Spray Deposition Quality in Coffee Production

Foliar spraying is one of the most universally practised agronomic interventions in coffee production, yet the quality of spray deposition remains one of the least standardised and most poorly understood variables in farm management. Spraying is the delivery mechanism for a substantial proportion of the inputs applied in coffee agriculture. Fungicides for the management of coffee leaf rust (Hemileia vastatrix) and other foliar fungal diseases, insecticides for pest pressure, and increasingly, foliar nutrition programmes targeting macro- and micronutrient supply during critical reproductive windows. In most producing regions, this means backpack sprayers, typically 16–20 litre units operated by hand or motorised pump, wielded by farm workers across terrain that is often steep, densely planted, and difficult to access. For some larger and flatter coffee growing regions, it means tractor-trailed specialised sprayers. The practice is near-universal. The understanding of what constitutes a quality spray application is not.

Introduction

 

Spraying is the delivery mechanism for a substantial proportion of the inputs applied in coffee agriculture. Fungicides for the management of coffee leaf rust (Hemileia vastatrix) and other foliar fungal diseases, insecticides for pest pressure, and increasingly, foliar nutrition programmes targeting macro- and micronutrient supply during critical reproductive windows. In most producing regions, this means backpack sprayers, typically 16–20 litre units operated by hand or motorised pump, wielded by farm workers across terrain that is often steep, densely planted, and difficult to access.

The practice is near-universal. The understanding of what constitutes a quality spray application is not.

In the majority of cases, farmers apply whatever nozzle is supplied with the sprayer, operate at whatever pressure feels comfortable, and follow label dilution rates with limited understanding of how droplet size, spray angle, walking speed, and canopy architecture collectively determine how much of an applied product actually reaches its intended target. The result is a system that is chronically wasteful with excess product lost to drift, run-off, or evaporation before it contacts the crop. Compounds that miss their biological target altogether because the operator did not know whether to spray the upper leaf surface, the lower surface, the shoot tip, or the fruit. And spraying programmes that fail not because what is added is wrong, but because the delivery is inadequate.

This problem is compounded by a structural knowledge gap. Most commercial spray nozzle design and application science has been developed for broadacre agriculture. Cereals, oilseeds, and row crops, where canopy architecture, wind exposure, and ground speed are fundamentally different from the conditions of a coffee farm. Guidance specific to coffee, particularly guidance that links application targets (the biological objective of the spray) to the physical parameters needed to achieve them (droplet size, volume median diameter, spray angle, pressure, adjuvant type), is scarce.

A secondary but equally important gap concerns tank mixing. Farmers routinely apply multiple compounds across a growing season in separate operations, a fungicide spray one week, a foliar nutrition programme the next, without awareness that many of these inputs can be combined in a single tank mix without antagonism or phytotoxicity. Often they can even lift the effect of one another. The labour cost of separate spray passes is substantial, particularly on smallholder farms in steep terrain, and represents a significant inefficiency that better mixing knowledge could address.

This paper describes field trials designed to begin filling these gaps. The trials characterises the effect of nozzle type, spray pressure and walking speed, and adjuvant use on deposition coverage and droplet size distribution across coffee canopy architecture.

2. Background

2.1 Spray deposition and droplet size

The effectiveness of any foliar application depends on two interrelated factors: how much of the spray reaches the target surface, and whether the droplet characteristics are appropriate for uptake or activity at that surface.

Droplet size is conventionally expressed as the Volume Median Diameter (VMD, in microns, µm), the droplet diameter at which half the spray volume is carried in smaller droplets and half in larger ones. In practice, droplet size governs three outcomes simultaneously: drift potential (finer droplets drift further), canopy penetration (finer droplets penetrate dense canopy more effectively), and surface retention (larger droplets are more prone to bounce and run-off on waxy or hairy leaf surfaces).

The American Society of Agricultural and Biological Engineers (ASABE) classifies sprays into size categories from Very Fine (<100 µm) through to Ultra Coarse (>665 µm). For foliar applications targeting leaf surfaces in tree crops, the practical optimum typically sits in the Fine to Medium range (100–300 µm). Coarser droplets, which is what most standard flat-fan nozzles produce at the pressures commonly used with backpack sprayers, trade penetration and retention for reduced drift risk, a trade-off that makes agronomic sense in open broadacre settings but is poorly suited to the dense, multi-layered canopy of a mature coffee plantation.

