Fungicide Application Season Is About to Start

Fungicide Application Season Is About to Start

Many Iowa corn fields will soon be tasseling, which means fungicide application season is almost here.

The 4th of July weekend is here, and it looks like we'll have a fair amount of corn tasseling in Iowa pretty soon—that means fungicide application season is about ready to begin. In talking with growers and ag chemical dealers the past week or so, based on all the rainfall across the state and the wet soils, this is shaping up to be a big year for fungicide application. They also say many farmers are "on the fence" and trying to decide whether to spray or not. Clarke McGrath, who writes the Corn/Soybean Insight column each month in Wallaces Farmer magazine, offers the following observations.

DISEASE WATCH: Conditions so far this summer are favorable for development of crop diseases. Plants in waterlogged soil and those with bruised stems and injuries from hail are more vulnerable. "We're not saying you should spray ever acre with a foliar fungicide. But farmers need to be watching fields for signs of disease," says ISU's Clarke McGrath.

"For soybeans, we are still pretty early, not much disease pressure yet," says McGrath, an Iowa State University Extension field agronomist based at Harlan in western Iowa. "And we are a little ways off from late R2/R3 growth stage of the bean plant, which is when we typically want to see the fungicides applied. So keep scouting the beans for diseases and other pests like aphids, Japanese beetles, etc. to be sure they aren't a problem yet. We haven't seen much activity over here in western Iowa with bugs either. If things look good now in your fields, my advice is to delay fungicide applications on soybeans until later."

Best time to apply fungicides for corn is fast approaching
What about applying fungicides for corn? What's the best timing? "Most everyone agrees that the number-one factor impacting fungicide application profitability is management of common diseases such as gray leaf spot, common rust, etc.," says McGrath. "If crop diseases are present, yield responses to applications are typically higher on hybrids that have low disease resistance scores. If disease levels are high enough, hybrids with solid disease resistance may respond well, too."


Warm, humid conditions around the time of grain fill favor the development of foliar diseases on corn. Crop history and crop residue levels can contribute, too. "With several pathogens that survive in corn residue, corn-on-corn and other high-residue systems can increase disease levels," he explains. "Geography can also influence disease. For example, southeast Iowa tends to be warmer and more humid than much of the state and historically has had higher levels of foliar diseases. While sometimes we see fungicide applications increase yields in fields with low disease pressure, increasing disease pressure is a better indicator to the potential profitability of treating with fungicide."

Timing of application is very important for fungicides
Application timing can influence the odds of a positive return. Combining label recommendations and field observations is critical, says McGrath. "If fungicide is applied too early, the residual effects of the product may be gone as diseases set in. If applied too late, the fungicide may not effectively control diseases that are already established. Most agronomists agree that the full tassel stage (VT) through blister stage (R2) is the optimum timing if a fungicide is needed."

Scout corn hybrids that have a moderate to low disease resistance more intensively, he advises. Scout more often if the weather's warm and humid, and if rainy weather is present or predicted for July and August. Watch corn-on-corn, high-residue fields closely. Also, late-planted corn is often more susceptible to diseases. "And don't cut the application rate of the fungicide product. Follow the label instructions," he adds. Click on this link More info in an interesting Iowa Soybean Association Newsletter article here.

Fungicide application—ground rig vs. aerial spraying
Of course in corn most applications are done by airplane, but in soybeans farmers have more flexibility with ground rigs, says McGrath. "So wheel the traffic impact on bean yields is a common question we get. Purdue has a tech sheet that talks about this issue—they put it together including Iowa data with help from ISU Extension ag engineer Mark Hanna." Click on this link to view the Purdue information, showing how wheel track damage made from ground applications can impact soybean yields.


Sprayer wheel traffic from first flower (R1) growth stage of soybean plants through harvest can damage soybean plants and reduce yield (Hanna et al. 2008). ISU research suggests that an adequate soybean stand (more than 100,000 plants per acre) planted in late April through mid-May can compensate for wheel tracks made when a field is sprayed at R1. Yield loss can occur, however, when wheel tracks are made at R1 or later in thin soybean stands (less than 100,000 plants per acre) or late planted soybeans. "After the various hail storms we've had this year, we have a lot of bean fields that have both thin stands and later planted beans," notes McGrath.

Regardless of stand, in the university studies the plants could not compensate for wheel tracks made at R3 (early pod development) or R5 (early seed development). The average yield loss per acre is based on sprayer boom width (distance between wheel track passes). "In the ISU trials yield losses averaged 2.5, 1.9 and 1.3% when sprayer boom widths measured 60, 90, and 120 foot, respectively," says McGrath. "Multiple trips along the same wheel tracks did not increase yield loss over the first trip."

Choose the application method that best fits your operation
So, to address the "ground vs air" question regarding fungicide product performance—research in the Midwest is limited. Some good work done by the Iowa Soybean Association's On-Farm network on corn showed only a .2 bushel per acre difference between ground vs air application of pesticide. "Experience tells us that both methods of application can be very effective on insect and disease pests in corn and soybean fields," says McGrath. "Operator/pilot skill and ensuring the right spray parameters for each type of application is a big player in how well the pesticides will work. Bottom line is you should choose the method to use that fits your operation the best, both are good options."

TAGS: USDA Extension
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