Drought affects crop conditions as well as the potential for disease development. Dr. Seth Naeve, Extension soybean agronomist, traveled to Minnesota recently to assess the soybean crop. Despite the drought, soybeans are looking reasonably good. However, we are living “paycheck to paycheck” regarding soil moisture and need rain for the crop to develop and yield.
After an early, dry spring, excessive rain at planting was followed by a premature rain shutdown, resulting in a perfect storm. The drought and its related plant stress are the primary challenges. Iron deficiency chlorosis (IDC), which affects a large portion of the acreage, is a second challenge.
Naeve observed that IDC-affected fields have expanded and are looking worse this year. He’s also seen IDC in areas like south-central Minnesota, where it is not an issue. Droughts and other plant stressors, such as herbicide and nitrogen carryover, increase IDCs. Growing plant populations, a recommended practice for managing IDC, has helped decrease severity somewhat, but iron chelate applications have shown greater effectiveness.
Planting an IDC-tolerant variety is a core recommendation. However, some background genetics have changed with new herbicide-tolerant traits, resulting in more IDC susceptibility in Minnesota varieties.
Unlike IDC, current conditions do not favor many soybean diseases. White mold should be non-existent under drought conditions, according to Dr. Dean Malvick, Extension plant pathologist. It requires relatively cool temperatures and wet conditions to develop, which have been rare in Minnesota this summer.
Even under widespread drought, areas in the state may have received adequate rains for white mold to develop. To manage the disease in high-risk fields, use resistant varieties and fungicide applications when conditions favor development. Fungicide applications prevent infection at growth stages R1 to R2 rather than stop it.
Charcoal rot, on the other hand, is favored by hot and dry conditions. While the disease is present in the state, it has never caused much concern. When the branch is split, it’s named for the black pepper-like specks in the lower stem or root. If you suspect charcoal rot, please send pictures to Malvick (dmalvick@umn.edu) to help determine how widespread this disease is.
A tar spot of corn was first found in Minnesota in 2019. Since then, it has spread rapidly and has been confirmed in 36 southern and central Minnesota counties. The infection has not been established in the state yet this year. Moderate temperatures, high humidity, and leaf wetness favor tar spots. Under the right conditions, an infection can develop from a few places to a fully involved leaf in a few weeks.
Corn leaves tend to have a lot of black spots, not all from tar spots. Frass can be mistaken for tar spot, but it can be rubbed off, unlike tar spot. Tar spot lesions are usually elongated and go through the entire leaf, with the area visible on both leaf surfaces.
Start scouting in July to assess where and when the tar spot is developing. Risk assessment also plays a role. For example, if a tar spot is in a new field in an area where it’s never been documented, the risk of severe disease is low. However, if the lot is in southeastern Minnesota, that risk level is higher, especially if weather conditions are favorable.
Fungicide can be effective but must be applied when the epidemic starts. While it won’t eliminate tar spots, it can significantly suppress and preserve yield. Fungicides should be used between VT (tasseling) and R3 (milk stage) if needed. Malvick has seen no data that supports initial applications after R3.
Naeve and Malvick agreed that fungicide after hail is proper only if fungal diseases develop. In soybean, diseases enhanced by storms aren’t managed effectively by fungicides. Goss’s Wilt and smut can develop in corn, neither of which is controlled by fungicides.
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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