Coffee presents additional complexity because the relevant target changes depending on what is being applied and for what biological purpose. A fungicide targeting H. vastatrix, which sporulates and infects from the abaxial (lower) leaf surface, must achieve meaningful penetration of the underside of leaves throughout the canopy, which is a demanding requirement that standard spray nozzles frequently fail to meet. A foliar nutrition programme delivering boron and zinc to support flowering requires coverage of young tissue and active meristems at shoot tips. A fruit-stage spray targeting berry borer (Hypothenemus hampei) or post-flowering nutrition requires coverage of the fruit surface itself. Each target demands a different combination of droplet size, spray angle, and application volume.

2.2 The role of adjuvants

Adjuvants are compounds added to a spray solution to modify its physical or biological behaviour at the target surface. They include surfactants (which reduce surface tension and improve spreading), stickers (which improve raindrop resistance and surface adhesion), penetrant adjuvants (which increase cuticle permeability and foliar uptake), and buffering or compatibility agents (which stabilise tank mixes and manage solution pH).

In the context of coffee foliar applications, adjuvants serve two primary functions. First, they improve surface retention and spreading on the waxy surfaces of coffee leaves, surfaces that are inherently resistant to aqueous spray solutions. Second, they can enhance the uptake of actives and nutrients across the cuticle, improving the efficiency of each application. The practical consequence is that adjuvant use at recommended rates typically allows application volume and active ingredient concentration to be reduced without loss of efficacy. A direct economic and environmental benefit.

Despite this, adjuvant use on smallholder coffee farms is uncommon, partly due to cost awareness and partly due to a lack of understanding of their function and how to use them.

 

2.3 The case for tank mixing

A standard coffee farm spraying programme may include four to six fungicide applications per season, four to six foliar nutrition passes, and additional insecticide applications as required by pest pressure. Executed as separate operations, this represents a substantial labour commitment, particularly significant in mountainous terrain where a single spray pass may require several hours of walking per hectare.

Tank mixing, which is the combination of two or more products in a single spray solution, offers the potential to reduce the number of spray passes without compromising the efficacy of individual components, provided that the products are physically compatible (no precipitation, stratification, or viscosity change), chemically compatible (no degradation of actives), and biologically compatible (no antagonism between modes of action and no phytotoxic interaction on the crop).

The primary risks in tank mixing are antagonism where one compound reduces the efficacy of another, and phytotoxicity where the combination causes crop damage at doses that would be safe when applied individually. Both risks are manageable through systematic jar testing prior to mixing and adherence to manufacturer guidance on product sequencing and concentration limits. Doing small tests on a few trees is a smart move.

2.4 The coffee canopy as a spray target

A mature arabica coffee canopy presents specific challenges for spray penetration. The multi-tiered plagiotropic branching structure creates a canopy of considerable density, and the orientation of leaves on productive branches means that upper surfaces intercept most of a downward spray while lower surfaces, which are biologically significant for fungal infection and several pests, receive substantially less coverage. Studies in other tree crops have consistently shown that standard hydraulic nozzles applied with conventional backpack sprayers achieve poor lower surface penetration in dense canopy conditions, and that nozzle selection and adjuvant use are the primary levers available to improve this.

3. Materials and Methods

3.1 Trial sites

The trials were conducted across four commercial arabica farms in three countries. Costa Rica (Central Valle), Colombia (Quindio), and Honduras (Marcala). Farm altitude ranged from 1,400 to 1,950 m a.s.l.. Varieties included Caturra, Catuaí, and Castillo. All sites were managed under standard good agricultural practice for their respective regions, with existing fungicide and foliar nutrition programmes in place prior to the trial.

3.2 Materials

Nozzle treatments

Three nozzles were compared:

• Standard nozzle (control): The flat-fan nozzle supplied as standard with the backpack sprayer used at each trial site (varied by brand, but all within the conventional flat-fan hollow-cone category typical of smallholder equipment in each country).

• POMA canopy nozzle: A specialised nozzle developed by POMA for full canopy coverage in tree crops, producing a finer droplet size and a wider spray angle to improve penetration into dense canopy structure.

• POMA fruit nozzle: A specialised nozzle developed by POMA optimised for fruit-surface coverage, producing a medium-fine droplet with a spray pattern designed to maximise contact with fruit surfaces across branch architecture.

3.3 Methods

Settings

Three pressure/walking speed combinations were tested for each nozzle:

• Setting A: 2.0 bar / 1.0 m/s

• Setting B: 3.0 bar / 1.0 m/s

• Setting C: 4.0 bar / 1.0 m/s

Adjuvant treatments

Each nozzle × setting combination was tested with and without a non-ionic surfactant adjuvant at 0.05% of the total spray volume.

Spray solution

All treatments used a standard foliar nutrition mixture as the spray base: urea (N source), zinc sulfate (zinc source), and Mantrac (manganese source), made up to volume in clean water with pH adjusted to 6.0.

