ASK THE AGRONOMIST BLOG

On June 23, Iowa State University Extension and Outreach announced that tar spot has been observed in six counties in central Iowa. Stine agronomists and sales reps have also received reports of suspected tar spot in corn throughout the Midwest and the Eastern Corn Belt. The disease, which was first discovered in the U.S. in 2015 in Indiana, traveled from Central America and Mexico. Since then, it’s moved from east to west and is quickly becoming a concern. The earlier it moves in during the season, the more time it has to spread and lead to yield loss.

Meaghan Anderson, field agronomist for central Iowa at Iowa State University Extension and Outreach, recently sat down with Stine Seedcast host David Thompson to discuss tar spot. In their conversation, which will air on Tuesday, July 18, Anderson notes that the severity of the disease this year will depend on the weather. “We’re certainly setting up to potentially have an issue if the weather does what we are worried it could do.”  

The disease thrives in moist environments with moderate temperatures. ISU Extension and Outreach notes, “the daily minimum or the daily mean temperature over the past 30 days was the most strongly correlated with tar spot development and increased severity. Moderately warm (below 70F) mean temperatures increased development and severity.”

How to identify tar spot in corn

Tar spot symptoms are typically observed later in the season during grain fill or around R3 to R6. The fungal disease appears as black lesions on the leaf called stromata. Anderson notes, “Stromata can break open and then they’ll release spores. Then those spores will get in the air or splash in raindrops and then move to new leaves and to new plants and elsewhere like new fields. They can consistently spread as weather conditions are good for it.”

Earlier in the season, you can find tar spot in the lower canopy. As the season progresses, it starts to infect the upper canopy. “This time of year, I recommend looking at the lower canopy, about knee high to try and find tar spot,” said Anderson.

Anderson also notes that tar spot is easily misdiagnosed. Growers might think the lesions are soil splashed onto the plant, dead bugs or other diseases that create similar shiny or semi-shiny black spots on the leaves. The main indicators that it’s tar spot, however, are 1. you can’t scrape tar spot off the leaf and 2. when you flip the leaf, you will be able to see lesions on the bottom. They impact both the top and bottom sides of the leaves. 

How do you control tar spot disease?

Tar spot treatment can depend on the severity of the disease. The best thing you can do is get out and scout. When scouting, use any local extension tools available that can help you diagnose the disease as tar spot and determine the severity of infection. The Crop Protection Network has a Disease Severity and Insect Defoliation Training tool to assist growers in diagnosing disease and defoliation ratings on field crops, including corn.

Anderson notes that scouting should begin just prior to tassel to make fungicide application decisions. Late in the season, fields should be re-scouted at around R5, the dent stage in corn. At this point, you can determine what’s happening at the ear leaf and upper canopy, which can tell you a lot about what happened earlier in the season. She adds that at R5, if you have more than 5% tar spot coverage on the ear leaf, it will likely impact yield. If you have less than 5% coverage, then it’s fairly unlikely that a fungicide would have been economically viable to treat it. She notes experts from Purdue Extension have experience in detecting and treating tar spot as it was first state where the disease was detected in the U.S., so growers can utilize their resources as well.  

If you need additional help with treating or identifying tar spot, contact your local Stine agronomist or sales rep for more information. Stine offers high-performing seed corn with the industry’s most desirable trait packages to combat disease. And stay tuned for Anderson’s full interview on tar spot on the July 18 episode of the Stine Seedcast.

The June 27, 2023, U.S. Drought Monitor Map reports that large areas of Nebraska, Kansas and Missouri, and some areas of Iowa, South Dakota, Oklahoma and Texas, are experiencing extreme to exceptional drought conditions. Many areas of the Midwest, South and East Coast are experiencing abnormally dry, moderate or severe drought conditions. The National Drought Summary states, “The Midwest and east-central Great Plains saw mostly worsening conditions and widespread crop stress and low streamflows after another week of mostly dry weather.”

Stine^® agronomists and sales reps have been fielding questions recently about managing Stine corn and soybeans during a drought.

