Figuring Out Fungicide Use

COLUMBIA, Mo. (DTN) -- Spray once when conditions are right for disease and don't expect any miracles -- that's the takeaway from the 2014 Corn Disease Working Group's (CDWG) regional corn fungicide summary.

"Where we see the most economically beneficial responses and the most consistent responses are when we're using fungicides in response to disease," Purdue University plant pathologist Kiersten Wise told farmers, academics and crop advisers at the University of Missouri's Crop Management Conference on Dec. 18.

"I also want you to understand the limitations to fungicides," Wise added. "We can't use fungicides to try to recover yield from other production problems."

Wise compiled fungicide efficacy data in corn from 2014 university trials across 15 states and Ontario, submitted by members of the CDWG. She considered the timing of fungicide treatments as well as the number of applications. In corn, the result was fairly conclusive: Single fungicide applications at tasselling stage (VT) were most likely to pay off, especially when the conditions were right for diseases.

Wise also analyzed soybean fungicide data from trials in Illinois, Indiana, Iowa and Nebraska, collected between 2008 and 2014. In most of those years, foliar diseases were not significant in soybeans in those states, and less than half of the soybean fungicide applications proved to be profitable, she concluded.

Since fungicide resistance is a growing problem, particularly in the South, using these products just once and only when you need them is increasingly important, Wise added.

"It's important to remember that if we keep using fungicides for physiological benefits or we spray at a set time point each year with no regard to disease, we might not have these fungicides around for a long time, because fungi can develop resistance to these products," she warned.

CORN

Clear trends emerged in the CDWG's corn data, which covered 2010 to 2014.

On average, early season (V4 to V6) and multiple fungicide applications (V4/V6 and VT) did not dependably produce yield responses high enough for farmers to break even on the expense, Wise concluded. However, the standard fungicide timing for corn at VT produced the most consistently positive yield responses among the data points. The effect was particularly strong in high disease years like 2014, where VT applications averaged a 7.6-bushel-per-acre bump across the U.S and Canada.

Region mattered, too. Southern growers saw a higher average yield response of 10.3 bushels in 2014 from VT applications compared to Midwestern growers, Wise noted.

The reason that double applications don't perform as well is not fully understood yet, but the conclusion on early applications makes sense, Wise said. Most fungicide products have only 14 to 21 days of activity in the plant. "It's a lot to ask of a fungicide to continue to provide disease control when you get to tasselling, especially when conditions are favorable for disease severity," she explained.

Disease levels also significantly influenced yield responses between the treated and untreated corn plots. During the 2014 season, when fungicide applications were made at VT, corn plots with less than 5% disease severity by the end of the season yielded on average only a 3 bushel gain. Yet corn plots with more than 5% disease severity averaged yield responses of more than 11 bushels. Keep in mind that price charts for Indiana growers suggest at least 7 bushels are needed just to break even on a fungicide application, Wise noted.

WHEN TO SPRAY

When considering a fungicide application around VT in corn, keep in mind these disease risk factors: susceptible hybrids, continuous corn, no-till or reduced tillage practices, and irrigation, Wise said. Most importantly, look for disease-favoring weather conditions -- ample moisture, moderate temperatures and high humidity, she added.

Most farmers prefer to spray fungicides on corn after pollination, but many might be waiting too long, Wise warned. Pollination is complete when silks are just beginning to turn brown. When the entire silk mass is dark brown and dry, the optimal fungicide window for disease control may have passed.

Wise recommended assessing pollination with the "shake test," where farmers carefully unwrap the husk, shake the young ear and see if silks fall away. You can see a video of Purdue Extension corn agronomist Bob Nielsen demonstrating the test here: http://goo.gl/….

SOYBEANS

With low foliar disease incidence in most of the years of the soybean data (2008 to 2014), the study essentially determined that in the absence of disease, soybean fungicide treatments are unlikely to pay off, Wise said.

She analyzed applications at R3 and applications added to insecticide passes. Insecticide treatments actually produced more of a yield response (2 to 3 bushels on average) in the university trials than fungicide treatments alone (1 to 2 bushels on average). When sprayed together, the insecticide-fungicide passes yielded just a 2.8 bushel response on average.

"Only a little less than half the time are you actually breaking even from these solo fungicide applications in soybeans," Wise concluded.

RESISTANCE

Fungicide resistance from overuse is a current, not future problem, Wise reminded conference attendees.

Already, frog-eye leaf spot fungus has been confirmed as resistant to strobilurin fungicides in several Midwestern and Southern states, she pointed out. The problem is more widespread in the South, but it has also been found in states such as Indiana, Illinois, Missouri and Kentucky, which don't always have frog-eye leaf spot pressure in soybeans, Wise said.

Resistance development depends on the fungus and its speed of reproduction, as well as fungicides' modes of action. Some, such as the strobilurin fungicides, are particularly prone to develop resistance. But resistance is also driven by how often farmers use fungicides.

The explosion of "new" fungicide brands on the market can give a false impression of abundance, Wise warned. "It might seem as though we have all these different products to work with, but the problem is we're really only operating with two to three modes of action," she noted.

Fungicide resistance can develop in both foliar and seed-borne fungi, so resistance is a concern with both foliar and seed-applied fungicides, Wise added.

Farmers should pay careful attention to how well their fungicide applications actually control disease in their fields and report any failures to their local Extension office immediately, she said.

Emily Unglesbee can be reached at emily.unglesbee@dtn.com

Follow Emily Unglesbee on Twitter @Emily_Unglesbee.

(PS/AG/CZ)

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