Atomiser optimisation for post-harvest fungicide application on plum pack lines

ABSTRACT

Atomiser systems of 23 pack lines were surveyed. Three systems, comprising mostly those using compressed air nozzles (60% of pack lines), those using compressed fluid nozzles (10% of pack lines) and systems using spinning disks (30% of pack lines) were identified.
Information that could assist in optimising the application of postharvest fungicides was gathered. The primary findings are presented under the five critical points identified as:

• spray deposition of fungicide onto the fruit
• condition of the pumps
• mixing of the fungicide
• suitability of the pipes transporting the fungicides
• concentration of the fungicide.

This short communication attempts to summarise information to assist packhouses with calibration and optimisation of atomiser systems, for control of decay on plums.

INTRODUCTION

Significant postharvest losses due to decay of stored plums may occur as a result of grey mould (Botrytis cinerea) and brown rot (Monilinia laxa). Application of fungicides can be highly efficient for control of postharvest decay of plums. Successful decay control, without exceeding maximum residue limits, relies on appropriate and optimised fungicide application, dosages and methods. Little research has been done in this field, since the first testing of atomisers for postharvest fungicide application on plum pack lines in SA (Taylor, Dodd and Chambers, 1992). Therefore, this survey was conducted on pack lines in plum packhouses in the Western Cape, in 2017 and 2018, to ascertain the conditions under which fungicide application technologies are currently used. This was done to identifying factors important for optimisation of the application systems that are used in the South African stone fruit industry.

MATERIALS AND METHODS

Atomiser systems of 23 plum packing lines were surveyed in 2017 and 2018, to determine which fungicide application technologies were prevalent, and to gather information on conditions and specifications applied.

FINDINGS AND RECOMMENDATIONS

Application systems
Three atomiser systems were identified, namely: compressed air nozzles (60% of pack lines), compressed fluid nozzles (10% of pack lines) and spinning disk systems (30% of pack lines). While all three systems have strengths and weaknesses, they all were capable of delivering satisfactory fungicide deposition. Therefore, a single system cannot be promoted for use in stone fruit pack-houses. It was clear that good calibration and regular maintenance of the atomiser systems play a critical role in ensuring effective functioning. The general design of the atomiser systems surveyed is shown in Figure 1.

Critical points
Presented below are the critical points of atomiser systems that are considered important for optimal fungicide deposition on plum pack lines, with as-sociated support information to assist users. More comprehensive details are available in Hortgro Re-port PL-16-EXP-PH03 (De Kock and Wilson, 2018).

Figure 1: The general design of the atomisers surveyed on plum pack lines.

CONDITION OF THE PUMPS:

• Pumps and connections should be fully serviced, regularly.
• Hourly checks of pumps and connections are required whilst the atomiser system is in operation.
• Ready access to a reserve pump is a worthwhile con-tingency measure.

MIXING OF THE FUNGICIDE:

• With Fludioxonil as an active ingredient, fungicides are formulated as a suspension concentrate and water insoluble particles settle out, if the solution is not frequently agitated.
• Mix the fungicide very well before application, by sha-king the product bottle vigorously before decanting. Do this to ensure the active ingredient is in full suspension.
• Mix the correct volume of fungicide with a small amount of water, stir well then add and mix with to the correct water volume, taking care to strictly follow label instructions.
• Make sure to agitate the fungicide solution properly before and during use, to prevent settling out of active ingredient at the bottom of the holding tank.
• Ideally, automatic stirrers should be installed into holding tanks to mix the fungicide solution throughout the application process.
• If fungicide solutions are held overnight in holding tanks, the product must be thoroughly mixed before the first application of the day, as well as after any stoppages.

SUITABILITY OF THE PIPES TRANSPORTING THE FUNGICIDE:

•Since the fungicide applied is generally a suspension concentrate, particles tend to deposit inside pipes, which may lead to clogging. But this can be overcome through preventative measures.
• Pipe diameters of >10mm were sufficient to overcome build-up of particles and subsequent obstruction of flow of the fungicide solution.
• Thoroughly flushing the system with warm water on a weekly basis should effectively remove product sediment from pipes.
• A filter that must be regularly cleaned can also be placed in the line between the pump and connecting pipes, in order to capture particles, if frequent blockages occur with nozzles.

