Farm Journal Test Plot demonstrates how the disease triangle impacts the effectiveness of fungicide application timings.
As producers become more proactive than reactive to disease management, the fundamentals of the disease triangle become increasingly important. The three sides of the disease triangle—a conducive environment, a present pathogen and a susceptible host—drive the decision of when (or even whether) to spray fungicides.
"Farmers ask if they could shift their application timing to V5 or split applications to V5 and R1," says Missy Bauer, Farm Journal Associate Field Agronomist. "But it really depends on the disease triangle. All three of these elements must be aligned to have disease present, and there must be some kind of disease pressure for a fungicide to be able to protect or increase yields."
Bauer recently concluded a three-year Farm Journal study examining the impact the disease triangle has on the effectiveness of fungicide and various application timings. The first year of the study included irrigated corn on corn and corn following soybeans, but the final two years focused exclusively on irrigated corn on corn, the environment ripe for higher disease pressure and in turn, a higher response to fungicide application. That’s because the disease inoculum can overwinter on old corn residue—and the steady availability of water is more likely to roll out a welcome mat for disease.
"Every year, we ran replicated, randomized trials to evaluate the hybrids’ response to fungicide and the timing of application," Bauer explains. "Each year, we used two hybrids—one that is identified by disease ratings as being more susceptible to disease and one with more resistance."
Three application timings at various growth stages were included in the trial: only V5, (five-collar), only R1 (silking) and both V5 and R1. Each was compared with a check of no fungicide. A high-clearance, self-propelled sprayer delivered all applications.
The wettest and most humid of the three years, 2010, provided the opportunity for the strongest response to fungicide use. When the two hybrids were averaged, compared with the check, the application at V5 was 6.2 bu. more; the application at R1 was 9.3 bu. more; and the two timings combined yielded 11.1 bu. more.
All Farm Journal Test Plots are harvested with calibrated yield monitors, and each bushel is weighed by grain carts outfitted with scales.
"This year  emphasized how important the disease triangle is to fungicide results. We saw early season anthracnose pressure, late-season heavy pressure of northern corn leaf blight and gray leaf spot," Bauer says.
The following year, 2011, brought overall moderate disease pressure. When the yields were averaged for the hybrids, compared with the check, the application at V5 was 1.3 bu. more; the application at R1 was 8.8 bu. more; and the two together were 6.9 bu. more.
In the drought of 2012, Bauer continued the study despite lower disease pressures. When both hybrids were averaged compared with the check, the application at V5 was 1.5 bu. more; the application at R1 showed no response; and the two timings were 1.8 bu. more.
"The hot, dry conditions with low humidity provided a poor environment for disease," Bauer explains.
Overall, the three test years with three wide-ranging in-season environments emphasized the importance of paying attention to the disease triangle when making fungicide decisions.
"You can’t adjust any crop management practice without paying attention to the agronomics behind the decisions," she adds.
Built-in defense. Hybrid selection also played a key role in the plot yields. That’s because one hybrid had a stronger response to the application based on the characteristics of the host.
For example, in 2010, the hybrid response to fungicide ranged from 6.7 to 12.1 bu., averaged across all three application timings. Within that, the individual hybrid response to timing varied from 0.3 to 14.1 bu.
In 2011, the hybrid response to fungicide ranged from 4.4 bu. to 6.8 bu., averaged across all timings. Within that, the individual hybrid’s response to application timing varied from no response to 8.9 bu.
In 2012, one hybrid showed no response to fungicide, while the other hybrid averaged a 4.2 bu. yield bump. That average includes everything from no response to a 7.6 bu. increase.
"It’s worth noting that the disease presence in the field and the susceptibility of the hybrid have a tremendous effect on the impact of a fungicide application," Bauer says. "Everything comes back to the disease triangle, and it’s important to understand how susceptibility factors play into that."
Harvest view. Fungicide impact carried through fall. "The higher the response to the fungicide, the higher moisture content we saw in the grain," Bauer says. "The response to fungicide kept the plant greener longer, which resulted in slower dry down. Keeping plants healthier for a longer period of time also helped kernel fill."
The yield component most affected by a fungicide application is the kernel depth. Having a high ear count allows for the application to have the greatest impact on yield, which is why Bauer advocates to know ear count.
"A fungicide application improves the kernels on the ears out in the field, but it won’t fix an ear count problem," she says.
"Looking at all three years, we can’t say there’s an effect to making two applications so that one plus one equals three," Bauer notes. "We’ve seen the most consistent yield response to the application at R1, with the exception of 2012, when there was little disease pressure."
That said, Bauer notes the application at V5 can’t be discounted. In 2010, with wet conditions and high humidity, the application at V5 showed a strong yield response, so keeping those plants healthy earlier in the season might have led to the higher yield response. Also, when scouting reveals early season disease pressure, a V5 application can provide early season protection.
No matter what, Bauer recommends not only scouting for disease but knowing the ear count before considering a V5 fungicide application.
In the end, the most successful fungicide decisions key off the disease triangle and recognize that the pieces of the puzzle vary year by year.
Thank You to Our Test Plot Partners
Each Farm Journal Test Plot is a cooperative effort. Our thanks go to: AGCO, Ed Barry, David Webster, Luke Olson, Reid Hamre and Lindsey Pettyjohn; Farm Depot and Mark Laethem; Orthman Manufacturing and Adam Souder; Unverferth and Jerry Ecklund; BASF; Bayer and Dennis Clark; Syngenta; North Concord Farms, Dick Dobbins Jr. and Kevyn Van Wert; B&M Crop Consulting, Vicki Williams and Megan Tomlin.
You can e-mail Margy Eckelkamp at firstname.lastname@example.org.
During the multiyear study, one of the key takeaways is the role of the environment in the disease triangle. The results varied from a wet year (2010), to a year with average disease pressure (2011) to a drought year with little to no disease pressure (2012). These average yield increases are for two hybrids used in the three treatments compared with a check of no fungicide.