Field Notes Talks About What to Expect After Heavy June Rains

Matthew Pfarr, UMN Extension Educator - Crops, Mankato Regional Office, Brad Carlson, UMN Extension Educator - Water Resources, Mankato Regional Office,  and Dean Malvick, UMN, Professor in the Department of Plant Pathology and Extension Pathologist, St. Paul Campus

The following information was provided during a 2025 Strategic Farming: Field Notes session. Use your preferred podcasting platform or listen online to a podcast of this Field Notes session hosted by UMN Extension crops educator Anthony Hanson.

Figure 1. Heavy rains have led to standing water in some fields across the state.

Nitrogen management

Timing of nitrogen applications

With rainfall events happening in the region, timing for nitrogen sidedress is a critical topic this week and into next week.  There are several corn nitrogen application options available: side dress liquid UAN, Y-drop liquid UAN, sidedress ammonia and topdress urea.  As we are getting past V4-V6 target timing for side dress nitrogen remember that there is no such thing as spoon feeding corn with nitrogen.  It’s either in the soil profile in a plant available form when the plant demands it - or it’s not.  Timing of application doesn’t matter so don’t delay unless there is strong justification for potential nitrogen loss.  Consider early sidedress just after planting corn as the risk of loss of May 20th side dress versus June 6th is very small - opt for putting on the nitrogen early when possible. As we look to future weeks (V10 to Tassel), the corn crop needs nitrogen in place for linear phase growth and uptake.  The absolute critical point for corn nitrogen applications before yield loss is V12.  

Trends in equipment

There has been a rise in recent years for crop producers owning equipment for in-season nitrogen, which has provided the ability to apply quickly without dependence on hired custom application.  Some producers use their spray equipment modified with Y-drop nozzles for application of 28% or 32% Urea Ammonium Nitrate (UAN).  Spin spreader box urea applications are sometimes completed by farmer equipment as well, but this is more commonly done by a commercial applicator.  It’s important to note that urea can cause streaking and burning to corn leaves when it enters the whorl, but there is no evidence of yield reduction.

How about nitrogen transformation inhibitors?

Nitrification inhibitors slow the conversion of ammonium to nitrate and are commonly used in the fall to protect nitrogen investment.  In-season nitrogen is typically not stabilized with nitrification inhibitors because the window of uptake by the plant is short.  However, urease inhibitors are used for in-season applications as they slow the splitting of urea into ammonia.  This helps offset the risk of volatilization as urea-based nitrogen needs measurable rain of a quarter an inch or more for effective incorporation. Light levels of precipitation or dew on surface-spread urea are worse than no rain in the short-term as they leave the splitting of the urea on the surface vulnerable to loss.  Urease inhibitors have come off patent which has dropped the price and made them more cost-effective to treat urea.  The use of urease inhibitors can be helpful in dry or unpredictable weather patterns. 

Nitrogen loss processes factor into application timing decisions

A common question we get is determining whether to wait to apply nitrogen before a large forecasted rain, especially when the corn looks nitrogen sufficient.  For instance, in the podcast we covered an example from a producer using liquid UAN to side dress corn.  The loss processes of nitrogen are water driven, and loss happens when the soil is completely saturated. Coming into the morning of June 25th, there’s decent soil capacity across a large part of Minnesota to absorb rainfall (or at least some of it) so the risk of loss is not high. The odds, particularly of leaching, aren't high at all. However, if the soil capacity is one to two inches of rainfall, and we get in excess of two inches, that’s when we see a risk of denitrifying a portion of nitrogen. Current soil temperatures of 80 degrees add more risk as denitrification is a biological process and can be accelerated by high temperatures.  Keep in mind that 25% of the nitrogen in UAN is already in nitrate which is the most easily lost form.  Comparatively, urea has less immediate risk for loss in wet conditions as it transforms first to ammonium and then to nitrate. 

Large rain events the week of June 16th centered on Willmar, MN

As mentioned the nitrogen loss processes are influenced by soil temperature, level of saturation, and importantly, length of time saturated. Drainage is our biggest ally in defending against rapid nitrogen loss.  However, in low basins that catch and hold water, there is apparent denitrification happening evidenced by yellow corn.  Research shows half of nitrogen in the nitrate form can be lost after 10 days of complete soil saturation. If there is a crop stand loss due to flooding and drown out in these areas the additional loss of nitrogen is a moot point. 

Even though the rainfall in the Willmar area was extreme, there hasn’t been enough time and thorough saturation to be talking about rescue nitrogen treatments across entire fields.  In cases where there is a question, utilize the help sheet on the nutrient management webpage to guide individual rescue nitrogen decisions.  Keep in mind if nitrogen rates were lower up front there may be more of a need.  The Pre-sidedress Soil Nitrate Test (PSNT) can provide some confidence with a high nitrate ppm value, but low nitrate ppm values can sometimes be exaggerated with immobilization tie-up by decomposing soil organic matter.  For further information consult the recent PSNT press release from Fabian Fernandez, University of Minnesota Professor in the Department of Soils, Water, and Climate. 

