The use of narrow-angle cone nozzles to spray cocoa pods and other slender biological targets

Abstract

Cone nozzles were investigated to maximise deposition of fungicides on cocoa pods to control diseases such as black and frosty pod rots. Currently most farmers use variable cone nozzles, but efficiency and work rate need to be improved. Droplet size spectra and spray angles were compared at different pressures (100–300 kPa). Narrow cones and fine droplet spectra achieved with smaller orifice plates appear to improve dose transfer efficiency to small targets, but they are subject to a greater risk of blockage. Disc (>D1.5) combinations should minimise the risk of blockage, but flow rates and cone angles are increased for a given swirl plate. The outputs produced by discs and swirl plates from different manufacturers may vary, so an international coding standard is needed.

In a field study, fluorescein tracer residues were assessed on cocoa pods in two trials as deposit (ml m⁻² of cocoa pod surface) per unit emitted (ml s⁻¹ equivalent to spraying approximately 1 m of branch). In each trial, a “standard” configuration (D2-45 at 300 kPa) that gave a 40° Cone of spray, as used in other field trials was compared with narrower cone angles and low flow rates. The combinations DC31-25 and D1.5-25 (both at 200 kPa) that produce spray angles of approximately 25° and 35°, respectively, achieved substantially better (approximately×3) deposits on pods per volume emitted than the D2-45. Such descriptive information on nozzle outputs should be helpful for a number of applications where efficient spray deposition is required on smaller biological targets, such as narrow branches and individual fruit.

Introduction

In terms of dose-transfer to the biological target, most pesticide application is highly inefficient (Graham-Bryce, 1977). However, relating ‘ideal’ deposits with biological effect is fraught with difficulty (e.g. Hislop, 1987). In spite of Hislop's misgivings, it is relatively easy to demonstrate that massive amounts of pesticides are wasted by run-off from the crop and into the soil, in a process that Himel (1974) called endo-drift. Minimising endo-drift and maximising dose transfer efficiency to the biological target will be of greatest interest to farmers, when reduced pesticide costs can be demonstrated. Since larger (>200 μm) droplets are especially prone to bouncing off plant surfaces (e.g. Brunskill, 1956) endo-drift is usually volumetrically more significant than exo-drift, caused by down-wind movements of smaller (<100 μm) droplets, beyond the crop boundary. In many countries down-wind exo-drift has become a major issue for research and regulation (e.g. the LERAP scheme in the UK), but reducing run-off should reduce contamination of soils and pollution of ground water with pesticides.

Most cocoa farmers are small holders, who usually minimise inputs for pest and disease management, and may do nothing when cocoa prices are low. However, pod diseases such as Phytophthora megakaryia (black pod in W. Africa) and Crinipellis (Moniliophthora) roreri (frosty pod rot in Latin America) have the capacity to reduce yields by up to 80%. Farmers spray for Phytophthora spp. on a regular basis, since copper compounds and other fungicides are efficacious (Evans and Prior, 1987); this study forms part of a current effort to mitigate losses caused by Crinipellis spp. Current research into improved cocoa disease control includes work on better application methods in order to: (a) provide practical and economic techniques to reduce the use of chemical fungicides and (b) to improve the effectiveness of experimental microbial and chemical control agents.

Application of large volumes of carrier water is inefficient in terms of dose transfer, and makes spraying tedious and labour intensive. In commercial operations, the cost of labour can be reduced substantially by reducing volume application rate, and thus increasing work rate (effectively by reducing the number of tank-loads per hectare). Although low-volume spraying with motorised mistblowers was specifically developed for cocoa (Clayphon, 1971), much pesticide application takes place using low-cost, manual (hydraulic) knapsack sprayers. Hislop (1963) compared knapsack mistblower application with a pressurised hydraulic sprayer and a tractor-mounted mistblower for black pod control in Nigeria. He concluded that the tractor-mounted machine was expensive to use and gave inferior disease control compared with portable sprayers, and suggested that even knapsack mistblowers may only be economic where labour is expensive or scarce. Some authors (e.g. Pereira, 1985) believe that mistblowers are now unsuitable for small-holder farmers, due to their capital cost and the maintenance required for 2 stroke engines. If necessary, treatment of tall trees can be achieved by the use of extended booms fitted to manual sprayers, but their pumps must be adequate to achieve sufficient pressure (>200 kPa) at the nozzle. Many locally available sprayers are fitted with variable cone nozzles that produce an infinitely variable range of droplet sizes and flow rates, and are arguably (e.g. Bateman, 2003) a contributory factor to reported poor or variable fungicide efficacy.

Cone nozzles are most appropriate for applying insecticides and fungicides to complex surfaces such as dicotyledonous foliage (Matthews, 1974). High pressures can be used to create small droplet sprays, sometimes reaching 1–2 MPa (150–300 p.s.i.), but manual equipment is more typically operated at 100–300 kPa (15–45 p.s.i.). Atomisation is achieved by swirling the pesticide mixture in the nozzle, then forcing it through a narrow circular hole. Spray droplet formation often occurs at the periphery of a cone-shaped liquid sheet (Fraser, 1956). Early droplet spectrum analysis, using a laser particle size analyser, was carried out by Combellack and Matthews (1981), and manufacturers such as Spraying Systems give data on flow rates and cone angles for nozzle configurations that are likely to be used in their principal markets (e.g. on tractor booms at pressures up to 2 MPa).