“There are a number of concerns growers need to be aware of when it comes to drought and crop production,” says Tom Larson, Stine’s director of agronomy. “Drought stress in plants obviously impacts plant performance, but growers also need to be aware of issues related to herbicide carryover.”

What are the effects of drought on plants?

The U.S. Drought Monitor notes that 70% of corn and 63% are soybean production areas are currently impacted by drought conditions. 

The negative effects of drought on crops can result in significant yield loss if dry conditions persist throughout the growing season, especially right before or during the critical pollination stages.  

Drought symptoms on corn

The initial symptom of drought stress in corn is leaf rolling. This typically occurs during the day. Long periods of drought during the vegetative stages can result in shorter, stunted plants and less leaf area. Growers may also note leaf discoloration. If drought persists through the pollen shed stage, yield loss can truly gain ground.

Dr. R.L. (Bob) Nielsen of Purdue University notes, “Stress during pollen shed and silking can cause more yield loss than almost any other period in the crop's development … During the silking and pollen shed period, severe stress may reduce yield up to 8 percent per day. During the 2 weeks following silking, severe stress may reduce yield up to 6 percent per day.”

Later in the season, if pollination was impacted by heat stress, it can lead to poor grain fill, aborted kernels and leaf tissue death.

“Now that we’re getting closer to the pollination stage in corn, that’s when heat stress can really take hold and damage yield,” said Larson. “Drought conditions can lead to delayed silking or stunted silks, which greatly impact the pollination process and grain fill. Fortunately, Stine has many corn hybrids that feature some drought resistance and are bred to perform under drought stress. While we’re hoping for rain, it will be interesting to see how these hybrids yield come harvest if the current conditions persist.”

Stine has several corn hybrids that have a longer flowering period, which can help insulate the crop from drought stress to a degree by having pollen shed over a longer period. Connect with your local Stine sales rep to explore these options for the 2024 season.

Drought symptoms on soybeans

“While soybeans are also subject to yield loss related to drought stress, they’re not affected as severely as corn,” said Larson.

Soybean drought stress can initially appear as leaf curling (or flipping) in the vegetative stages. Continued drought stress can further impact the plant by stunting leaves and early flowering, which affects pollination.

Experts from Iowa State University Extension and Outreach note that the “Soybean is most susceptible to water deficits

during germination and the reproductive growth stages. Soybean yield loss from water deficits is caused by a reduction in flower number, pod number, and seed fill during the reproductive stages of growth.”

Drought stress can also lead to fewer seeds per pod, smaller seed size and seed abortion in soybeans.

Drought and herbicide carryover

“Herbicide carryover should be a consideration for growers during a drought year,” says Larson. “Plants under stress don’t metabolize chemistries properly, so it takes longer for it to flush through their systems. Herbicides carried over from the previous year may be present in the soil as this year’s crop grows. If this year’s crop doesn’t have tolerance to that chemistry, you could have a big issue on your hands.”

Herbicides applied in the spring or early summer have more time to break down in the soil. However, herbicides that are sprayed in season post-emergence have a shorter half-life and may cause issues for the next growing season. 

“For any corn herbicides that were sprayed late last season, we’re seeing some of that chemistry impact this year’s crop as the chemical has not broken down by now,” said Larson. “To ensure this year’s herbicides don’t carry over into next year, you need to time your herbicide applications and select your chemistries wisely. Look for something that you know won’t impact next year’s crop or choose a corn or soybean product that has tolerance to that chemistry. Your Stine agronomist or sales rep can help you with these decisions.”

To learn more about drought’s impact on corn and soybean crops and the potential for herbicide carryover, contact your local Stine sales rep or regional sales agronomist today.

A recent article by Farm Journal titled “Seize Your Corn Yield Potential with Sulfur’s Amazing Superpowers” highlights sulfur’s use in agriculture and its supporting role with nitrogen in crop production. We at Stine have always stressed the importance of sulfur as part of a super management approach to nutrient management.

