The drought affects crop conditions as well as the potential for disease development. Dr. Seth Naeve, Extension soybean agronomist, traveled Minnesota recently 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 rain for the crop to develop and yield.
After an early, dry spring, excessive rain at planting was followed by an early shut down of rain, resulting in a perfect storm. The drought and its related plant stress is the primary challenge. Iron deficiency chlorosis (IDC), which affects a large portion of acreage, is a second challenge.
Naeve observed that IDCaffected fields have expanded and are looking worse this year. He’s also seen IDC in areas, like south-central Minnesota, where it normally is not an issue. Drought and other plant stressors, such as herbicide and nitrogen carryover, contribute to IDC’s increase.
Increasing plant populations, a recommended practice for managing IDC, has helped decrease severity to some degree, but iron chelate applications have shown greater effectiveness.
Planting an IDC tolerant variety is a core recommendation. However, with new herbicide- tolerant traits, some of the background genetics have changed, 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, conditions that have been rare in Minnesota this summer.
Even under widespread drought, there may be areas in the state that 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 are used to prevent infection at growth stages R1 to R2, rather than to 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. It’s named for the black pepperlike 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 (dmalvick@umn.edu) to help determine how widespread this disease is.
Tar spot of corn was first found in Minnesota in 2019. Since then, it has spread rapidly and been confirmed in 36 counties in southern and central Minnesota. Infection has not been confirmed in the state yet this year.
Tar spot is favored by moderate temperatures, high humidity and leaf wetness. 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, not all from tar spot. Frass can be mistaken for tar spot, but it can be rubbed off, unlike tar spot. Tar spot lesions are usually elongated and they also go through the entire leaf, with the spot visible on both leaf surfaces.
Start scouting in July to assess where and when tar spot is developing. Risk assessment also plays a role. For example, if tar spot is in a new field in an area where it’s never been documented, risk of severe disease is low. However, if the field is in southeastern Minnesota, that risk level is higher, especially if weather conditions are favorable.
Fungicide can be effective, but it needs to be applied when the epidemic starts. While it won’t eliminate tar spot, it can significantly suppress it and preserve yield. If needed, fungicides should be applied between VT (tasseling) and R3 (milk stage). Malvick has seen no data that supports initial applications after R3.
Naeve and Malvick agreed that fungicide after hail is useful only if fungal diseases are developing. In soybean, diseases enhanced by hail aren’t managed effectively by fungicides. In corn, Goss’ Wilt and smut can develop, 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!
