Selecting the right planting date can be a useful management tool to prevent some soybean diseases in regions where the planting season is long. Use of early or delayed planting dates may increase, reduce, or not affect soybean disease at all, depending on the type of disease. This method, unfortunately, has limited use for disease management in Iowa because the state has a narrow planting window for maximum yield. ISU agronomists (see 1995 ICM, page 73) have demonstrated that the level of success in achieving maximum yield decreases as planting date is delayed. However, those who farm large acreages, or have fields with a history of frequent disease problems, may find that arranging the time in which certain areas are planted may help reduce the risk of disease. For example, if one of your five fields has had a previous problem from Phythium damping-off, plant the problem field last to reduce disease risk.
Studies by plant pathologists show that early planting increases the risk of seedling blight, sudden death syndrome, purple seed stain (also call Cercospora blight), and pod and stem blight, which results in Phomopsis seed decay in storage. Seedling blight and sudden death syndrome are concerns to some Iowa growers, and purple seed stain and pod and stem blight usually are minor problems in Iowa. In other regions where a longer planting season is available, delayed planting is effective in managing all of these diseases. Even in Iowa, I have observed that the first two diseases are much lighter when soybeans are planted mid-May or later, rather than when fields are planted in late April or early May.
Fusarium solani and Pythium, which cause sudden death syndrome and seedling blight, respectively, require cold soil temperatures when they attack soybean seedlings. Early planting in cold, wet soil can increase the disease risk if these pathogen are present. Be aware that the fungi would not cause much damage in cold soil when moisture is not excessive. Seedling diseases should not be a concern in a dry spring.
Purple seed stain and pod and stem blight are more severe in early maturing varieties of soybean than the later maturing varieties. In contrast, brown stem rot may occur less in early planted soybean and be more severe in later maturing soybean because cool temperatures during the late growing season are favorable to the development of this disease. Further, Phytophthora root rot, bacterial blight, brown spots, stem canker, and white mold are not affected by planting dates.
This article originally appeared on page 40 of the IC-476 (6) -- April 22, 1996 issue.