In field-grown indeterminate tomato systems, canopy closure transitions from a semi-open row configuration to a dense vertical leaf volume when leaf area index (LAI) exceeds 3.5 m²/m²; Mediterranean and subtropical processing-tomato trials under drip irrigation commonly report LAI values between 4.0 m²/m² and 6.0 m²/m². Under those conditions, the upper leaf stratum acts as an aerodynamic filter: a spray droplet must traverse successive leaf layers with interleaf spacing frequently below 0.10 m, while air jets from axial-fan or tower air-assisted sprayers lose velocity through momentum exchange with foliage. The maintenance of spray coverage in the lower canopy is therefore not a sole function of nozzle classification; it is governed by droplet diameter, droplet velocity, fan outlet air speed, air volume rate, travel speed, and local leaf area density. Droplet size distribution is conventionally expressed through the volume median diameter (VMD, equivalent to Dv50), number median diameter (NMD), and relative span (Dv90-Dv10)/Dv50; a narrow relative span below 1.4 is generally associated with less droplet segregation by size during canopy transport, while a broad relative span above 2.0 may contain a large population of drift-prone fine droplets and a smaller population of runoff-prone coarse droplets. These distribution descriptors are necessary because the target VMD alone does not define the proportion of droplets in the critical range that can penetrate dense foliage, and air-assisted applications to dense tomato canopies are therefore typically specified by a target VMD band rather than by a single nozzle size or pressure setting.
Field measurements with water-sensitive paper and fluorescent tracer deposits in staked tomato systems indicate that a Medium-to-Coarse spray quality under ASABE S572.1, corresponding to a VMD of 226 µm to 400 µm, produces a measurable improvement in lower-canopy coverage when the air stream carries droplets beyond the external leaf boundary. Droplets in this band retain sufficient inertia to avoid complete streamline deflection at the first leaf surface, while maintaining a high enough droplet number concentration per unit spray volume to create deposit densities above 50 droplets/cm² on upper and middle leaflets. For a water droplet with diameter d and velocity U approaching a leaflet of characteristic width L, the Stokes number St = (ρd²U)/(18 μL) indicates whether impaction dominates; at typical air velocities of 5 m/s and a leaflet width of 0.03 m, a 250 µm droplet yields a Stokes number above 1.0, whereas a 50 µm droplet yields a Stokes number below 0.5, meaning the smaller droplet follows streamlines and may bypass the leaf entirely. In contrast, droplets above 450 µm tend to impinge on windward leaf surfaces and form coalesced deposits that run off under high spray volumes; droplets below 150 µm decelerate rapidly, and collected deposits on abaxial surfaces in lower strata become highly variable. The classification ranges in ISO 25358:2018 are obtained from static nozzle tests under standardized pressure and spray pattern conditions; they do not directly measure the effect of air shear in an air-assisted boom. A nozzle classified as Medium at 0.4 MPa may therefore exhibit a measured VMD 20 µm to 40 µm lower when exposed to fan outlet velocities above 30 m/s. Because the droplet size class is a reference condition and not an in-field guarantee, field verification with the actual tank mix and fan setting is required before using the classification for canopy coverage decisions.
Table 1 presents the six major spray quality classes under ASABE S572.1 and their general operational interpretation for air-assisted dense tomato canopies.
| Spray quality class | Reference VMD band (µm) | Operational interpretation for air-assisted dense tomato canopy |
|---|---|---|
| Very Fine | <145 µm | Not suitable for open-field lower-canopy targeting without shrouded or electrostatic discharge; high drift fraction under wind above 3 m/s |
| Fine | 145–225 µm | May improve spatial distribution on abaxial surfaces under low wind, but lower-canopy deposit becomes variable with high air velocity and elevated temperature |
| Medium | 226–325 µm | Primary working band for air-assisted tomato sprayers; best compromise between drift control and deposition if fan outlet velocity is maintained above 10 m/s at target depth |
| Coarse | 326–400 µm | Useful for dense canopies with LAI above 5.0 m²/m²; reduced drift but lower droplet number concentration at constant volume rate |
| Very Coarse | 401–500 µm | Limited to high-volume protectant applications; risk of runoff and uneven lower-canopy coverage unless air momentum is high and leaf surfaces are not near retention capacity |
| Extremely Coarse | >500 µm | Generally not appropriate for dense tomato foliage because coalescence and deposit mass per droplet exceed the upper leaf retention capacity before lower canopy coverage is reached |
ASABE S572.1 reference values may not equal field VMD under air shear; they are intended for nozzle selection, not as a substitute for in-field deposit verification.
