Many growers used foliar fungicides on soybeans in 2005, and there were many anecdotal reports of yield gains. The reports of yield gains have piqued grower interest in foliar fungicides, and many will consider trying foliar fungicides in 2006. However, a word of caution must be offered about these anecdotal reports. We often hear about the positive responses from the latest input, but what about all of the neutral or negative yield responses?

What is the mechanism behind the yield gain?

The reasons often cited for increases in yield are protection from plant diseases, increases in plant growth efficiencies, and increases in stress tolerance. There are many foliar diseases in Minnesota that can infect soybeans. It is believed that the majority of these diseases usually have minimal or uneconomic impacts on soybean yield. Some of these diseases, such as, Septoria brown spot may be managed with foliar fungicides. Increases in plant growth efficiencies and stress tolerance appear to have been associated primarily with the strobilurin fungicides. Increases in plant growth efficiencies are thought to be due in part to inhibition of respiration and increases in stress tolerance. It is believed that some combination of these mechanisms are responsible for positive yield responses.

Aside from anecdotal evidence what do we know about foliar fungicides and yield response?

Crop management decisions based on anecdotal evidence are often costly, and decisions with substantial economic risk are best based on results from replicated research trials. Data from replicated foliar fungicide trials conducted at 65 locations across the north central region indicate applications of strobilurins (e.g. Headline and Quadris) had a positive economic return about a third of the time, however, a third of the time they had positive effects on yield but were below an economic return, and a third of the time they were associated with a yield loss. In the same set of experiments, triazole fungicides (e.g. Folicur, Laredo, Domark, Tilt) resulted in an economic return about a quarter of the time.

Are there other concerns with application of foliar fungicides?

Application and misapplication of fungicides have the potential to cause crop injury. However, it is difficult to quantify the impact of this sort of crop injury. Application of fungicides can also have negative impacts on beneficial entomopathogenic fungi. Entomopathogenic fungi (EF) are known to play a role in reducing soybean aphid populations, and fungicide application may affect EF populations which could have an impact on soybean aphid populations. Finally, there is the threat of developing fungicide resistance from the overuse of fungicide products, and there is the possibility of creating some other unintended environmental impact. Fungicide labels should be consulted prior to application to help reduce the risk of unintended consequences.

Bottom line

Foliar fungicides potentially provide an opportunity to increase soybean yield through protection from plant diseases, increases in plant growth efficiencies, and increases in stress tolerance under some conditions. Much work is underway and is planned to understand when and where economically positive benefits from foliar fungicide use on soybeans are most likely to occur. Unfortunately, the inconsistent results from fungicide trials make it difficult to successfully implement a foliar fungicide program that reliably improves yields in Minnesota. Even with the threat of Asian soybean rust, the inconsistency of soybean yield response to foliar fungicides makes their use an economically risky proposition.

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