Fungicide Applications Depend on Soybean Growth Stages and Presence of Disease

We have had lots of inquiries this past week on the benefit or lack-there-of from fungicide applications on soybean in Ohio.  There are several factors that I have found in the past that can influence this return on investment:   growth stage of the plant, conditions that are favorable for disease and the presence of inoculum.  Below I have outlined how these may influence the outcome for a couple of our most yield limiting late season diseases:  Sclerotinia stem rot (white mold) and frogeye leaf spot. 

Sclerotinia is not in every field in Ohio; we are targeting historic fields that were planted with a highly susceptible variety.  If a variety has a good level of resistance, then it is probably not going to give a very large return or saved yield following fungicide applications – to the same level that a highly susceptible variety might – which can be as much as 50% of the yield saved.  In addition, if the field has been too wet to spray – and it is now in full flower or past flower, it is too late.  The infections would have occurred and all of the fungicides labeled for Sclerotinia must be on the plant prior to inoculation or the spores landing on the plant.

When I evaluate fungicides for efficacy towards frogeye, I use a variety that is super susceptible.  On the company rating scales where 9 is the worst, we are right up there with a 7, 8 or a 9.  When this happens, lesions will start to be pretty easy to pick out around flowering.  We then apply fungicides at the R3 and can save some yield.  When we wait until the R5 with just a few lesions here and there, we do not see any increase in yield and often there are still very few lesions on the new leaves.  One year was a bit odd:  I thought we were going to have a great study, but it turned hot and dry, no dews, around the R4 growth stage – and there were no new lesions, the new foliage was clean.  Later in the season, it got moving again.  If you can easily find frogeye in your fields in 2015, we are still recommending one fungicide application at the R3/R4 growth stage.  We are also moving more towards a triazole type of chemistry due to good efficacy, change up the mode-of-action this year in case we have strobilurin resistant strains, as well as reduced cost of material.  If it turns hot & dry, you may not need this.  If we have cool nights, in the untreated strip checks (yes, please leave a couple in your fields), there should be the continued development of disease.

Some cautions:  1) watch out for the look-a-likes.  The herbicide damage to soybeans used to go after  weed escapes can look very similar to the lesions caused byCercospora sojina (frogeye leaf spot).  If it is disease, you should be able to see little “whiskers” – conidia - on the underside of the lesions.  For herbicide damage it will also be very widespread – all at a similar level in the canopy.  For frogeye – this is sporadic and has very uneven distribution in the field.

2) Work with your company contact to know the true resistance level of the variety.  If the variety has a few spots, but is believed to have good resistance, this will prevent too many more spots from developing as well as reduce the amount of Sclerotinia stem rot that develops.  The reason you purchased that top notch variety is so you would not have to worry about the added costs to your production for the fungicides – so don’t spray in these situations.  Your homework is paying off.

The take home message is: only spray if the field has a history of Sclerotinia stem rot or lesions of frogeye leaf spot are already present, the variety is susceptible, and you can get the fungicide on at the appropriate growth stage.

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About the C.O.R.N. Newsletter

C.O.R.N. Newsletter is a summary of crop observations, related information, and appropriate recommendations for Ohio crop producers and industry. C.O.R.N. Newsletter is produced by the Ohio State University Extension Agronomy Team, state specialists at The Ohio State University and the Ohio Agricultural Research and Development Center (OARDC). C.O.R.N. Newsletter questions are directed to Extension and OARDC state specialists and associates at Ohio State.