Foliar Fungicide

For over a decade, foliar fungicides that are effective against myriad diseases that infect soybeans have been available.

In this project, the foliar fungicide input was the only one of the five that, when omitted, resulted in a yield reduction, which was about 3 bu/acre. This was so even though threshold levels of foliar diseases did not appear in any year of the study.

Thus, these results do not indicate whether this yield increase resulted from the control of low levels of several diseases that were present at below-threshold levels, or from the “plant health” effects that are touted by manufacturers and researchers.

This yield enhancement by foliar fungicides when only low levels of foliar diseases were present confounds recommendations. First, sound IPM practices dictate that pesticides only be applied when the targeted organism is present at a yield-limiting level. Second, repeated use of the fungicides when not needed to control diseases increases the selection pressure on fungal pathogens, and this will hasten resistance development such as that now occurring in the frogeye leaf spot causal organism.

According to the project’s Principal Investigator, “There will be a time when farmers will need these chemistries to defend against serious disease problems. Burning through them now for marginal economic gain may not be a prudent long-term strategy”.

Take-Home Message

Foliar fungicides should be an input component for soybeans growing in a high-yield environment when diseases are present at yield-limiting levels. The small yield and/or economic gain realized from their use when only low levels of diseases are present such as in this study will hasten resistance development in fungal organisms. The long-term negative effect from this latter occurrence far outweighs any small short-term gains realized from their annual use as a “plant health” enhancer.

In the Midsouth, foliar diseases frequently are present at yield-limiting levels. Thus, applying foliar fungicides is likely to be a more common practice for Midsouth soybean producers. This will entail using a thorough scouting protocol to ensure that the anticipated yield gain from controlling these diseases is worth the possible hastening of resistance development that may occur from the increased frequency of their application.

Composed by Larry G. Heatherly, Oct. 2012, larryheatherly@bellsouth.net