Early season diseases apparent this year 

This week is an in-between period separating early season diseases from those that appear later in summer.  One early season disease of note is Pythium root rot.  Pythium symptoms include plant early damping off or seed rot in the ground. The majority of seed treatments typically provide activity against Pythium root rot with several active ingredients available.  We're nearly past the window of concern for Pythium with no major stand reductions attributed to the disease.

Rhizoctonia root rot observed in Waseca trials 

Where we've had enough rain combined with warm soil temperatures, keep an eye out for Rhizoctonia root rot in soybeans.  Conditions have been ideal in some areas as evidenced by University of Minnesota Rhizoctonia trials in Waseca.  Several soybean plots have seen considerable stand loss attributed to the disease. 

Phytophthora root rot remains a season-long soybean challenge with new resistance concerns

In addition to increased Rhizoctonia reports, Phytophthora root rot is something to monitor as well this year. Phytophthora is promoted by wet, virtually saturated soil conditions under warm temperatures.  This is one of the few diseases that we can see throughout the entire soybean growing cycle.  Phytophthora in soybeans is largely managed by genetic resistance.  Every soybean variety has two data points for resistance - RPS genes (major genetic resistance) and field tolerance or partial resistance.  These data points are important in determining how a soybean variety will react and survive in a field that's subject to severe Phytophthora stress.  Reports indicate that the Phytophthora pathogen population in soils across the state is gaining abilities to overcome RPS resistance genes. If you have evidence of Phytophthora in your soybeans take note of which resistance genes you have and consider rotating sources of resistance in the future.

Sudden death syndrome (SDS) in soybean

Leading the way for later season disease is SDS.  We’ve had very good conditions for SDS development in geographies with wet soils after planting and periodic rainfalls.  Research observations on SDS note that though ideal conditions may be present for early season infection, disease usually doesn’t develop if weather dries out severely in late July or early August.  This makes SDS very difficult to predict.

Weather considerations on foliar disease development

It’s been a favorable year so far for pathogens.  Dry winter weather may have led to better survival of fungi as they have had less competition in the cool, dry soil.  Also, the wind and precipitation to date have moved soil and spread soilborne fungal diseases effectively.   The uptick in moisture in season is also a concern. Wet residue in the field can help pathogens that are surviving on that residue produce spores and then spread into the new growing foliage.  If the ground layer is damp, so is the foliage.  Initial foliage infection may be occurring now but we won’t see the symptoms until later when the disease is visible to the eye.  

Crazy top disease on Corn

Some ag professionals are aware of a corn disease called crazy top which occurs under very wet conditions.  Even in standing water conditions, the pathogen can spread to corn plants by entering the leaf whorl through high flood water or rain splashing.  The most characteristic symptom of crazy top is the development of a bushy mass of leafy structures at the top of the plant instead of a normal tassel.  Affected plants may also be stunted or barren.  There have been a few reports of crazy top in Minnesota so it might be something to watch for in the areas that receive a very large amount of rain. 

Tar Spot

Tar spot is one of our marquee diseases in recent years due to it’s recent geographical spread.  Later season weather will once again drive the severity of this disease in 2025.  We need some level of moisture to develop this disease.  In the drought year of 2023 we saw very little tar spot. In 2024 we saw a lot more rain and we saw a lot more tar spot; however, Tar Spot didn't occur everywhere there was a large amount of rain.  This adds to previous observations that tar spot needs a specific combination of enough moisture and also cool enough temperatures to reach damaging levels. We're developing and improving our ability to predict tar spot severity, but there's still a lot we don't understand. What we do know is based on the distribution of where the disease was found last year - the pathogen is surviving in fields across most of Minnesota. So the risk is greater, for example, in western Minnesota than it was a year ago because the pathogen is probably there in a number of fields.  That doesn't mean it will cause problems and yield loss. But it's more likely we'll see it. Use IPM Pipe Tar Spot webpage to track where tar spot has been reported so far this season across the nation on a county level.

White mold

What happens in July and early August is going to drive white mold development in soybeans as well. The soybean plants are getting fairly growthy right now and are close to filling rows.  The white mold pathogen survives in the soil and when the soil is shaded and moist enough, it will germinate and produce this little mushroom-like structure (apothecium, plural apothecia) under the plants.  Spores produced will then go on to the foliage (especially the dying flowers) and start infection. We need a combination of factors, primarily flowering under cool enough temperatures (<80 degrees) and enough moisture to keep the soil and plants moist to get white mold developing.  The time is getting close for initial infection, which is the best time for fungicide application.  When considering a fungicide application, don’t apply too late and or too early.  R2 is the optimal time for a single fungicide application. 

Questions 

Carlson and Malvick addressed key audience questions including “Determining whether to wait when corn looks N sufficient before a large forecasted rain”, “Implications of heavy rains such as the recent 8+ inch rainfall near Willmar, MN.”, “Did the dry winter help decrease disease pressure”, “pathogens waterborne versus airborne”, “any updates on corn tar spot”

Thanks to the Minnesota Soybean Research and Promotion Council and the Minnesota Corn Research and Promotion Council for their support of this program.