Improving Spray Coverage In Field Tomato

Dr. Jason S.T. Deveau, Application Technology Specialist, OMAFRA – Simcoe; Michael Celetti, Plant Pathologist Program Lead, OMAFRA – Guelph

Spraying field tomato in August is difficult – period. The crop canopy gets dense and holds in humidity, making it an ideal place for nasty diseases such as late blight. Adding insult to injury, it is difficult to penetrate a dense canopy with fungicide spray and this often leads to inadequate disease control.

Working in a market garden operation in Bolton, the spray coverage from four different nozzle configurations was compared. After calibrating the sprayer to ensure it was operating optimally, we used the growers typical spray parameters: a travel speed of 4.5 kilometres per hour (2.8 miles per hour), an operating pressure of about 4 bar (60 pounds per square inch), a boom height of 45 centimetres (18 inches) above the ground, and a sprayer output of 550 litres per hectare (about 60 gallons per acre).

To monitor spray coverage, water sensitive paper was placed throughout the tomato canopy. This paper is bright yellow until it contacts moisture (such as spray droplets) when it turns blue (see figure 1). As a generality, good coverage is about 85 distinct droplets per square centimetre in the hardest-to-reach portion of the canopy. For each trial, we placed papers on the upper surface, deep inside the canopy near the ground and within clusters of fruit.

Figure 1 – Water-sensitive paper at top of tomato canopy
Figure 1 – Water-sensitive paper at top of tomato canopy

This particular sprayer was equipped with an air assist sleeve (see Figure 2) that blew a curtain of air into the canopy at about 100 kilometres per hour (65 miles per hour) as indicated by an air speed monitor placed at the air outlet. The purpose of the air assist sleeve is two-fold: to rustle the leaves and make holes in the canopy for spray to enter, and to prevent small droplets from drifting by blowing them downward. We sprayed using the four different nozzle configurations, with and without air assist.

Figure 2 – Boom sprayer with air assist sleeve operating
Figure 2 – Boom sprayer with air assist sleeve operating

Here is a relative description of what we observed (see Table 1):

Table 1 – Relative Coverage and Drift from Four Nozzle Configurations with or without Air Assist

Nozzle Type / Sprayer Output With Air Assist Without Air Assist
80 degree flat fans /

~550 L/ha (60 g/ac)

Good coverage in upper canopy

Moderate/poor canopy penetration

Low drift

Good coverage in upper canopy

Poor canopy penetration

Moderate drift

80 degree air induction flat fans /

~550 L/ha (60 g/ac)

Inconsistent upper canopy coverage

Poor canopy penetration

“No” drift

Inconsistent upper canopy coverage

Poor canopy penetration

“No”/Low drift

Twinjet dual 80 degree flat fans /

~550 L/ha (60 g/ac)

Good coverage in upper canopy

Moderate/poor canopy penetration

Moderate Drift

Good coverage in upper canopy

Poor canopy penetration

Moderate/High drift

Hollow cones /

~750 L/ha (80 g/ac)

Good coverage in upper canopy

Good canopy penetration

Low drift

Good coverage in upper canopy

Good/Moderate canopy penetration

Very High drift

After inspecting the papers deep in the canopy, we agreed that the air assist did not appear to improve canopy penetration, but it did do a great job of reducing drift by driving small droplets downward. Water sensitive paper cannot resolve droplets smaller than 50 µm, so it stands to reason that because the droplets were not drifting, they must have been blown into the tomatoes. In other words, even though we couldn’t see it, coverage was likely improved with air assist.

The air induction nozzles performed poorly in this situation, giving splatters of spray rather than even coverage, and resulted in very little canopy penetration. Twin fans and conventional flat fans were both inconsistent with inner-canopy coverage. The twin fans contributed to higher drift but otherwise produced coverage similar to the conventional flat fans.

The best results were achieved with hollow cones. Admittedly, the tips we used had an output of about 750 litres per hectare (80 gallons per acre) which was higher than the other nozzles by about 200 litres per hectare, but the spray distribution throughout the canopy was superior, with very little indication of drenching or run-off (see Figure 3). When we attempted lower volumes using the hollow cones the spray still gave consistent coverage throughout the canopy, but it did not achieve the threshold of 85 drops per square centimetre. Further, winds were only about 5 kilometres per hour (3 miles per hour) during the trials. Using such low volumes in windier conditions means less spray will get to the crop before it is whipped away on the wind.

Figure 3 – Acceptable spray coverage deep in canopy using hollow cone nozzles
Figure 3 – Acceptable spray coverage deep in canopy using hollow cone nozzles

In conclusion, the penetration of the spray droplets into a dense canopy such as that of tomatoes at this time of year is extremely important to control tomato diseases such as late blight. The hollow cone tips created smaller droplets at higher outputs than flat fans and improved canopy coverage and penetration. They are, however, very prone to drift and their use is not recommended without an air assist sleeve to counter the spray drift.

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