No, Wooster, OH, didn't see any Asian soybean rust infestations this year.

“If we had, we would have made national news,” says Richard Derksen, USDA Agricultural Research Service (ARS) agricultural engineer. However, that didn't deter Derksen and his colleagues at USDA and Ohio State University (OSU) from determining what spray equipment would have worked optimally, had their fields been infected.

The Ohio researchers tested 10 spray application methods to find out what equipment performed best at applying spray droplets and covering soybean leaves near the middle and bottom plant canopy. Since rust starts low on the plant and progresses upwards, the best chance to control the disease is to attack it near the ground, before it causes yield damage, explains Erdal Ozkan, OSU Extension agricultural engineer. The difficulty, however, is determining the best spray equipment that will penetrate a tall, dense canopy and also provide good spray coverage to the lower leaf tissue.

“Penetrating a tall canopy and delivering adequate spray coverage to the middle and bottom leaves can be difficult,” says Derksen. “However, from what we've seen, it helps to open the canopy up somewhat to get the spray down deeper than standard nozzles and spray booms are typically able to penetrate.”

During the Ohio spray study, soybean plants were at the R5 growth stage — a time when plants are still flowering and susceptible to many fungal diseases, including rust. The soybeans averaged 3-4 ft. high and were densely planted. In order to measure spray deposits and to calculate spray coverage near the lower leaf canopy, the researchers placed water sensitive paper and sheet metal plates throughout test plots at about 1 ft. and 2 ft. heights.

“The single most important factor affecting the control of Asian soybean rust is to get a thorough coverage of the plant with the fungicide, which is much different and more challenging than spraying for weeds and insects,” emphasizes Ozkan. “In most spraying situations, we'll be fairly satisfied if we can achieve a good uniformity of coverage on the horizontal plane. Unfortunately, with soybean rust we are going to be concerned with both the horizontal as well as the vertical distribution of the fungicide on soybean leaves.”

In the study, the spray rigs operated at about 7 mph at a 15 gpa application rate, unless a slower speed was required to obtain a fine spray quality.

Three different tools consistently performed best at penetrating the dense canopy and providing spray coverage to the middle and lower leaf area in the study: An air-assisted sprayer, a premixed air and liquid sprayer, and a conventional spray boom with mechanical “canopy opener.”

The “canopy opener” is a horizontal bar attached to the spray boom that travels 1 ft. below and 1 ft. in front of the spray nozzles. USDA-ARS and OSU engineers designed the canopy opener to mechanically tip the top portion of soybean plant over enough for spray droplets to reach the middle and lower leaf area.

The mechanical canopy opener ranked second in the study to a Jacto air-assisted sprayer in providing the best spray coverage to the bottom canopy. It ranked third best in providing spray coverage to the middle canopy. Only the Jacto air-assisted sprayer and a Top Air boom sprayer equipped with AirJet nozzles (premixed air and liquid) performed better in providing spray coverage to the middle canopy.

Still, the engineers aren't ready to recommend this prototype mechanical canopy opener to all farmers just yet. “The canopy opener was good, but it was only tested on a 12-ft. boom length,” says Heping Zhu, another USDA-ARS agricultural engineer, who provided the idea to develop the canopy opener. “It might not be feasible on a longer boom. It might bounce too much or break without strengthening the boom supports.”

Even more importantly, the Ohio researchers are reluctant to make any recommendations for now because they haven't tested these spray systems on a rust-infected field. “The question that we still need to answer is how much spray deposition and coverage is enough, particularly in the lower canopy, to control Asian soybean rust,” says Zhu. “If a conventional spray boom and conventional spray nozzles provide enough coverage, then there would be less need to invest in other, non-traditional spray equipment.”

Air-assisted spray technology does have the potential to more efficiently apply rust control products and reduce drift than a conventional sprayer, however. “We do know that you get a good spray deposition and you could possibly reduce pesticide usage if you are using an air-assisted sprayer to control rust,” says Zhu, “but for now, we don't have the efficacy data to back that up.”

The study did provide enough deposition and coverage data, though, to predict which spray application tools would likely do best at controlling rust. “It's very clear that an air-assisted sprayer is the best equipment option we have, if our goal is to achieve the best coverage of soybeans with fungicides,” says Ozkan. “Unfortunately, a sprayer with air-assist technology may add from $10,000-15,000 to the price tag of the equipment. However, this one-time cost may well outweigh the income lost due to soybean rust in just one growing season.”

The researchers also collected enough data to predict which spray tools will likely fail to deposit enough fungicide in the middle and bottom leaf area to adequately control rust.

“The flatter, more horizontal spray patterns aren't as effective in penetrating tall, dense crop canopies,” says Zhu. “Sprays that are directed across the top of the canopy have a greater chance of the foliage intercepting the spray before it can penetrate.”

According to the study, a hollow cone nozzle (TX-18), a dual nozzle body (Turbo Duo with QJ90-TT11002) and a twin-pattern nozzle (TwinJet60-8004) failed to perform as well as extended range (XR) flat-fan nozzles in penetrating the canopy and covering the lower and middle leaf areas. “The vertical velocity was slower for the Turbo Duo and TwinJet nozzles than for extended range, flat-fan nozzles,” says Zhu, “because they were not directed straight down into the canopy.”

