There are commercially available, water-soluble, non-toxic fluorescent dyes that can be used to assess where sprays have deposited. Fluorescent dyes can highlight where individual droplets have landed within the crop, into stubble or onto the soil. To be able to see the individual droplets requires the use of a black light ultraviolet (UV-A) torch or hand-held UV-A lamp that causes the individual droplets to glow in the dark.

One of the benefits of assessing spray droplets on an actual plant is the ability to see where the spray droplets have landed. Quite often droplet movement will be influenced by the size and shape of the plant and the individual leaves, which can be difficult to replicate using artificial collectors such as water-sensitive paper.

Fluorescent dyes can also be useful for evaluating how thorough a decontamination procedure has been and for training staff on safe mixing procedures.

Useful comparisons that can be made using fluorescent dyes and UV-A light sources

Assessing penetration into crop canopies and standing stubble

Changes to the nozzle type and design, spray quality, total application volume, pressure at the nozzle, boom height, spraying speed and wind speed at the time of spraying can all influence how far into the crop canopy droplets may penetrate.

Penetration can be particularly important when applying some fungicides to prevent or control diseases that may establish on the lower parts of crop plants.

When assessing the impact of any changes to the set-up of the sprayer on penetration into the crop canopy, it is a good idea to only change one thing at a time, trying to keep all other factors as constant as possible. Always try to compare like with like. For example, when assessing the impact of increasing the application volume, try to do this with nozzle types that produce the same spray quality at a similar pressure; this way you are comparing the effect of total application volume, not other factors. A similar approach can be taken to assessing the impact of spraying speed or spray quality (droplet size).

Assessing spray coverage and droplet density on various parts of the plant

One advantage of using a fluorescent dye is the ability to look at where the spray droplets have deposited on different parts of the plant. Using a black light UV-A torch, the coverage of the entire plant can be evaluated. This technique allows the applicator to look at the undersides of leaves, vertical stems and the ground, all at the same time. Using knowledge of where the product needs to land, along with knowledge

of which sprayer set-ups can maximise the spray deposits in these locations, will improve application results.

Limitations of fluorescent dyes

Fluorescent dyes are useful for determining where droplets are landing. However making direct comparison of spray coverage will generally require the applicator to make ratings or rankings of the coverage in the field (in the dark) and record these for future reference, as it can be difficult to capture clear images in the field.

The main drawback with this technique is the temporary nature of water-soluble dyes and pigments, as the assessments of coverage need to be made relatively soon after application to avoid dew or rain washing the dye off the plants. Working in the dark can also create its own issues, unless plants are removed from the field and assessed in a darkened room or area.

Where to obtain fluorescent dyes

One of the best fluorescent dyes available was developed by the South Australian Research and Development Institute (SARDI) and can be obtained through trialsupplies.com.au.

Fluorescent dyes may seem expensive to purchase. Obtaining enough fluorescent dye to treat an area up to one hectare may cost more than $150, depending on the size of the container purchased. Before purchasing or using the dye the spray operator needs to carefully plan what comparisons are going to be made and maximise the number of comparisons that can be made from one tank mix. Before starting the application, it is important to have all of the equipment required on hand, such as extra sets of nozzles.

Find out more

For more information: SARDI's fluorescent dye and UV torch can be purchased through Trial Supplies Trial Supplies

Where to obtain UV-A torches or lamps

For use in the field, spray applicators can obtain light-emitting diode black light UV-A torches relatively cheaply or by searching online for suppliers. UV-A torches are useful for looking at individual plants, however they may not illuminate a large enough area to compare several plants at a time.

Specialised handheld lamps and floodlights (e.g. the ‘Lambino’ lamp), which are able to illuminate much larger areas in the field, can also be purchased. However, these are typically too expensive for individual spray operators to consider purchasing. Lambino lamps normally have to be purchased from specialist distributors of UV light sources or through scientific equipment suppliers. It may be possible to access one on loan via chemical company representatives or machinery dealers, or for a grower group to purchase one to be shared between group members.