Within a closed tomato canopy, air velocity decays with insertion depth because foliage extracts momentum from the jet; a tower sprayer with a 0.8 m diameter axial fan operating at 540 rpm PTO speed may generate outlet air velocities of 30 m/s to 45 m/s at the fan discharge, but the local velocity at 0.6 m to 1.0 m inside the row can fall below 8 m/s. This decay increases the importance of droplet diameter. Larger droplets in the Very Coarse range may be less sensitive to velocity decay, but they also reduce droplet number concentration per unit spray volume; a 500 µm droplet contains approximately 64 times the volume of a 125 µm droplet, so the number of droplets per nozzle at a constant volume rate falls sharply. At a volume application of 300 L/ha, a monodisperse 300 µm droplet population yields approximately 213 droplets/cm², whereas a 500 µm population yields approximately 46 droplets/cm²; real polydisperse spectra reduce these counts further because the larger droplets occupy most of the spray volume. The resulting deposit pattern on the abaxial side of lower leaflets may therefore fail to reach the 50–70 droplets/cm² threshold often cited for protectant fungicide coverage. Operators may compensate by increasing application volume, but dense leaf surfaces have a finite retention capacity, and volume rates above 400 L/ha can produce runoff on upper leaves before the lower canopy receives a sufficient deposit. Sprayer forward speed also controls the air and liquid dose per linear metre; at 6 km/h the canopy receives half the air exposure time compared with 3 km/h, which can reduce the penetration window for the coarse droplet fraction and increase the reliance on higher air volume to maintain lower-canopy transport.
Among air-assisted sprayer configurations used in tomato production, axial-fan tower machines, multi-head cannon sprayers, and sleeve-boom airblast units with vertical manifolds differ in how liquid is introduced into the air stream. The first configuration effect is the relative velocity between the liquid sheet and the air jet. When hydraulic flat-fan or hollow-cone nozzles discharge a liquid sheet into an air stream moving at 35 m/s, secondary atomization may reduce the final droplet diameter below the static nozzle classification. Air-induction nozzles partially buffer this effect by entraining air into the liquid before discharge; the resulting droplets have a coarser VMD under static conditions and tend to show less shift under high air shear. However, air-induction sprays may produce fewer small satellite droplets, which can limit abaxial retention if the canopy is extremely dense. Comparative observations from air-assisted spraying in vineyard and orchard canopies indicate that the relationship between static VMD and deposit uniformity is strongest when the static VMD is between 275 µm and 350 µm; transfer to tomato canopies is not direct because tomato leaf area density is higher and the crop height is lower, and published data for this specific configuration is limited. In canopies with LAI above 5.0 m²/m², a VMD closer to 400 µm may be required to overcome aerodynamic filtering, but the risk of coalesced runoff on the outer canopy increases. Liquid pressure at the nozzle should remain between 0.3 MPa and 0.6 MPa for conventional flat-fan nozzles; raising pressure to 1.5 MPa reduces droplet diameter and increases drift without necessarily improving lower-canopy coverage. Pressures below 0.2 MPa can collapse the pattern distribution of some air-induction nozzles, creating streaks in the spray swath and leaving untreated vertical bands inside the row. The same air-shear effect that reduces VMD also narrows the effective forward speed window: if travel speed is too high relative to fan output, the lower canopy receives insufficient air momentum, while if travel speed is too low, upper-canopy wetness may exceed retention capacity before the inner leaflets are wet.
On abaxial leaf surfaces of trichome-bearing tomato cultivars, the retention function departs from the smooth lamina model used in many spray retention studies. The adaxial surface of many tomato cultivars has a lower trichome density and a cuticular wax layer, while the abaxial surface carries glandular and non-glandular trichomes that intercept impacting droplets and may alter the contact angle and adhesion. A droplet that reaches the lower canopy with a diameter between 200 µm and 350 µm may be captured on a stem or petiole rather than on the abaxial leaf surface if the air jet is not aligned with the row profile. Mid-canopy placement of water-sensitive paper in commercial tomato field evaluations frequently shows a strong vertical gradient: upper samples may exceed 100 droplets/cm², while lower samples remain below 30 droplets/cm². This gradient is less a function of nozzle selection than of air carrying capacity. If the air stream cannot maintain a local velocity of at least 10 m/s at the target leaf, particles with VMD below 200 µm are increasingly carried around leaves rather than impacting them. Conversely, if the local velocity exceeds 25 m/s, smaller droplets may impact with sufficient energy but the same airflow can push leaf surfaces together, reducing the exposed area of abaxial surfaces and preventing penetration into the innermost leaflets. Air outlet direction therefore has to be matched to the row profile: in a staked tomato row with vertical trellising, the air stream should be directed slightly upward from below the mid-canopy to open the lower leaf boundary, while in a bush-type field crop the airflow may need a more horizontal delivery to avoid lifting the upper canopy and closing the lower gaps.