Deposition assessment

Spray deposition was quantified using two methods: water-sensitive paper (WSP) cards placed at standardised positions within the canopy (upper and lower leaf surfaces at three canopy heights: top third, middle third, and lower third of the tree, and a fluorescent tracer added to the spray solution, with subsequent fluorescence quantification from a UV Photo Box. The primary outputs for Trial 01 were:

• Spray coverage (%) on adaxial (upper) and abaxial (lower) leaf surfaces across canopy thirds, as well as on fruits.

• Spray uniformity index (coefficient of variation across canopy positions)

• Effect size of adjuvant addition on each metric above

4. Results

4.1 Droplet size by nozzle and setting

Table 1. Volume Median Diameter (VMD, µm) by nozzle type and pressure setting

All three nozzles showed the expected inverse relationship between pressure and VMD. The standard nozzle produced droplets classified as Medium to Coarse across all settings (278–349 µm), while the POMA canopy nozzle produced Fine droplets across all settings (122–175 µm). The POMA fruit nozzle produced Fine to Medium droplets (178–225 µm). At Setting B, the POMA canopy nozzle produced droplets 54% smaller by VMD than the standard nozzle (148 µm vs. 325 µm),  a difference of sufficient magnitude to substantially alter canopy penetration behaviour and surface retention characteristics.

4.2 Spray coverage by nozzle, setting, and adjuvant

Table 2. Adaxial (upper) leaf surface coverage (%) — mean across canopy thirds

Table 3. Abaxial (lower) leaf surface coverage (%) — mean across canopy thirds

Lower surface coverage was constrained across all nozzle types, reflecting the inherent challenge of penetrating a dense, multi-layered coffee canopy with a hand-held sprayer. Nevertheless, the POMA canopy nozzle produced a consistent and statistically significant improvement over the standard nozzle on abaxial surfaces. 17% vs. 12% without adjuvant, and 19% vs. 14% with adjuvant at Setting B. While these absolute values underscore the difficulty of achieving complete lower surface coverage in coffee canopy conditions, the relative improvement of 42–57% over the standard nozzle is of direct practical significance for fungicide applications targeting H. vastatrix, which infects exclusively via the abaxial surface.

Table 4. Fruit surface coverage (%) — POMA fruit nozzle vs. standard

4.3 Effect of adjuvant across nozzle types

Adjuvant addition produced a consistent positive effect on surface coverage across all nozzle types and settings, contributing a mean uplift of approximately 2–3 percentage points on both adaxial and abaxial surfaces. While modest in absolute terms, this improvement is consistent across all treatment combinations and represents a meaningful gain relative to the narrow absolute coverage ranges observed. The relative benefit of adjuvant use was comparable across nozzle types.

Table 5. Spray uniformity (coefficient of variation, %) across canopy positions — Setting B with adjuvant

Uniformity data reveal a dimension of performance not captured by mean coverage values alone. The POMA canopy nozzle achieved substantially lower coefficients of variation on both surfaces. 15% adaxially and 17% abaxially, compared to 22% and 28% respectively for the standard nozzle. This indicates that the canopy nozzle not only deposits more on the abaxial surface on average, but does so far more consistently across canopy positions. For fungicide applications where protection depends on continuous coverage rather than average coverage, this uniformity advantage is agronomically significant: an uneven distribution of deposits, regardless of the mean, creates unprotected windows through which infection can establish.

5. Discussion

5.1 The nozzle is the primary intervention point

The results confirm that nozzle type is the single most influential variable in spray deposition quality. The VMD data tell the clearest story. The standard nozzle at Setting B produced droplets of 325 µm while the POMA canopy nozzle at the same setting produced droplets of 148 µm, a 54% reduction. This difference in droplet size has cascading effects on canopy penetration, surface retention, and the uniformity of deposition across canopy positions. All of which matter more than any other parameter in determining whether a sprayed compound reaches its biological target.

The gap in abaxial surface coverage, 12% vs. 17% without adjuvant, 14% vs. 19% with adjuvant, is consistent with the droplet size data. It is worth contextualising these absolute values: coverage in the 12–19% range reflects the genuine difficulty of penetrating a mature arabica canopy with a hand-held backpack sprayer, regardless of nozzle type. This is not a failure of a particular nozzle; it is a feature of the system. What matters agronomically is the relative difference and the uniformity of what is deposited. A POMA canopy nozzle achieving 19% abaxial coverage with a CV of 17% provides meaningfully better fungicide protection than a standard nozzle achieving 14% coverage with a CV of 28%, even though neither figure looks impressive in isolation.

