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Field Notes talked soybean conditions and crop diseases

Phyllis Bongard, Educational content development and communications specialist

Iron deficiency chlorosis.
Photo: Angie Peltier
The drought is affecting not only crop conditions but the potential for disease development. Drs. Seth Naeve, Extension soybean agronomist, and Dean Malvick, Extension plant pathologist, addressed how soybeans are faring in the state and disease concerns during the July 19 Field Notes session. They were joined by moderator Liz Stahl, Extension crops educator, for the wide-ranging discussion.

Soybean update

Naeve travelled around Minnesota these past two weeks assessing the soybean crop. Despite the drought, soybeans are looking reasonably good. However, we are living “paycheck to paycheck” in terms of soil moisture and need timely rain for the crop to develop and yield.

After an early, dry spring in southern Minnesota, excessive rain at planting was followed by an early shut down of rain. These conditions have resulted in a perfect storm of challenges. The primary challenge is the continued drought and the plant stress it causes. Iron deficiency chlorosis (IDC), which affects a large portion of our acreage, is another major challenge.

Iron deficiency chlorosis (IDC)

Naeve observed that IDC-affected field areas have expanded and are looking worse this year. He’s also seen IDC in areas, such as south-central Minnesota, where it normally has not been an issue.

What’s behind the expansion and increased severity of IDC this year? Plant stress is a major driver, so drought contributes to its increase. Besides drought, other plant stressors can play a role in the increased IDC severity, too. These include carryover of both herbicides and nitrogen.

Because 2022 was a dry year, some soybeans have been visually affected by herbicide carryover in the state. Naeve speculates that low levels of herbicide residual remaining in the root zone could stress the plants further creating more IDC in those fields. In a normal year, rain would have diluted the herbicide concentrations and with a normal rooting profile, soybean roots would have grown through that zone. This year, the rooting profile appears to be stuck in the zone where herbicides could be concentrated.

Nitrate carryover can also impact IDC. Like herbicides, conditions were conducive for nitrogen (N) to carryover. Corn yields were lower than expected due to the dry conditions in 2022, so nitrogen use would also have been lower. If not lost, that N could carryover and impact IDC.

Finally, stunting caused by any of these stressors keeps the canopy open, leading to difficult-to-control weed flushes.

IDC management

On Naeve’s Minnesota tour, IDC seemed well-managed where recommended practices had been used. Increased populations helped decrease severity to some degree, but iron chelate applications were more effective for treating IDC.

Planting an IDC tolerant variety has been a core recommendation for decades. However, with the new herbicide-tolerant genetics, some of the background genetics have changed. This has resulted in more IDC susceptibility in varieties adapted to Minnesota.

Impact on yield

IDC is expected to have an impact on yield this season. Low yielding, IDC-affected areas with 10 and 20 bushels will cap yield potential for the entire field.

For more information, see Managing iron deficiency chlorosis in soybean.

Challenges in no-till

Reports from several farmers point to an unusual year for no-till challenges. The spring moisture pattern penalized the crop, possibly through nitrogen tie-up or herbicide carryover or something related to residue breakdown. It’s possible that a lack of mixing and diluting herbicide residuals in a no-till system led to some carryover issues.

Soybean diseases

White mold

While this is the time of year when white mold tends to get started, it should be non-existent under drought conditions. It requires relatively cool temperatures and wet conditions to develop, conditions that have been rare in Minnesota this summer.

Ideal conditions for this disease include a saturating rain followed by periodic rains to stimulate the fungus in the soil to grow, release spores and infect leaves. Even with the widespread drought, there may be areas in the state that have received adequate timely rains for white mold to develop.

To manage white mold in high risk fields, use resistant varieties and fungicide applications when conditions favor disease development. To be most effective, fungicides need to be applied at R1 (beginning flower) or R2 (full flower) stage. Fungicide applications are used to prevent infection, rather than to stop it.

For more information, see Sclerotinia stem rot (white mold) on soybean.

Stem diseases

Charcoal rot is favored by hot and dry conditions. While the disease present in the state, it has never caused much concern. It’s named for the black pepper-like specks that can be seen in the lower stem or root when the stem is split. If you suspect charcoal rot, please send pictures to Malvick ( to help determine how widespread this disease is in the state.

Pod and stem blight is recognized by rows of linear spots on infected stems. It is favored by warm, wet conditions, but stress may have more of an impact later on in the season.

Tar spot in corn

Tar spot was first found in Minnesota in 2019. Since then, it has spread rapidly and been confirmed in 36 counties in southern and central Minnesota. While there was a report of disease from south central Minnesota, tar spot has not yet been confirmed in the state this year. Several counties in Iowa have reported the disease, but at very low levels.

Tar spot is favored by moderate temperatures, high humidity and a fair amount of leaf wetness. Since the disease is relatively new to the state, Malvick is taking a closer look at how conditions interact to enhance development. Under the right conditions, an infection can develop from a few spots to a fully involved leaf in a matter of a few weeks.


Corn leaves tend to have a lot of black spots, but not all are from tar spot. Bug frass is often mistaken for tar spot, but it can be rubbed off while tar spot fruiting bodies cannot. Tar spot lesions are usually elongated, not round or spherical. They also go through the entire leaf, so you can see the spot on both the top and bottom leaf surfaces.


The key to managing tar spot is figuring out where and when it’s developing. Start scouting in mid-July to document spread of the disease and to time potential fungicide applications. Perceived risk also plays a role in treatment decisions. For example, if tar spot is found in a new field that’s located in a part of the state where it’s never been documented, risk of high levels of disease developing is low. However, if the field is located in southeastern Minnesota, that risk level is much higher, especially if weather conditions are favorable.

Fungicides can be effective, but they need to be applied when the epidemic starts. While fungicide won’t eliminate the disease, it can significantly suppress tar spot and preserve yield. If needed, fungicides should be applied between VT (tasseling) and R3 (milk stage).

To date, Malvick hasn’t seen data that shows any benefit for initial applications after R3 stage. Similarly, if the disease initially develops after R4, the risk of significant yield loss due to tar spot appears low.

A final management key is to avoid susceptible hybrids in the future; look at hybrid trials when they’re available to see which ones are holding up against the disease.

Hail and fungicide applications

Naeve and Malvick agreed that applying fungicides following hail is useful only if significant fungal diseases were developing before the hail damage occurs. In soybean, the diseases that are enhanced by hail aren’t managed effectively by fungicides. In corn, hail opens the crop to Goss’ Wilt and smut, neither of which is controlled by fungicides.

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