6.3.2 The need for nozzle standards in cocoa growing areas
Unfortunately, many manual sprayers used by smallholder cocoa farmers worldwide are fitted only with variable cone nozzles, and few farmers know which setting to use. When screwed down to its minimum setting (i.e., a very fine spray), they produce a hollow cone spray, comparable in quality to standard fixed geometry cone nozzles.
However, even unscrewing the outer cover slightly to produce a spray jet (as commonly done when attempting to treat high branches of tree crops) results in a dramatic increase in droplet size [1].
Spray quality matters: a relatively small number of large droplets may represent a large proportion of the spray volume (that could have been turned into a large number of more efficient small droplets).
These larger droplets are highly likely to run off leaves, fall back onto the ground (‘run-off’ or exo-drift) and be wasted. This is a contributory factor to poor or variable efficacy.
The figure below illustrates the enormous variability of variable hollow cone nozzles: with a sample of three nozzles, taken from cocoa-growing areas.
Measurements were taken at a relatively high pressure (500 kPa) to emulate farmer practice when attempting to achieve a very fine spray or long throw in the ‘jet’ mode.
Not only is there a 2 to 5-fold increase in Volume Median Diameter (VMD) [2], but there is also 60-80% variation in flow rate.
Output (L min-1) and spray characteristics of three variable cone nozzles, used in different cocoa growing areas, using water with a surfactant at 500 kPa. (Note: different number rotations required to change from full cone [minimum] to maximum liquid jet settings). Spray quality is described by measured VMD (diamonds) with the 10% and 90% percentiles by volume (D[v,0.1] and D[v,0.9] as bars).
It follows that accurate calibration is impossible with variable cone nozzles. Reliably achieving recommendations on effective dosage is obviously unattainable with such equipment.
Worldwide, millions of dollars have been spent over the past 30 years in order to improve nozzle design and there are a number of established international standards for hydraulic nozzles, such as those that define their fitting to nozzle holders (ISO 8169: 1984).
Work has been done to develop a fixed geometry ‘cocoa nozzle’ by assessing suitable nozzle settings for increasing spray deposition on pods or other relatively narrow targets such as branches [3].
A narrow cone of fine spray can be achieved using disc and core nozzle combinations where a relatively small disc plate is ‘overloaded’ with an over-rated swirl plate (in terms of the more normal 80 by the manufacturer). In controlled tests, fitting combinations such as a D1.5-25 (or a D3-45 if blockage is likely to be a problem) should greatly improve dose transfer efficiency of contact fungicides for cocoa pod diseases such as Phytophthora spp. and Moniliophthora roreri.
Validation tests have been variable, with good results achieved with farmers (who presumably wish to save on fungicide bills), but difficulties with operators ‘trained’ to ‘spray to run- off’: with this idea in mind, the technique simply slowed them down waiting for run-off to occur!
Such technology is of limited value unless accompanied by training: with emphasis on dose transfer efficiency and saving money on pesticides.
Unfortunately, in many cocoa growing areas, it can also be difficult to find equipment that complies with basic requirements spraying equipment such as ISO 8169 compliant nozzle holders, so farmers are unable to benefit from the R&D described above.
