Equipping and operating sprayers to control insects and diseases in soybeans

Essential information for maximizing chemical control of insects and diseases, including white mold, in large and dense soybean canopies.

In order to maximize insect and disease control in soybeans, insecticide and fungicide droplets need to penetrate large and dense soybean canopies and thoroughly cover the leaves and stems. This article provides current Michigan State University Extension recommendations for achieving these goals.

Spray volume has the greatest impact on canopy penetration and leaf coverage. Spray volumes of 15 gallons per acre are required when applying insecticide and fungicides to soybeans through growth stage R3 (pod development). After R3, 20 gallons per acre will improve coverage.

Droplet size is the second most important factor affecting canopy penetration and leaf coverage. Research has shown that fine to medium droplets having volume median diameters (VMDs) ranging from 200 to 350 microns will provide the optimum canopy penetration and leaf coverage. All nozzle manufacturers use a common spray quality classification system which divides droplets into eight droplet size categories (Table 1.) The colors listed in Table 1 should not be confused with the color of the nozzle itself; the colors listed refer to the droplet size range and the color of the nozzle refers to the capacity.

Table 1. ASABE standard 572.1 (spray quality categories)

Droplet category

Symbol

Color

Extremely Fine

XF

Purple

Very Fine

VF

Red

Fine

F

Orange

Medium

M

Yellow

Coarse

C

Blue

Very Coarse

VC

Green

Extremely Coarse

XC

White

Ultra Coarse

UC

Black

Source: Droplet Chart/Selection Guide, Virginia Tech, Publication 442-031, P. Hipkins and R. Grisso.

Ground speed is an important consideration as it affects spray volume and vertical droplet velocity. Ground speeds of less than 10 mph are recommended.

Nozzle pressure must be considered as it affects droplet size, spray volume and droplet velocity. In general, nozzle pressures of 40 pounds per square inch (psi) are recommended. Higher pressures are OK as long as the optimum droplet size spectra is produced.

Nozzle pattern is an important factor. Research conducted by Erdal Ozkan, agricultural engineer at Ohio State University, showed that nozzles producing a single flat fan pattern provided better canopy penetration than nozzles or combinations of nozzles producing a twin-fan pattern when used in large and dense soybean canopies. Venturi or air-induction nozzles should not be used for insecticide and fungicide applications.

Consider spray volume, droplet size, ground speed and operating pressure when selecting spray nozzles. Select nozzles that produce droplet sizes near the fine end of the medium (yellow) category and deliver 15 gallons per acre (GPA) at your desired ground speed and operating pressure. The information in Table 2 shows that a sprayer traveling at 10 mph equipped with XR11005 nozzles and operated at 40 psi will deliver 14.9 gallons per acre while producing fine to medium droplets. All nozzle manufacturers provide similar information for each of their nozzles.

Table 2. Relationship between spray volume, ground speed, pressure and droplet size for Teejet XR8005 and XR11005 nozzles

Tip

PSI

Droplet size

6 mph

8 mph

10 mph

80 degrees

110 degrees

XR8005 and XR11005

15

Course

Medium

15.3 GPA

11.5 GPA

9.2 GPA

20

Course

Medium

17.3 GPA

13.0 GPA

10.4 GPA

30

Course

Medium

21 GPA

16.0 GPA

12.8 GPA

40

Medium

Medium

25 GPA

18.6 GPA

14.9 GPA

50

Medium

Medium

28 GPA

21 GPA

16.6 GPA

60

Medium

Fine

30 GPA

23 GPA

18.1 GPA

Source: Catalog 51, Spraying Systems Co.

Boom height controls spray pattern uniformity and droplet velocity. Operating the spray boom at the correct height is essential. Ozkan recommends setting the target area as midway between the lowest leaves on the plant and the top of the canopy when spraying large, dense soybean plants. Use the manufacturer’s recommendations for your nozzle spacing and nozzle spray angle to determine how high to set your boom above the target area. For example, a boom equipped with 110 degree flat fan nozzles spaced 20 inches apart should be operated 16 to 18 inches above the target area – midway between the lowest leaves and the top of the canopy. This would be 7 to 9 inches above the top of the canopy in 24-inch tall soybeans assuming the lowest true leaves were 6 inches above the ground.

Taking the time to equip and operate your sprayer properly will improve insect and disease control in large and dense soybean canopies. For information on other factors to consider before applying a fungicide for white mold management in soybeans, see “Applying fungicides for white mold management in soybeans.”

This article was produced by the SMaRT project (Soybean Management and Research Technology). The SMaRT project was developed to help Michigan producers increase soybean yields and farm profitability. SMaRT is a partnership between Michigan State University Extension and the Michigan Soybean Checkoff program.

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