Conditions that lower the effective droplet diameter below 150 µm include high nozzle pressure, low liquid flow rate, high fan outlet velocity, and hot dry ambient air. Droplets in the Very Fine and Fine fractions under ASABE S572.1 have a short atmospheric lifetime. At 25°C and 50% relative humidity, a 100 µm water droplet may evaporate within several seconds when transported over long distances; at 35°C and 30% relative humidity, the lifetime is substantially shorter. Air-assisted sprayers that do not use shrouds or tunnel recovery systems release these droplets into the ambient wind; the drift-prone fraction can move beyond the treated row before depositing. In a dense tomato canopy, upper leaves may still intercept a fraction of these smaller droplets, but lower-canopy deposition becomes highly variable because the droplets follow streamlines rather than impinging. The use of electrostatic charging, shrouded booms, or recycle tunnel sprayers can modify this behaviour; without such engineering controls, a target VMD below 150 µm is generally incompatible with lower-canopy coverage in open-field air-assisted applications under wind speeds above 3 m/s. Drift measurement under ISO 22866 requires downwind collectors; the standard does not measure canopy penetration, so a low-drift result does not by itself demonstrate lower-canopy coverage. The relevant operational boundary is that fine-droplet bands should be reserved for enclosed or shaded structures where air movement is bounded and collector data confirm deposition at the target depth. Operators should also avoid combining fine-droplet nozzles with high fan outlet velocities above 35 m/s because the additional air shear can shift an already fine spectrum further downward and increase the respirable and drift-prone fractions without improving impaction on inner leaflets.
For air-assisted applications, droplet size should be measured with laser diffraction equipment conforming to ISO 13320:2020, with the laser beam aligned in the air stream where possible. Static nozzle classification alone is insufficient because air shear at the nozzle exit and the resulting droplet trajectory interactions are not captured by standard patternator tests. A field verification protocol for dense tomato canopies may include collectors placed at 0.3 m, 0.9 m, and 1.5 m above the bed, with at least three replicates per row and a minimum of 20 water-sensitive paper samples per treatment. Image analysis of water-sensitive paper by pixel classification or droplet counting software can provide deposit number density and percent area coverage, but the method is sensitive to light conditions during scanning and to the colour range of the paper. Water-sensitive paper should not be used as the sole quantitative measure if ambient relative humidity is above 90% or if spray volume exceeds the runoff threshold, because the paper saturates and individual droplet stains merge. For quantitative deposit mass, fluorometric tracers such as Brilliant Sulfaflavine or sodium fluorescein can be extracted and measured by fluorometry. The relationship between droplet count and deposit mass is not linear because overlapping droplets and coalesced deposits reduce the number of distinguishable impacts while maintaining high mass. Consequently, a lower canopy might show adequate mass per unit area but inadequate spatial coverage, especially for contact fungicides. ISO 22522:2007 provides a framework for field measurement of spray distribution in tree and bush crops; tomato row crops require adaptation of the sampling grid to the lower, middle, and upper canopy strata because the standard was not developed for low-profile, non-woody row vegetables. Sprayer calibration should include individual nozzle flow checks with a stopwatch and volumetric flask; deviation among nozzles above 10% of the manufacturer’s rated flow at the set pressure indicates replacement or cleaning. Worn brass nozzles can increase flow by 10–15% before visible pattern distortion occurs, shifting the VMD while the operator may not detect the change from the cab.
Because protectant fungicides and copper-based formulations require redistribution and contact with sporulating areas, biological efficacy in dense tomato canopies imposes a droplet-count constraint rather than a mass-only constraint. A droplet count of at least 30–50 droplets/cm² on abaxial leaf surfaces is often cited in efficacy trials, but the exact threshold varies with pathogen and formulation; systemic active ingredients may tolerate more gaps because translocation compensates for non-deposited areas. Air-assisted sprayers with a droplet VMD between 250 µm and 350 µm have been used in commercial tomato disease management programmes, but published data for this specific configuration is limited. The operational boundary is that droplet sizes above 400 µm may increase deposit mass on outer foliage while producing insufficient droplet number density in lower strata, while sizes below 150 µm may improve spatial uniformity under low wind, but they are not reliable in open-field conditions. Tank-mix adjuvants that reduce dynamic surface tension may shift the droplet size distribution toward smaller diameters, so nozzle classification should be re-checked with the actual spray mixture rather than with water alone. Coverage decisions should be based on water-sensitive paper or tracer data collected at the lower canopy, not on catalogue spray quality class; otherwise the upper canopy may appear evenly covered while the lower canopy remains below the threshold for disease control.
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