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More on Foliar Fungicide Applications to Soybean

On several occasions, I have posted information on this website about using foliar fungicides to enhance soybean yield. Results reported in a recent Crop Science article [Vol. 57, p. 983-992, 2017] titled “Corn, soybean, and wheat yield response to crop rotation, nitrogen rates, and foliar fungicide application” by S. Mourtzinis et al. provide further insight into this practice. Important points and pertinent results from that research follow.

   Field trials were conducted from 2013 through 2015 within a long-term corn-soybean-wheat rotation site established in 2002 in Wisconsin.

   Two foliar fungicide treatments–fungicide [Headline in 2013 and Priaxor in 2014-15] applied at the R3 growth stage, and no fungicide.

   Across the 3 years, the foliar fungicide treatment resulted in an average of only 2 bu/acre yield increase vs. the untreated control, or a 3.3% increase.

In an Oct. 2012 blog posted on MSSOY, the following points were made pertaining to the results from USB’s Kitchen Sink Project.

   Foliar fungicide input was the only one of the five high-yield inputs 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. At southern locations (2 in Arkansas, 3 in Kansas, and 2 in Kentucky) in the study, the decrease was only about 2 bu/acre.

   The very small yield enhancement by foliar fungicides when only low levels of foliar diseases were present highlights two things. First, sound IPM practices dictate that pesticides only be applied when the targeted organism is present at a yield-limiting level, and these results confirm that. 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”.

   The Take-Home Message from that report is foliar fungicides should be an input component for soybeans growing in a high-yield environment only 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 touted “plant health” enhancer.

   In the Midsouth, foliar diseases frequently occur 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.

There is no doubt that foliar diseases can and do cause significant soybean yield losses in the Midsouth. There is also no doubt that using foliar fungicides to manage or control some of these diseases has become an important input component for soybean production, especially in potentially high-yielding environments. However, the blanket use of these products should now be evaluated differently because of the following important occurrences.

   First, resistance to fungicides is confirmed in several fungal pathogens, most notably the resistance of the frogeye leaf spot pathogen, Cercospora sojina, to the strobilurins (FRAC Code/Group 11). This has likely resulted from the overuse of these products, to include their application in situations where the target disease was not at a yield-limiting level or the application was made under the assumption that a “plant health” benefit would be gained.

   Second, results from the above cited USB project show that this blanket use of these fungicides will only occasionally result in a positive return on investment. The authors concluded that yield protection, not yield enhancement, should be the goal when using products to manage foliar diseases in soybean. As stated in a previous blog that summarized the results from this project, “foliar fungicides should be applied to soybeans in a high-yield environment when diseases are present at yield-limiting levels. Using these products on a regular basis as a yield enhancer is not economically feasible, and will hasten resistance development in fungal pathogens”. Also, some prominent foliar diseases are not controlled by foliar fungicide. Click here for a detailed discussion about using fungicides for managing soybean diseases, and here for a detailed discussion about fungicide resistance management.

A June 18 2016 post on the MCS blog site is an article titled Soybean R3/R4 Fungicide Selection by Dr. Tom Allen, Plant Pathologist at the MSU-DREC. Pertinent points from that article are highlighted below.

   The automatic fungicide application at R3/R4 should be targeted to soybeans in “high-yield” environments. Past experience indicates this automatic fungicide application will result in a 3 to 6 bu/acre yield increase (this is higher than the impact shown in the above-cited research). This yield increase likely occurs only where the foliar fungicide application actually controlled disease(s) that were present.

   A strobilurin product applied to a frogeye leaf spot (FLS)-susceptible variety may not provide intended results due to widespread strobilurin resistance within the FLS fungal population.

   Know the disease package of a particular variety before making an application of a strobilurin-only product. Click here to check disease ratings of varieties grown in the MSU variety trials.

   If a variety is known to be susceptible to FLS, consider a tank-mix or premix of fungicides with multiple modes of action. Click here for a White Paper that discusses this topic in detail and contains a list of soybean foliar fungicides along with their mode of action and efficacy ratings against prominent soybean diseases.

   Fungicide applications made closer to R4 should provide residual activity past R5, or into the seed filling period. Most fungicides applied at labeled rates should provide 17 to 21 days of residual activity. Note that this protection does not continue past about stage R5.5.

   Pay special attention to rate of application since this will determine the length of residual activity.

   The application volume should be chosen to achieve maximum coverage since the systemic activity of both strobilurin and triazole fungicides is limited to movement around the area where the spray droplet is deposited.

Check the above White Paper for the below additional points to consider for control of soybean diseases.

   Several important diseases of soybean, e.g. stem canker, sudden death syndrome, and charcoal rot, have no curative control. They may only be prevented or managed by selecting less-susceptible or resistant varieties, or rotation to a non-host crop.

   Seed and seedling diseases can be effectively prevented by using the appropriate seed-applied fungicide.

   Soybean rust can be managed with applications of preventive and curative fungicides timed according to expected occurrence of rust in the area of concern.

   If an FLS-tolerant/resistant variety is grown, applying a stand-alone strobilurin fungicide is an acceptable practice to manage other diseases or as an automatic fungicide application.

Here is a final point to consider.

The widespread soybean seed rot that occurred in the Midsouth in 2017 raises an interesting thought. Would it be more advantageous to save any blanket application of a foliar fungicide until near maturity (with observed preharvest interval for labeled fungicides) to ensure protection against organisms associated with this malady vs. making that application earlier to obtain only the small yield increase that may or may not occur? It seems that the large economic loss that is occurring in 2017 from uncontrolled seed rot at maturity may be greater than that from not making the earlier prophylactic application. Fungicides with different modes of action can be rotated for this purpose. If this is not actionable or feasible, then it is likely that genetic resistance to pathogens that cause late-season seed rot may be the only successful control option for this problem.

Composed by Larry G. Heatherly, Sept. 2017, larryheatherly@bellsouth.net