on 75 µm sieve |
| pH of aqueous dilution | ISO 10523:2008 | 5.5–7.5 |
| Simulated rainfastness retention | EPPO PP 1/239(2) | Report retained % after 10 mm simulated rainfall; product-specific threshold |
Rainfastness retention failures on commercial orchard sprayers are most often associated with batch-to-batch variability in the free dispersant concentration rather than with an absolute deficiency in active ingredient. A 5% over-dosage of the nonionic wetting agent in the let-down phase can shift the rainfastness retention by more than 15% under a 10 mm simulated rainfall. To minimize this variability, in-process control should include a surface tension measurement at 20°C with a du Noüy ring tensiometer per ASTM D1331-20, targeting 28 mN/m to 32 mN/m for the final spray dilution. The same measurement should be repeated with the recommended tank-mix adjuvant concentration to identify whether the combined surfactant level crosses the rewetting threshold. If the measured surface tension falls below 26 mN/m, the rainfastness risk is elevated. The formulation is incompatible with concentrated amine-based pH modifiers, which can cause deprotonation of the dispersant and a sudden increase in viscosity. Pre-drying of the formulation is not required before application, but storage below 0°C must be avoided because ice crystal formation can rupture the adsorbed dispersant layer and reduce rainfastness after thawing.
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