Jollands and Jollands (1984) appreciated that more targeted application on cocoa trunks and pods could be achieved with careful selection of cone nozzles. They investigated the efficacy of a metalaxyl (a systemic phenylamide fungicide) and cuprous oxide mixture for control of black pod (P. palmivora). Reducing volume application rates (VAR) to as little as 65 l/ha, using a TX2 ‘Conejet’ giving a 67° Cone angle, had no effect on yield and significantly reduced the spread of black pod lesions. However, Matthews (1974) pointed out that low flow rate nozzle tips with narrow orifice sizes are prone to blockage, especially with tank mixtures containing particulate suspensions. In recent field trials, a directional spray was simulated in a reproducible way with a D2-45 fitted to a 300 kPa pressure regulating valve to give a VAR of 200 l/ha at a flow rate of 760 ml/min (Hidalgo et al., 2003). Even though this hydraulic nozzle combination had not been optimised, it achieved better cocoa pod disease control than a motorised mistblower fitted with a rotary atomiser. It appeared that a greater proportion of the spray cone had hit the biological target (flowers, pods, etc.) with less wastage of fungicide (copper hydroxide in water).

Cone nozzles are available in a number of forms:

• Standard (integral) hollow cone tips (Spraying Systems: ‘ConeJet’ and ‘VisiFlo’TX range; or Hypro Lurmark ‘HollowTip’: HCX series); these are cylindrical in form and protrude from the nozzle holder. The original Spraying Systems ‘Conejet TX’ nozzle was designed to produce an 80° spray at 689 kPa (100 p.s.i.), and ‘TY’ nozzles a narrower 65°Cone. Narrow cone TY-3 nozzles were found to be optimal on tail-booms for cotton spraying since smaller sizes were prone to blockage and larger sizes would result in unacceptably high VAR^s (Matthews, 1974).

• Full (or solid) cone tips: are more rarely encountered and are manufactured for spot applications: e.g. the Hypro Lurmark ‘FulcoTip’. Applications include pest colonies and post emergence contact herbicide spraying of plant suckers, with manually operated sprayers. They typically produce an 80° angle of larger spray droplets than the two types above, under equivalent operating conditions.

• Disc and swirl plate types: give a similar hollow cone of spray and as its name suggests come in two parts. These were originally labelled by Spraying Systems Dx-yz where D describes the disc (e.g. D2-25). Hypro Lurmark have a similar coding system, starting with the 1 mm DC02 disc (although smaller sizes are available) and similar swirl plates: thus the equivalent of the D2-25 is DC02/CR25. Commonly used orifice discs range from the D1 to the D10 (3.97 mm), the numbers from D3 upward originally signifying 64th^s of an inch, but this is not the case with D1, D1.5 and D2 (see Table 1). Discs are usually made from stainless steel (numbered) or engineering plastic such as polyacetal, which may be colour coded, although unfortunately not according to any international standard. Swirl plates (often called ‘cores’ in the USA) are made in engineering plastic, stainless steel or brass and are numbered, most commonly: 13, 23, 25 and 45. For hollow cone nozzles, the first digit signifies the numbers of slot-shaped diagonal holes; the second digit indicates their size relative to other swirl plates. A hole drilled in the centre of the plate, creates a full cone spray, but the numbering system here is arcane (M.G. Baxter, Spraying Systems, pers. comm.). If a liquid jet is required, a disc can be used alone—with the swirl plate removed. In combination, discs and plates can give a range of spray angles (from 15° to 115°), depending on the combination of disc, swirl plate and pressure used.

• Proprietary cone nozzle designs are fitted as standard to certain manual sprayers. These include a Chinese design (shown in Fig. 1) fitted to low cost sprayers, which are also exported to adjacent Asian countries. Although they are not widely known in the west, the Chinese Academy of Agricultural Sciences estimates that 80 million of these nozzles are in service at any given time.

• Variable cone nozzles are often fitted to cheaper lever-operated knapsack and garden sprayers and can be set from a fine hollow cone to a jet of spray by adjusting the depth of the swirl chamber by a screw thread. Because they are infinitely variable, the spray pattern cannot be duplicated and they are not recommended for professional use (see below).

Bateman (2003) measured droplet size spectra produced from a variable cone nozzle. When screwed down to its minimum setting it produced spray sizes that gave a comparably good-quality spray relative to certain fixed cone nozzles: such as the D2-25 or a Chinese design (commonly encountered in Asia). However, even unscrewing the sleeve slightly, to produce a narrow spray jet (as frequently done when attempting to treat high branches of tree crops) resulted in a dramatic increase in droplet size, and thus potential endo-drift. Suitably fine droplet size spectra are often achieved at normal, low (<300 kPa) pressures with cone nozzles, fitted as standard by manufacturers of manual sprayers (e.g. Bateman et al., 1999). However their angles of spray (normally >70°) are visibly too wide for target cocoa pods, hence much fungicide is wasted.

This paper provides a survey of the spray output from a number of potentially useful, lower-range cone nozzles. I have focused on disc and swirl plate nozzle combinations since these appear to be the most widely and easily available means of achieving narrow cone sprays in cocoa producing countries. The objective was to identify nozzle combinations that achieve <40° (ideally <30°) spray angle with a relatively fine droplet size spectra, followed by field studies on spray recovery with cocoa pods.