Rice Leaf Sheath Spray Retention with Wetting Agents in Flooded Paddy Microclimates

Retention of fungicidal spray deposits on the leaf sheath of Oryza sativa under flooded paddy conditions is governed by the interaction of formulation-induced wetting, droplet impact dynamics, canopy architecture, and the high-humidity boundary layer generated by standing water. In both indica and japonica production systems, the leaf sheath is the primary infection court for Rhizoctonia solani Kühn (teleomorph Thanatephorus cucumeris), and spray deposits that fail to reach the lower canopy are directly correlated with reduced disease suppression. Published field evaluations of sheath blight control in paddy rice repeatedly show that deposits on vertical leaf sheaths can be 30–50% lower than deposits on horizontal leaf blades when no wetting agent is used. The flooded paddy microclimate amplifies this deficiency because water vapour flux from the floodwater maintains canopy relative humidity above 85–95% during much of the day and night; dew periods lasting 4–8 h are common in tropical and subtropical rice-growing areas. Under these conditions, spray droplets evaporate more slowly than in upland crops, but the same humidity promotes coalescence on hydrophobic surfaces and can lead to runoff before the active ingredient partitions into the cuticle. Wetting agents are therefore evaluated not solely by equilibrium surface tension, but by their ability to reduce dynamic surface tension within the 20–150 ms contact time of a droplet impact event, as specified in surface tension test methods such as ASTM D1331-14 and OECD 115.

In field practice, spray retention on rice leaf sheaths is measured by adding a tracer dye to the tank mix, spraying a defined plot, washing the excised sheath tissue, and quantifying the recovered dye by spectrophotometry or fluorometry. The measured deposit is expressed as mass of tracer per unit projected sheath area or per unit fresh biomass, and the coefficient of variation commonly exceeds 20–40% in flooded rice because of edge effects, operator walking speed, and heterogeneous canopy density. Wetting agents influence retention through two competing mechanisms: a reduction in surface tension lowers the energy barrier for spreading and increases the wetted area per droplet, but the same spreading can reduce the critical droplet size at which runoff occurs. The balance depends on leaf angle, sheath surface roughness, spray volume, and the presence of a pre-existing water film from dew or guttation. For rice sheaths positioned near the water line, the air boundary layer is saturated and the drying time of a 200 µm droplet may exceed 10–20 s, whereas the same droplet on an upland leaf may dry in under 5 s; this extended drying period increases the probability of coalescence and drip-off from the sheath tip.

Why does prolonged dew rewetting shift deposit redistribution on rice leaf sheaths?

Dew formation on rice sheaths in flooded paddies occurs when the nocturnal canopy temperature falls below the dew point of the saturated air layer above the floodwater. Leaf wetness sensors placed at 0.3–0.5 m above the water surface typically record wetness onset between 22:00 and 02:00 local time, with wetness persistence until 07:00–09:00. During this period, dried spray deposits on the sheath surface absorb moisture and form a continuous liquid film that is not chemically equivalent to the original spray droplet. The film has lower surfactant concentration at the air–water interface because the surfactant mass is diluted by condensed water, and it can mobilize active ingredient from the deposit. This rewetting process can either improve distribution into the leaf sheath collar region or cause transfer of active ingredient to the floodwater, depending on sheath angle and the presence of the ligule and auricles as flow paths. For systemic fungicides such as azoxystrobin or trifloxystrobin, dew-driven redistribution can increase phloem-adjacent contact, but published data for this specific configuration in flooded paddy rice is limited. For contact fungicides such as mancozeb or copper-based products, dew redistribution generally reduces residual activity on the sheath because the active ingredient is diluted and washed into the water layer. Wetting agents with a high cloud point and a low critical micelle concentration tend to retain surface activity after dilution by dew, whereas short-chain ethoxylates may desorb from the interface and lose effectiveness. The flooded paddy therefore demands a wetting agent that maintains dynamic surface tension below 35 mN/m after dilution by 10–20% dew water, not merely in the concentrated tank mix.

