How are dosages determined?

We examine the science behind the numbers. By Anna Mouton.

Growers know the dosage rates for chemicals because the numbers are provided with the products. But how do scientists evaluate deposition parameters when they are researching ways to optimise spray application? Fresh Quarterly delved into some of the fascinating methods used by Philip Rebel of specialist agricultural consultancy ProCrop while working toward his master’s degree.

Evaluating deposition in the orchard

Researchers in Australia have developed a yellow fluorescent pigment that can be added to the tank mix for evaluating spray deposition. Trial orchards can be sprayed with the fluorescent pigment using the same equipment and methods as on any normal working day. A representative sample of leaves is removed after spraying and examined under an ultraviolet light source that reveals the yellow fluorescent pigment.

Rebel fortunately did not have to spend years hunched over a microscope to measure fluorescent deposits. Modern technology allows researchers to photograph leaves and use software to analyse the images. The software can determine the percentage of the leaf area that is covered in yellow fluorescent particles and thereby calculate deposition quantity — the amount of spray deposited on the leaf.

Deposition quality — the distribution of drops on the leaf — can likewise be worked out with the help of software. Values for uniformity are obtained by comparing the deposition quality between different leaves in the sample.

The advantage of evaluating deposition in the orchard using this method is that it is quantitative — it generates numbers that can be statistically analysed to give an unbiased evaluation of spray applications. Another advantage is that leaves can be sampled from many positions allowing deposition in the top of the canopy to be compared to that in the bottom and deposition inside the canopy to that on the outside.

Linking deposition to disease control

Knowing that a tree is being adequately covered with plant-protection products is not the same as knowing it is protected from disease. Rebel conducted a series of laboratory trials to gauge what deposition values translated into control of apple scab when applying mancozeb.

Growing a commercial orchard under laboratory conditions wasn’t an option so Rebel created a mini-orchard using seedling apple trees to produce young leaves that are susceptible to apple scab infection. He developed a spraying system that mimicked spray application in the real world. Yellow fluorescent pigment was used to evaluate deposition in the same way as he used it for his orchard experiments. Rebel was able to use deposition quantity to infer mancozeb residue levels for his different treatments with the help of previous correlations made between mancozeb and the yellow fluorescent pigment.

Rebel applied five different concentrations of mancozeb to his seedlings. He also had untreated controls.

The seedling apple trees were then challenged with the apple scab fungus — Venturia inaequalis — to test the protective effect of the range of mancozeb levels. This required growing the fungus on artificial media to generate spores. The fungus was cultured on potato dextrose agar which really is made from potatoes — but researchers use a commercial powdered medium instead of boiling potatoes.

The next step was measuring the severity of infection. Apple scab infection can be scored using a visual rating scale, but different people will tend to produce different scores. In theory you could use the same image-analysis techniques that Rebel applied to quantifying yellow fluorescent pigment. In practice this doesn’t work because apple scab lesions contrast poorly with normal tissue.

Researchers have previously found that leaf tissue damaged by apple scab has a lower temperature than healthy tissue. This is because apple scab damages the cuticle resulting in higher transpiration rates. Rebel tried thermal imaging to quantify apple scab damage which worked well but seemed to overestimate the level of infection.

As a comparative method Rebel also directly assessed the amount of apple scab fungus in leaves using qPCR — short for quantitative polymerase chain reaction. This technique quantifies the amount of fungal genetic material in a sample as a proxy for the fungus itself. Measurement by qPCR proved more accurate than thermal imaging.

The combination of deposition and residue data generated from yellow fluorescent pigment measurements and infection data generated by qPCR enabled Rebel to establish how well apple scab was controlled with different spray deposition quantities. This information can be used to evaluate whether spray deposition quantities in orchards are likely to be sufficient for control of apple scab.

This work was carried out by Rebel and co-workers under the leadership of Prof Adéle McLeod of the Department of Plant Pathology at Stellenbosch University. It was supported by ProCrop and funded by Hortgro Pome.

Image: Good spray deposition quantity and quality is characterised by even distribution of droplets on the target.

Supplied by Philip Rebel | ProCrop.