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Research helps guide decisions about fungicides in soybeans

Spraying a fungicide on soybeans for a disease like frogeye leaf spot can be profitable in some varieties and not in others. The key, according Bob Williams, Extension area specialist in northwest Tennessee, is knowing the difference, information which is now available through the University of Tennessee Extension Service.

Williams speaking at the Milan No-Till Field Day, presented research collected over the last few years, which is being compiled to develop a ratings system for 277 soybean varieties.

Soybean varieties in maturity groups 3-5 are being evaluated for their level of resistance to diseases as well as the effect of fungicide on frogeye leaf spot and other foliar diseases. The research is supported in part by the Tennessee Soybean Promotion Board.

In the research, compiled by UT Extension plant pathologist Melvin Newman, varieties which indicated a high level of resistance to a specific disease received a zero rating, while highly susceptible varieties received a 10 rating. Categories were developed with a zero rated as none; 1-3, low; 4-6, moderate; and 7-10, severe. Soybean varieties with a severe rating would have the least amount of disease resistance.

Here are some of their findings:

In 2005, none of the 40 Group 3 varieties in the research scored a zero when resistance to frogeye leaf spot was measured.

The four varieties with a low rating averaged 36 bushels per acre. The 25 varieties in the moderate category had an average yield of 35 bushels while 11 varieties in the severe category made 30 bushels per acre.

The results indicated that spraying a fungicide on Group 3 varieties might not always pay for the cost of the fungicide, even in severely infested plots.

Researchers also selected 40 varieties widely planted by farmers in west Tennessee in maturity groups 3 and 4, to determine resistance levels and the benefit from spraying a fungicide at R-3. “We were under the threat of rust last year, so we did include a triazole with the application.”

In side-by-side plots for each variety were plants that had been sprayed and plants that had not been sprayed.

There were 12 Group 3 varieties, 12 early Group 4s and 16 late Group 4s. No Group 3 varieties were rated in the zero category for frogeye leaf spot, five were in the moderate category and seven were in the severe category.

When the Group 3s were sprayed, all the moderate and severe varieties moved into the low category.

Yield increased from 34 bushels per acre in the unsprayed category for Group 3s to 40 bushels in the sprayed category. This would indicate that spraying Group 3s might not be feasible.

The difference in yield from high to low in all other maturity groups ranged from 9 bushels to 16 bushels per acre.

But there was one environmental factor that should be considered, according to Williams. “Last year, it paid more to spray the Group 4s, but that was because of Hurricane Katrina. It wreaked havoc and devastation in a lot of places south of us, but it made west Tennessee a soybean crop.”

Williams said the Group 3s weren't able to take advantage of the moisture in the way the early and late Group 4s were. We had a 6-bushel yield response in the Group 3s, but if we had gotten the rains 10 days to 12 days earlier, it might have been 12 bushels.”

For Group 4s, the highest-yielding varieties in unsprayed plots nearly always occurred where disease ratings were low. “Spraying a fungicide on those varieties with a low disease rating did not provide a large enough increase to pay for the cost of the fungicide on many of these varieties. You might select one of these varieties and forget spraying it.”

Williams noted that one variety with a severe disease rating was one of the lowest yielding varieties in the test last year where that rating was taken. “But when we sprayed that variety with a fungicide at R-3, we increased yield from 37.5 bushels to 61 bushels. Is that a financial impact? You bet. Will that pay for a little fungicide? You bet.

“That's the way we want to see you use the data, to help you make an informed decision. But this is not the gospel. There are a lot of different strains of frogeye. This is a good tool to help you make a decision about which fields you may need to spray and which ones may not need a spray.”

The studies did show some anomalies. One variety had a low disease rating, “but the data showed that when you spray that soybean variety, you still get a tremendous increase in yield. Why? Obviously, we're controlling something else. We don't understand everything that's happening, but we are getting a substantial yield increase from spraying this variety.”

Growers can obtain a copy of the ratings on the 277 varieties through the University of Tennessee Extension Service. Go to for more information.

Williams also provided some tips on understanding frogeye leaf spot and the timing of fungicide applications. “Young tender leaves are very susceptible to infection from frogeye leaf spot spores, much more so than an older leaf. Frogeye is not like rust, it's not going to move 300 miles on a rain. It's going to move from right underneath the plant. It's borne in soil residue.

“This year, we've had some rain, but it has not fallen frequently. We're seeing less frogeye than we've seen in west Tennessee in several years, but it's there and if we get the right conditions, it's going to pop up on us.”

Williams said it takes 12 to 14 days after an infection before you visually see a frogeye spot. “So you almost never see young leaves with a spot on them. They may be already infected, but it will take that long before we see it.

“When we spray a fungicide, we want to spray as many of the leaves as we can. You can spray a soybean too early and have frogeye come back all over you in 14 days because the rest of the plant isn't protected.

“You want to delay your fungicide application as long as you possibly can, until you get the optimum benefit. With a good fungicide spray lasting 14 days to 21 days, you could come near to having one fungicide application that will cover and protect your entire plant.”

But “if you're at R-3 and you have two frogeye spots per leaf, it's time to get the spray rig. The R-3 stage on an indeterminate variety is when you have quarter-inch long pods four nodes above the first fully expanded leaf.

“If frogeye is not a big worry for you, but sudden death syndrome or stem canker is, you can select a variety with resistance to SDS or stem canker. It may have a weakness on frogeye, but you can spray a fungicide and eliminate the frogeye problem.”

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