Minimize disease with planting dates

In regions with extended planting seasons, the date of planting can be a useful management tool to prevent some soybean diseases. Early or delayed planting may increase, reduce, or not affect soybean diseases, depending on the type of disease. The accompanying table summarizes the effects of planting date on several soybean diseases in Iowa, assuming the pathogens are present in your soybean field.

No effect. Some diseases, such as the root rot phase of Phytophthora, bacterial blight, brown spot, and stem canker, are not affected by planting dates.

Direct effect from early planting--increased risk. When infection of a disease occurs at the seedling stage, the planting date directly affects the risk. Early planting increases the risk of Pythium seedling blight, sudden death syndrome, and maybe pod and stem blight. Fusarium solani and Pythium, which cause sudden death syndrome and seedling blight respectively, require cold soil temperatures to attack soybean seedlings.

Direct effect from early planting--decreased risk. By planting early, however, you can escape seedling blight caused by Rhizoctonia and Phytophthora because infections by these two fungi occur in warm soil temperatures. Be aware that fungi do not cause much damage if soil moisture is not excessive. Seedling diseases are not a concern when spring conditions are normal to dry.

Indirect effects. Planting date also indirectly affects the occurrence of some diseases, such as white mold, brown stem rot, and pod and stem blight. Infections of these diseases do not occur at the seedling stage but at growth stages related to the planting date. When the growth stages meet or miss weather conditions that are favorable for disease development, planting date makes a difference in the disease. For example, more white mold was observed last year in late-planted soybeans because of the cool, wet weather we experienced during the later flowering seasons.

In recent years, an increased incidence of Cercospora leaf spot (also called purple stain seed) has been observed in Iowa. Studies from other states indicate that more severe disease develops in early-maturing varieties compared to later-maturing varieties. In contrast, brown stem rot often is less severe in early-maturing varieties and more severe in later-maturing varieties.

In Iowa, the use of planting date is to minimize disease is limited by the narrow planting window for maximum yield, especially in the northern Iowa. ISU agronomists have demonstrated that the level of success to achieve maximum yield decreases as planting date is delayed (see 1995 ICM, page 73). However, producers who farm large acreages or have fields with a history of frequent disease problems may want to arrange planting schedules to reduce the risks of disease. For example, if you have five fields, one with past damping-off problems from Pythium, plant the problem field last to reduce the risk of disease problems. In Iowa, I have seen great reductions in the occurrence of a few diseases, such as Pythium damping-off and sudden death syndrome, even in a short planting window. One example is that sudden death syndrome usually is light when soybeans are planted later than mid-May.

Effects of planting date on some principal soybean diseases in Iowa.

Disease Conditions for infection Growth stage for infection Effect of planting date
Damping-off by Pythium cool and wet before v2 Early planting increases risk.
Damping-off by Rhizoctonia warm before v2 Late plantings often have more problems.
Damping-off by Phytophthora warm and wet seedling stage Late plantings often have more problems.
Sudden death syndrome cool and wet early growth stage Early planting increases risk.
Brown stem rot cool and wet all vegetative growth Risk is more severe in late-planted soybeans but varies.
White mold cool and wet flowering stage Risk varies with weather in flowering stage.
Pod and stem blight cool and raining pod setting Risk often varies with weather at pod setting.

This article originally appeared on pages 48-49 of the IC-478 (6) -- April 28, 1997 issue.

Updated 04/27/1997 - 1:00pm