Epicuticular wax morphology on Oryza sativa leaf sheaths differs from the adaxial blade surface in both density and crystal habit, and this difference has direct consequences for wetting hysteresis. Leaf blade surfaces are frequently reported with water contact angles above 120° and sometimes above 140° because of dense platelet-shaped wax crystals, whereas the abaxial and sheath surfaces often show lower contact angles due to reduced wax load and a higher density of stomata and silica bodies. Published contact angle values for leaf sheath surfaces under flooded paddy conditions are limited; extrapolating from leaf blade data can overestimate the need for low surface tension because droplet adhesion on the sheath is partly controlled by surface roughness and trichome density rather than by wax chemistry alone. When a droplet impacts the sheath, the dynamic contact angle hysteresis determines whether the droplet pins or slides. For a water droplet on a rice leaf sheath with a static contact angle near 100–120°, the advancing contact angle may exceed 130° and the receding contact angle may fall below 60°, producing a hysteresis of 70° or more. This hysteresis is reduced by alcohol ethoxylate surfactants and trisiloxane surfactants, but the reduction is not always proportional to equilibrium surface tension. A surfactant that produces a very low receding contact angle can create a trailing film that drains from the sheath, lowering retention per unit area even though the wetted area appears larger. For this reason, retention studies should measure mass per unit area rather than visual coverage, because visual coverage can increase while mass retention decreases. The practical operational boundary is that wetting agents should not reduce the receding contact angle below 10–15° on the target sheath if the goal is maximum active ingredient retention under high humidity, because the deposit becomes too mobile and runs off into the floodwater.

Droplet Size Classes, Nozzle Selection, and Canopy Penetration in Flooded Rice

Nozzle selection in flooded paddy rice is constrained by the need to penetrate a dense canopy while limiting spray drift from small droplets. The droplet spectra from flat-fan, air-induction flat-fan, and rotary cage nozzles are classified under ASABE S572.3 into fine, medium, coarse, very coarse, and extremely coarse categories. A conventional flat-fan nozzle operated at 3.0 bar with a spray volume of 150–250 L/ha typically generates a volume median diameter in the medium-to-coarse range, while an air-induction nozzle at the same pressure shifts the spectrum toward very coarse or extremely coarse droplets. In flooded rice, very coarse droplets may bounce from vertical leaf sheaths because their kinetic energy exceeds the surface adhesion energy, particularly when the sheath is dry. The addition of a wetting agent reduces droplet surface tension but does not necessarily eliminate bounce; it shortens the contact time and increases the maximum spreading diameter, which can dissipate energy through viscous friction. Field experience with tractor-mounted boom sprayers on dry paddy soils before flooding shows that air-induction nozzles improve lower-canopy deposition compared with fine sprays, but after flooding the high humidity reduces the evaporative loss that would otherwise consume fine droplets before they reach the sheath. Backpack mist blowers and UAV-mounted centrifugal nozzles produce droplets with smaller VMD and higher droplet density, and they are increasingly used in Asian paddy production. However, the downwash from multirotor UAVs can flatten the rice canopy and expose the sheath, while also creating recirculation zones that increase droplet drift and reduce deposition uniformity. Published field data indicate that UAV spray deposition on the lower sheath can vary by 30–60% across a single pass due to rotor vortex effects and canopy movement. This variability must be accounted for when comparing wetting agent performance; a wetting agent that improves retention on a static horizontal target may not improve retention on a moving vertical sheath in a rotor downwash.

Representative ranges compiled from controlled laboratory studies; field retention in flooded paddy canopies shifts with humidity, dew duration, leaf age, and formulation coformulants.

When organosilicone surfactants reduce dynamic surface tension below 22 mN/m, retention on rice sheaths may become runoff-limited

Organosilicone trisiloxane surfactants are characterized by their ability to lower the equilibrium surface tension of water to 20–22 mN/m at concentrations as low as 0.025–0.1% v/v. This is lower than the surface tension of many epoxy and wax surfaces, and it enables rapid spreading across rice leaf sheaths. However, the flooded paddy environment introduces a practical conflict: the same superspreading that produces visual coverage can reduce the amount of active ingredient retained because the liquid film becomes very thin and drains under gravity. The critical use rate at which retention peaks for trisiloxane surfactants on rice leaf sheaths is usually between 0.05% and 0.1% v/v; above this range, retained mass per unit area may decline rather than increase. This behavior has been documented in controlled droplet retention tests using dye tracers on excised rice leaves, but published field data for flooded paddy sheath tissue is limited. The mechanism involves a collapse of the receding contact angle to values below 10°, which reduces the capillary force holding the droplet to the inclined surface. On a vertical sheath, the gravitational force per unit wetted area exceeds the pinning force and the droplet slides. In addition, organosilicone surfactants can penetrate the leaf cuticle and alter cuticular wax fluidity, which may increase active ingredient uptake but also increases the risk of phytotoxicity under high temperature and high humidity. Field observations in tropical paddies have noted marginal leaf necrosis when trisiloxane-containing formulations are applied at more than 0.1% v/v during periods of high solar radiation and air temperature above 35 °C. For this reason, label instructions for organosilicone adjuvants often specify a maximum use rate of 100 mL/100 L and warn against application when leaves are wet for more than 2 h or when the crop is under heat stress.