This has direct implications for fungicide efficacy. H. vastatrix infects exclusively via the abaxial surface; a spray programme that leaves large, systematically unprotected areas of that surface, as the standard nozzle's uniformity data indicate, is structurally vulnerable to disease establishment regardless of the chemistry applied.

5.2 Pressure settings

The relationship between pressure and VMD is actually already well-established. Higher pressure produces finer droplets. In the context of backpack sprayer applications in coffee, finer droplets improve canopy penetration but also increase drift potential, a concern both for off-target movement of agrochemicals and for loss of spray volume before it contacts the canopy. This is a trade-off that needs to be considered in the individual production system.

5.3 Adjuvants

The consistent positive effect of adjuvant addition across all nozzle types and surfaces, a mean uplift of approximately 2–3 percentage points on both adaxial and abaxial surfaces, confirms the agronomic case for adjuvant use as a standard component of coffee spray programmes. The gain is modest in absolute terms, but it is consistent and cost-negligible relative to the value of the compounds being delivered.

The more important finding is what adjuvants cannot achieve. They cannot compensate for fundamentally mismatched droplet size from a poorly selected nozzle. The abaxial surface coverage of the standard nozzle with adjuvant (14%) remained below that of the POMA canopy nozzle without adjuvant (17%). The uniformity advantage of the canopy nozzle, a CV of 17% abaxially versus 28% for the standard, is not addressed by adjuvant use at all. The sequence of priority is therefore unambiguous: correct nozzle first, correct settings second, adjuvant third.

5.4 Matching spray parameters to biological targets

A theme running through the data is the importance of matching spray parameters to the specific biological target of each application. This is not currently reflected in standard extension advice in any of the three trial countries, where spray guidance is typically limited to dilution rate and timing, with no specification of nozzle type, pressure, or adjuvant requirements.

The following target-parameter relationships emerge from the trial data and the broader spray science literature:

Fungicide (leaf rust — abaxial infection target): Requires fine droplets (VMD 150–200 µm), wide spray angle, and adjuvant use to achieve meaningful abaxial coverage. The POMA canopy nozzle with an adjuvant is the appropriate tool. The standard nozzle is not fit for purpose for this application.

Foliar nutrition (shoot tip and young tissue target): Requires fine to medium droplets (VMD 150–250 µm) with good canopy penetration. The POMA canopy nozzle with an adjuvant is appropriate. Adjuvant addition improves cuticle penetration and uptake, particularly for compounds with limited mobility such as zinc and boron.

Fruit-stage applications (fruit surface target): Requires medium droplets (VMD 180–230 µm) with spray geometry optimised for fruit-surface contact rather than leaf penetration. The POMA fruit nozzle with adjuvant achieved 24% fruit surface coverage compared to 19% for the standard nozzle with adjuvant, a 26% relative improvement that, combined with more uniform deposition geometry, translates to more consistent coverage across the fruit-bearing branch architecture.

6. Practical Recommendations

Based on the findings, the following recommendations are proposed for coffee spray programmes in the smallholder and commercial contexts, where a backpack sprayer is used, represented by the trial farms.

Nozzle selection: Replace standard backpack nozzles with purpose-designed nozzles appropriate to the application target. The performance differential between standard and specialised nozzles is too large, and too consequential for product efficacy, to be managed by any other parameter adjustment.

Pressure and walking speed: Operate at medium pressure (3.0 bar) and a consistent walking speed of 1.0 m/s. Mark pressure gauges or settings at the correct setting. Where terrain makes consistent walking speed impractical, reduce application volume targets accordingly.

Adjuvant use: Include a non-ionic surfactant adjuvant at the recommended concentration as a standard component of all foliar spray applications. The cost is negligible relative to the agrochemical cost of the spray being applied and the labour cost of the application.

Application targeting: Apply fungicides for rust management with equipment capable of meaningful abaxial surface coverage. Apply foliar nutrition with equipment capable of canopy penetration. Do not use the same nozzle and settings for all applications and assume equivalent results.

7. Conclusions

The data presented here make a straightforward case. The majority of spray applications in coffee are underperforming, and the primary cause is not the product being applied but the delivery. Nozzle selection, operating pressure, and adjuvant use are accessible, low-cost interventions that collectively produce large improvements in deposition quality. In the case of fungicide applications targeting the abaxial leaf surface, the gap between standard and optimised spray equipment is not agronomically acceptable.

Tank mixing, approached systematically through jar testing and adherence to compatibility findings (or testing), offers a practical route to significant labour efficiency gains without compromising compound efficacy or crop safety.

Both of these considerations point in the same direction: substantial yield and quality improvements, and reductions in agrochemical inputs and labour costs, are available to coffee farmers without changes to the products they use. Only to how they are delivered.