Big corn crop, soybean rust: Ag engineer offers fungicide rules

The Mid-South's massive shift to corn coupled with early Asian soybean rust nipping at the soybean crop's heels mean fungicide applications are on the list of farmer concerns. Roberto Barbosa says there are some fundamental rules for the application of fungicides, whether the target is ASR or a corn disease.

“What's very critical is coverage,” says the LSU AgCenter agricultural engineer. “With ASR and many other diseases, the disease typically starts at the bottom of the plant and works its way up.”

  • The first rule: get the fungicide to the field on time. Many of the prescribed fungicides for ASR have protective modes of action. Others are curative. “What's usually recommended is a fungicide be applied prior to the disease's arrival. So it's absolutely critical to get the timing right.”

  • The second rule: ensure good product distribution. Good product distribution on the lower canopy is critical but that presents a big challenge. “The canopy acts as a barrier, an umbrella that catches most of the treatment. The lower you go in the plant, the less product hits the target.”


In many ground sprayers lacking proper maintenance, Barbosa and colleagues find leaks, plugged or worn nozzles, even different nozzle types along the boom. Good maintenance is key to achieve proper product distribution.

“Good maintenance on the sprayer includes checking nozzle flow accuracy and boom uniformity. A good way of doing that is to compute an average boom flow and replace any nozzle with an output of plus/minus 10 percent difference from the average.

“Also, check for worn nozzles and clean all screens and filters. But be careful how these things are done.

“Clean the equipment with an old toothbrush. Avoid contamination by never using your hands or mouth to clean sprayer parts. Be very careful with the source of water used. Screens and filters are there for a reason. Using clean water will increase operation efficiency and equipment longevity. Just don't go into the field with a sprayer that isn't in very good operating condition.”

Check application rates constantly. “Use your preferred method of calibration, but do it several times during the season. The use of an electronic rate controller doesn't exempt you of calibrating your equipment.”

Droplet size

Droplet size is also a key to proper fungicide application. Droplet size is dependent on nozzle type and operating pressure and can range from extremely coarse to fine. Extensive research has shown that a good deposition is achieved — especially in the mid- and bottom-canopy — medium-sized droplets.

“We're very careful not to rule out certain nozzle types. We're not saying, ‘You must use Nozzle A over Nozzle B.’ But we are advocating whatever nozzle chosen must be able to produce droplets in the medium range (250 microns to 350 microns). If you're producing that kind of droplet, the chances of getting a good distribution increase tremendously.”

What are the pros and cons of a medium droplet versus a fine or coarse droplet? “If you select a nozzle with an orifice that produces too many fine droplets, you'll increase the chances of drift. That's one of the first things a producer wants to avoid. He wants to put the product where it needs to be rather than be blowing it around. That's like blowing money into the air, not to mention environmental and legal problems.”

Large droplets

On the other end of the spectrum are large, coarse droplets. These droplets generate poor (uneven) distribution. Parts of the crop will be treated and others won't.

“Always remember: when droplet size is increased so does the volume carried in the droplet. So if I go from a 250-micron droplet to a 500-micron droplet — from a medium to coarse — I'm doubling the diameter. But the volume has been increased by eight-fold. That won't generate the necessary number of droplets per centimeter-square.”

Barbosa says there are several questions typically linked to that fact. First is what pressure should be used.

“If the correct droplet size is being generated (250/350 microns), increased pressure can aid the droplets to get deposition in the canopy.

“But if you generate an improper droplet size, even an increased pressure won't aid in the deposition. This is a game where you must make sure you're using the right nozzle that's generating the right droplet.”

But, as a rule, pressure alone won't translate into better deposition without proper droplet size.

“That must be made very clear. I don't want anyone getting the notion ‘well, regardless of the nozzle, I'll just crank it up to 100 psi to get needed deposition.’ That is not true.”

A more controversial issue involves numerous reports claiming if the volume of application increases then, by default, the deposition will be increased. That's not always true, insists Barbosa.

“Again, you must be generating the correct droplet size. I've seen producers doing very good jobs at 5 gallons per acre and others doing good jobs with 15 gallons per acre. The problem in using such a wide volume range is it must be analyzed by each particular operation. Can he afford to spray 15 gallons per acre when applying a fungicide?

“Depending on the timing, you want the application to be a fast, efficient process. Tripling application rates means it's likely there'll be more refilling time and efficiency will drop. So, there's a trade-off and I've visited with many farmers using just 5 gallons per acre.”

One way to ensure the sprayer generates the desired droplet size is to add drift reduction agents. “Those products help by decreasing the production of fine droplets and increase the medium volume diameter of the spray.”


There have been studies done in South America and the United States checking the optimum volume for application of fungicides. Initially, the thought was the applications needed as much water as possible to help guarantee distribution.

But that didn't prove true in the field, says Barbosa. “We've seen applications using low volumes — 3 gallons per acre to 5 gallons per acre — with very good distribution. Again, the key thing is a combination of good volume and the right droplet size.

“One thing they've found is when you go below 5 gallons per acre, you need to use a crop oil. You want to delay the droplet evaporation process — especially here in Louisiana where it's so hot.

“BASF, for example, with Headline recommends up to a pint per acre of crop oil in aerial application rates of less than 5 gallons per acre.”


Aerial application also benefits from the calibration process and checks for uniformity in the deposition. “We conduct calibration clinics in Louisiana for aerial applicators. The objective is to help them set up their equipment properly.

“A concerned farmer should require his aerial applicator to participate in these clinics annually.”

Water-sensitive cards are a very simple method to check fungicide distribution throughout the canopy. The cards are very effective in demonstrating spray penetration. Crop consultants should place them throughout the canopy prior to a spray.

“We can do a more comprehensive analysis of the cards, if needed. They just need to go to the county agent or me directly. We have all the equipment here to run that analysis.”

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