Foliar fungicide applications on soybean

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Most areas of eastern and central Kansas have had plenty of rain since mid/late July and the soybean yield potential went from poor-yielding hay to 40 bushels/acre. Recent rainfall and cool temperatures (for August), have resulted in a flush of newly developed nodes, flowers and developing pods in the upper canopy of most soybean fields. Now the question is: With all the rain and cool weather, should we be protecting the yield potential with a foliar fungicide? The answer is: Possibly.

We have conducted several soybean fungicide and insecticide trials across eastern and central Kansas the last few years. The decision to spray is not always an obvious one.  Sometimes we can get a large yield increase (greater than 5 bu/acre) while sometimes there is no response (or even a negative response). To make the situation even more confusing, the yield response of soybean to a foliar fungicide doesn’t always seem to be tied to the visible presence of a disease, either. However, current weather conditions present an opportunity to potentially observe a yield increase.  

Yield results from fungicide tests

When yield potential was low at 8 environments experiencing heat and drought in 2012, we found yield unchanged with a fungicide application alone (Tables 1 and 2). In 2010/2011 when yield potential was greater, there was again no significant difference in soybean yield from a fungicide. Depending on the price of fungicide and application cost, it may be hard to get a return on investment from fungicides with anything less than a yield increase of 2 bushels per acre, especially with soybean hovering around $12/bu.

Table 1. Effect of fungicide and fungicide+insecticide applied on soybean at the R2-R3 stage:

K-State tests, 8 environments in 2010-11 and 2012

 

2010-11

2012

 

Yielda

Difference

Yield

Difference

 

Bu/acre

Check

39.5 b

0

26.9 a

0

Fungicide

41.2 ab

1.7

27.5 a

0.6

Fungicide + insecticide

42.0 a

2.5

27.6 a

0.7

aMeans followed by the same letter are not significantly different at p=0.05.

 

Table 2. Effect of foliar fungicides on soybean yield when applied at the R3-R4 stage:

K-State tests, 11 environments, 2007-2012

Site

Year

Untreated Check

Fungicide at R3-R4

 

 

Yield (bu/acre)

Leavenworth Co.†

2007

40.4

45.2

Marshall Co.

2007

53.7

53.6

Republic Co. (May planted)

2010

54.1

55.0

Republic Co. (June planted)

2010

45.2

45.5

Brown Co.

2011

62.0

58.8

Franklin Co.

2011

21.0

23.2

Shawnee Co. (rainfed)

2011

55.4

58.2

Shawnee Co. (irrigated)

2011

62.7

66.2

Jefferson Co.

2012

39.7

41.3

Pottawatomie Co.

2012

33.0

33.3

Riley Co.

2012

39.3

39.1

Average

 

46.0

47.2

Average

 

46.0

47.2

† The Leavenworth County site was the only site planted in 7.5-inch rows and where the yields were statistically different at the p=0.05 level of significance. All other sites were planted in 30-inch rows.

If a producer chooses to take a chance on a fungicide, applications should be made around the R3 growth stage, near the end of blooming. Soybean R3 growth stage is when a 3/16” pod has formed at the node of the 4th fully emerged trifoliate from the top of the plant. The type of fungicide is also important. Most soybean yield increases in K-State trials have been seen with the strobilurin class of fungicide as opposed to the generally less expensive triazoles. Products containing strobilurin fungicides include Headline, Priaxor, Quilt XCEL, Quadris, Stratego YLD, Approach, and others.

One caveat to applying strobilurin fungicides is they will often cause a delay in maturity. This can be important as we have some later-planted soybean in Kansas this year and development is already behind. If a significant enough delay in maturity occurs with a fungicide application, then this could put the crop at higher risk of yield reductions from an early freeze this fall. 

Therefore, planting date and relative maturity of the soybean planted may be an additional factor to consider when deciding whether to spray with a strobilurin fungicide. Delayed maturity may be less of a risk on earlier-planted and earlier-maturity soybean than on later-planted and later-maturity varieties.

Considerations for insecticide applications

Beginning pod formation (R3) is a critical time for soybean yield determining factors. At this stage and beyond, insect feeding becomes increasingly important to monitor.  Detrimental pod-feeding insects to scout for include corn earworm, stinkbug, and bean leaf beetle. If and when fields ever dry out, producers should be scouting for these insects and decide whether an insecticide is warranted. 

The economic threshold for corn earworm is 1 per foot of row (30-inch rows). For stinkbug and bean leaf beetle, the threshold is 1 insect per 3 feet of row (30-inch rows). If these thresholds are met, an insecticide may be justified. A significant yield increase of 2.5 bu/acre was found for fungicide+insecticide applications in 2010/2011 (Table 1). The general pod and leaf feeding insect pressure at the time of application in those tests was 1 insect per 2 feet of soybean row across the 8 environments. 

In 2012, the fungicide+insecticide treatment provided no yield increase. However, pod and leaf feeding insect pressure at the time of application in 2012 was only 1 insect per 200 feet of soybean row across the 8 environments, well below treatment thresholds. This demonstrates the value of scouting for insects prior to insecticide applications in soybean.

Summary

Ultimately it will be up to the producer and whether to gamble on a fungicide or insecticide application. The cost of product and application charge may exceed $20/acre so the producer will need to see more than a 1.67 bu/acre yield increase to break even at $12/bushel soybean prices. Potential yield loss from wheel traffic of ground spray equipment is also a factor and will need to be considered when calculating break-even yields. Always read and follow all label restrictions and directions.


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