Spray quality also made a difference. “Smaller droplets typically mean slower-moving droplets,” says Derksen. “A conventional boom spray with small droplet sizes will produce a lot of drift-prone droplets.”

Among the conventional boom sprayer treatments, a medium spray quality, produced with an XR8004 nozzle, delivered the best spray coverage to the lower leaf canopy than fine or coarse spray qualities. In fact, the medium spray quality from the XR8004 nozzle on the conventional boom performed equally to the Top Air sprayer in providing spray coverage to the bottom canopy. In comparison, the conventional boom sprayer with an XR8002 nozzle, producing a fine quality spray, provided the worst spray coverage on the bottom leaf canopy of any spray equipment tested.

When examining canopy penetration alone, however, “the study shows that an extended range, flat fan nozzle is a pretty good choice,” says Derksen. “Air-assisted and standard flat-fan nozzle treatments provided the best overall canopy penetration.”

In general, big droplets also performed better than small droplets at penetrating the upper foliage. “Smaller droplets are better for improved spray coverage, but not necessarily for penetrating the crop canopy,” says Derksen. “A very fine spray quality, without air assistance, is very difficult to get down deep in the canopy.”

Yet, when taking into consideration spray coverage as well as penetration, big droplets failed to perform as well, even when combined with air assist technology, says Derksen. “Big spray droplets are more difficult to move around compared to smaller droplets,” he points out. “So, it's better to use the small droplet sizes with air assist technology.”

Speed is another variable that could greatly alter spray coverage and penetration. “Speed kills performance,” says Derksen. “If you go too fast, deposits are reduced lower in the canopy.”

Although this year's Ohio spray study provided a significant amount of new information on equipment performance, the researchers say more study would help to determine better recommendations. “Different spray techniques provide a different quality or amount of spray deposition inside the canopy,” says Zhu, “but we're still learning what coverage and deposition is needed to adequately control the disease.”

Until rust actually appears in one of their studies, spray equipment researchers don't know for sure what will really happen. For example, soybean “fields with taller, denser canopies might require more fungicides than fields with shorter, thinner canopies,” says Zhu. “And you might be able to use less fungicide with proper equipment that provides more spray coverage in the bottom canopy.”

Still, the study gave a good indication of how best soybean rust could be managed, if it occurred here, says Derksen. It also provided good information on spray tools to avoid when dealing with this disease.

Nozzles, Sprayers And Operating Conditions Used In Spray Tests
Treatment Pressure (psi) Speed (mph) Flow (gpm) VMD (micron)* Spray Quality
Jacto air-assisted sprayer 154 7 0.35 118 Fine
TopAir sprayer 42 7 0.35 N/A Medium
XR8004, boom sprayer 31 7 0.35 321 Medium
XR8002, boom sprayer 42 4 0.20 180 Fine
XR8005, boom sprayer 20 7 0.35 384 Coarse
Turbo Duo, boom sprayer 31 7 0.35 376 Medium
TwinJet, boom sprayer 31 7 0.35 286 Medium
Hollow cone, boom sprayer 54 7 0.35 171 Medium
Canopy opener, boom sprayer 31 7 0.35 321 Medium
*Volume medium diameter (VMD) is droplet size measured in microns.
Source: Ohio State University

Aphids, rust and stem rot — good-bye?

If you're anxious about soybean aphids, Asian soybean rust or Sclerotinia stem rot (SSR) in soybeans, don't worry. This year's spray equipment studies in Ohio may help pave the way for more effective control of all three pests, says Richard Derksen, a USDA Agricultural Research Service agricultural engineer, based in Wooster.

Just as rust starts on the lower leaf canopy and progresses upward, so do other soybean diseases and insects. For example, “Soybean aphids are particularly a problem down in the lower canopy and on the underside of leaves,” says Derksen. “So attaining good spray coverage on the underside of leaves could be valuable for both soybean aphids and Asian soybean rust control.”

The results of this year's rust study should help to determine what equipment works best to reach the underside of leaves and therefore help to control both pests, he points out. “The canopy opener might help with coverage on the underside of leaves, because it bends the top of the plant forward and changes the surface angle of the leaf during the spray application,” says Derksen. “The whole plant is bent over a little bit.” He adds that air assist equipment also helps deliver spray product to the underside of leaves. “It stirs up the leaves and makes them flutter,” he says, “which creates an opportunity for cover-age on more surfaces during the spray application.”

Just as in previous spray studies in Ohio, this study showed that slower travel speeds and air-assist equipment provided the best spray deposits for SSR control, compared to conventional broadcast application methods, says Derksen. This year, however, the canopy opener showed another possible use for stem rot control. This year's study also showed that an extended range, flat-fan nozzle, operated at a medium spray quality, also did well in penetrating a tall, dense crop canopy and providing spray coverage throughout the canopy, which could also prove valuable for stem rot control.