“Without adequate amounts of sulfur, it becomes increasingly difficult to achieve top-end yields,” says Brian Hartman, Stine’s national corn product manager. “While the three main players in nutrient management tend to be nitrogen, phosphorus and potassium, sulfur is just as important. It works in tandem with nitrogen, so we always recommend growers consider sulfur if they’re applying nitrogen.”

WHAT IS THE ROLE OF SULFUR IN PLANTS?

The role of sulfur in plants is important to chlorophyll production and photosynthesis. Essentially, it helps convert sunlight into energy and supports the plant’s nutrient uptake.

“Sulfur has a direct role with nitrogen; it helps metabolize nitrogen, which also happens to be the No. 1 nutrient in crop production,” says Hartman. “Sulfur is also known to help protect crops from disease. They call this S-dependent resistance.”

Farm Journal Field Agronomist Ken Ferrie notes that sulfur and nitrogen are “structure-building components, so sulfur (like nitrogen) is required early in the season. The plant needs sulfur to build the factory that will produce the fruit.” He explains that insufficient sulfur will result in stunted plant growth or delayed crop maturity.

Sulfur is a macronutrient, so plants deplete it in larger quantities. Typically located in the organic matter of the soil, it cannot be translocated through the roots to the rest of the plant until it has mineralized and converted to sulfate. Then, the nutrient is mobile and acts like nitrogen. To mineralize sulfur into its sulfate form, organic material makeup in the soil must decompose. If this doesn’t happen, that’s when sulfur deficiency can occur.

DEFICIENCY OF SULFUR IN PLANTS

Sulfur deficiency can closely mirror nitrogen deficiency, so it’s important when conducting soil samples to ask the labs to also test for sulfur. There are a few different reasons why your soil may lack sulfur, including taking out the previous year’s crop.

“Harvest can remove approximately 8 to 17 pounds of sulfur per 100 bushels of corn,” says Hartman. “For soybeans, it can remove approximately .2 pounds per bushel. You need to put sulfur back in to replace what you took out with the previous crop.”

Ferrie notes in the Farm Journal article that a 200-bushel corn crop can remove up to 34 pounds of sulfur per acre.

Reduced greenhouse gas emissions are also suspected to cause deficiency as less sulfur dioxide in the atmosphere means less sulfur is deposited back into the soil. Roughly 20 years ago, we may have gotten about 70 to 90 pounds of sulfur per acre from the atmosphere. Now, with the controlling of air pollution, we receive about five pounds per acre.

The evolution to more plants per acre is also a factor.

“The higher the planting populations and the more plants you have per acre, the more nutrients are consumed,” says Hartman. “While we still encourage planting in high-population environments as it equates to higher yields, we also recommend a super management approach that includes applying additional sulfur to support this increase.”

Deficiency symptoms of sulfur in plants

Experts from the Iowa State University Extension and Outreach note that deficiency symptoms of sulfur are sometimes mistaken for nitrogen deficiency. Classic symptoms of nitrogen deficiency include “yellowing of lower (older) leaves, from the leaf tip to the base down the midrib.” In sulfur deficiency, symptoms include “yellowing of new leaves (in the whorl, sometimes with interveinal striping), with lower (older) leaves remaining uniform green.”

HOW TO APPLY SULFUR TO PLANTS

Start with soil sampling. Sometimes, labs will not test for sulfur, so you must request a sulfur test with your regular nutrient test. From there, soil fertility specialists recommend amounts to include in your application. Right now, recommendations would be for in-season applications of sulfur.

Sulfur can be applied to plants in a few different ways. 

In-season. Getting the sulfur to the plant as soon as it needs it is imperative. Corn takes up 38% of sulfur during the final three stages of growth. Consider a side-dress or over-the-top application with rates of phosphorus, potassium and nitrogen along with the sulfur. Sulfur should always be added to an in-season nitrogen fertilizer application.

Fall application. Put back what you take out after harvest with a fall soil application. This allows the nutrients more time to break down into the sulfate form to use with next year’s crop. Our experts recommend using ammonium sulfate or ammonium thiosulfate at one pound for every five to 10 pounds of nitrogen.

To learn more about the value of sulfur in crop production, contact your local Stine® agronomist or sales rep for more information.