Foliar Fungicides on Soybeans

Many North American soybean trials have shown a yield boost with the use of a foliar fungicide, even in the absence of significant foliar diseases. This yield boost may be a function of controlling bean diseases that have previously been ignored. Or it may result from “plant health” benefits from the application of the fungicide.

“Plant Health” Benefits

These “plant health” benefits are usually associated with the strobilurin fungicides (Headline and Quadris). Several effects have been observed, including a reduction in the amount of ethylene in plants late in the season, delaying plant senescence. Fungicides may also improve carbon assimilation, improving plant growth efficiency and increasing tolerance to stress during flowering and pod fill. Although, all the effects on the plant are not fully understood, it is clear that foliar fungicides impact the plant enough to affect yield.

Ontario Strip Trials

Are these yield benefits large enough to warrant spraying in the absence of major disease outbreaks? In both 2005 and 2006, large scale replicated field trials were conducted to assess the possible yield benefits of strobilurins fungicides on soybeans. On-farm strip trials were set up by OMAFRA, local Soil and Crop Improvement Associations, Ontario Soybean Growers and various agri-businesses across a wide variety of soil types, environmental conditions and geographic locations. Data was collected from 19 sites in 2005, and 31 sites in 2006. The majority of trials were sprayed with the fungicide at the R2 soybean plant growth stage (full bloom), as promoted in plant health literature.

Inconsistent Yield Gains

Graph #1: Soybean Yield Response to Foliar Fungicides in Ontario (2005)

Average Yield Gain of 3.6 bu/ac

Link to data equivalent table

Graph #2: Soybean Yield Response to Foliar Fungicides in Ontario (2006)

Average Yield Gain of 0.8 bu/ac

Link to data equivalent table

Economics

The results from these trials showed some yield benefit to soybeans when applying these products on a large percentage of the plots. In 2005, the average yield gain was 3.6 bu/ac, with 17 of the 19 trials (89%) showing some yield increase. In 2006, the average yield gain was only 0.8 bu/ac, with 21 of the 31 trials (68%) showing a yield gain.

The problem is that yield gains are both inconsistent and usually not high enough to be economical. The cost for strobilurins (Headline and Quadris) is approximately $16.00 per acre, excluding application costs. Assuming an application cost of $8.00 per acre, a tramping loss of 1.0 bu/ac and $7.00 soybeans, a 4.4 bu/ac yield increase would be required to break even. ($16.00 product + $8.00 application + $7.00 tramping loss).

In 2005, only 42% of the trials (8 of the 19) had a yield benefit of over 4.4 bu/ac. In 2006, only 38% of the trials (8 of the 21) had yield gains high enough for a positive economic return. In other words, 58% and 76% of the trials respectively had an economic loss.

The inconsistency of yield gain found in Ontario is similar to foliar fungicide trials conducted across many US States. Considerable research is now underway to understand when and where positive economic returns can be found with the use of foliar fungicides. Yield response may be associated with the amount of stress a plant is under, but even this theory has yet to be proven. Economic yield results have been too inconsistent when applying foliar fungicides to warrant their application in the absence of significant disease pressure.

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