Conventional alcohol ethoxylate nonionic surfactants provide a wider operational margin in flooded rice because their equilibrium surface tension remains above 28 mN/m and their receding contact angle on rice sheaths generally remains above 20°, which maintains droplet pinning. Their primary limitation is dynamic surface tension; at droplet impact timescales of 20–150 ms, alcohol ethoxylates may reduce surface tension only to 35–45 mN/m, which is insufficient to wet highly hydrophobic blade surfaces but is often adequate for the less waxy sheath. Blends of alcohol ethoxylates with short-chain alkyl polyglucosides are used in paddy fungicide formulations to achieve a compromise between rapid interfacial adsorption and low runoff risk. The addition of a high-molecular-weight anionic or nonionic polymer to the tank mix can further increase deposition by increasing extensional viscosity, which suppresses droplet bounce and reduces mist formation. However, polymer adjuvants must not be combined with amine-based fungicides or high-calcium paddy water without a compatibility test, because the resulting complexes can precipitate in the spray tank and block nozzles. Production-scale experience with knapsack sprayers in South and Southeast Asian rice fields shows that tank mixes with hard water containing more than 200 mg/L calcium carbonate equivalent reduce the cloud point of some nonionic surfactants, causing phase separation in the tank and uneven deposition. In these waters, the use of chelating agents or the selection of anionic wetting agents with high electrolyte tolerance is necessary.

Measuring Spray Retention Under Paddy Conditions Without Losing Field Reality

Field measurement of spray retention on rice leaf sheaths requires a tracer that is stable under high humidity, high UV, and overnight dew. Water-soluble dyes such as Brilliant Sulfoflavine and Tartrazine are widely used because they do not penetrate the rice cuticle and can be recovered by washing the sheath in a known volume of deionized water. The extraction recovery is typically validated by spiking untreated sheath tissue with a known mass of dye and measuring recovery after the same washing procedure. ISO 24253-1:2015 provides a framework for measuring spray deposition on field crops, although its practical implementation in flooded paddies requires adapting the sampling grid to the water layer and the narrow row spacing. The standard measurement is expressed as deposit per unit projected sheath area, but the projected area does not account for the concave curvature of the sheath, which can trap spray liquid and cause higher local deposits near the leaf collar. A more robust metric is deposit per unit fresh biomass, corrected for the water content of the sheath, because rice sheaths can absorb moisture from the humid air and change mass by 5–10% within hours. Field trials in paddy rice routinely show coefficient of variation for retained deposit exceeding 25%, and statistical comparisons between wetting agent treatments require at least 8–12 replicate plots per treatment to detect a 20% difference. This high variability is due to operator speed, nozzle height, boom bounce, and the patchy leaf wetness created by dew and fog. Because of this variability, controlled laboratory studies on excised sheaths are used to screen wetting agents, but their absolute values cannot be transferred directly to flooded field conditions. The operational boundary is that laboratory retention data should be generated on sheaths with a controlled water film corresponding to dew or guttation, because dry-sheath data overestimate retention.

Operational boundaries for wetting agent use in flooded paddy rice are set by water hardness, dew duration, leaf angle, and nozzle spectrum rather than by a single surface tension value. A wetting agent that performs well on rice leaf blades in controlled laboratory tests may fail on the sheath under flooded paddy conditions if the receding contact angle drops below 10–15°, if the tank water hardness exceeds 200 mg/L calcium carbonate equivalent, or if the spray is applied during a heavy dew period. Conversely, wetting agents that only reduce equilibrium surface tension to 30–35 mN/m may be sufficient for sheath coverage if the nozzle spectrum and spray volume are selected to deliver medium-to-coarse droplets with enough canopy momentum. Air-induction nozzles and polymer additives should be tested with the specific fungicide formulation because the addition of a wetting agent can alter the breakup length of the liquid sheet and shift the droplet size distribution by 10–20%. Field validation should include leaf washing within 1–2 h after application before dew redistribution, because deposits measured after overnight dew do not represent initial retention. If washing is delayed beyond this period, retention data should be reported as post-dew residual deposition rather than initial retention.