CONCENTRATION OF THE FUNGICIDE:

• It is important to use fungicides as stipulated on the label, to ensure decay control efficacy and to prevent exceeding the Maximum Residue Level (MRL), which is5.0 mg/kg fruit for Fludioxonil.
• Fungicide residues on fruit must be determined as part of essential regular atomiser system calibrations, to establish if sufficient fungicide is being applied and to ensure that levels are within the MRL requirement.
• If residues are on the low side (< 0.5 mg/kg fruit), it’s best to increase the application volume slightly before increasing concentration (De Kock, 2019).

SPRAY DEPOSITION OF THE FUNGICIDE ONTO THE FRUIT:

•It is important to calibrate atomiser systems, to achieve the desired performance. This is required at the start of the season, and at regular intervals thereafter. Also re-calibrate, if any substantial equipment changes are made.
• Good coverage is essential to achieve effective decay control, since the fungicide solution must reach target areas such as stem-end cavities and other depressions such as wound indentations. This is why dip systems are more effective for decay control, but in the case of plums, impractical to use (Taylor, Dodd and Chambers, 1992).
• To assist with calibration of atomisers the spray deposition uniformity can be assessed using water-sensitive paper cards that turn from yellow to blue on contact with solution droplets (Figure 2).
• To evaluate spray deposition by the nozzles or disks located on the spray boom, the water-sensitive cards should be placed on the pack line, in three positions representing both sides and the centre of the spray area over the roller conveyer system. See the example of cards placed adjacent to each other for purpose of the photo, which in this instance, indicates sub-standard deposition (Figure 3). The same treatment duration as would be the case with fruit must be applied (typically 8-13 seconds on the pack lines surveyed).

Figure 2: The surface of the yellow water-sensitive paper cards turns blue through liquid contact.

Figure 3: An example of fungicide spray deposition at the sides and middle of a pack line, where nozzle placement seemed fine for optimal delivery, but the boom was not enclosed in a chamber and was in proximity to an open door. Spray drift caused by air flow resulted in greater deposition of product on the right hand side of the conveyor belt. The average Fludioxonil residue on fruit in this case was 1.72 mg/kg fruit.

• The flow rate of fungicide through the atomiser heads on the same boom may vary substantially. Consequently, manual settings must be carefully done during calibration.
• Use of a single atomiser head was not sufficient to give adequate fungicide deposition on any commercial plum pack line.
• To optimise spray deposition from a uniformity point of view, it may be necessary to adjust the number of atomiser heads, the spacing of the atomiser heads and the height of the heads above the pack line.
• It is important to enclose the atomiser heads in a cabinet or chamber (Figure 1) to prevent spray drift, which can be caused by air flow (Figure 3).

AIR FLOW

• In the case of Fludioxonil, a flow rate of 1.2 L fungicide mixture/ ton of fruit is required, with the mixture made up as per the label instruction.
• It is imperative that the atomiser heads switchoff each time the pack line stops, to avoid over- application, but that agitation of the fungicide in the holding tank continues. Failure to do this could result in exceeding the MRL.
• It must also be mentioned that uptake of fungicide, directly from wet rotating conveyors, also contributes significantly to coverage on the fruit surface and, hence also to the ultimate residue levels. Therefore, it’s imperative that both deposition and residue are checked, to optimise atomiser systems.
• In Figure 4, an example is shown of good spray deposition achieved using atomisers with nozzles driven by a hydraulic compressed fluid system. For comparative purposes, an example of spray deposition from spinning disk atomiser heads is shown in Figure 5.

Figure 4: Spray deposition on a pack line where an atomiser with nozzles driven by a hydraulic compressed fluid system was used. While there was slightly less deposition in the top-right of the area beneath the applications chamber, deposition was good overall. Fludioxonil residues of 1.89 -1.06 mg/kg fruit were recorded.

Figure 5: Spray deposition on a pack line where an atomiser with a spinning disks system was used. The spray deposition pattern is clearly different from the compressed air and/or fluid nozzles. The droplet size was larger than delivered with nozzles, and in this instance the distribution was even, but insufficient. The Fludioxonil residue on the fruit was 2.19 -1.79 mg / kg fruit.

ACKNOWLEDGEMENTS

Thanks to the South African Stone Fruit Producers Association (SASPA) for financing this research, and HORTGRO for placing the research at ExperiCo (Agri-Research Solutions).

V.A. DE KOCK:
ExperiCo (Agri-Research Solutions), Stellenbosch, SA; email: info@experico.co.za I.

WILSON:
ExperiCo (Agri-Research Solutions), Stellenbosch, SA; email: info@experico.co.za
IDA WILSON CONSULTING, Stellenbosch, SA; email: ida@idawilsoncon-sulting.com