The warmer-than-usual spring weather has allowed for early soybean planting this year, much earlier than in previous years. Early planting along with the predicted dry growing season could affect soybean disease incidence.
In a normal season, early planting often is associated with higher seedling disease risk because soybean seeds stay in cool and wet soil longer than later-planted soybean seeds. Pathogens that infect plants at the seedling stage and show up in late summer also may have higher risk if soybeans are planted early. For example, sudden death syndrome caused by Fusarium solani and seedling damping-off caused by Pythium require cold soil temperatures when soybeans are in the seedling stage. Soybeans have a higher risk of developing these two diseases if seeds are planted early in cool and wet soils. This spring, however, the risk of these two diseases should be low for early-planted soybeans because of the dry and warm soil conditions.
This season, early planting could affect the risk of bean pod mottle virus, a relatively new disease in soybeans. This disease has been a concern in reducing seed quality in the past 2 years. The virus can be transmitted by infected seeds, a small portion of which may develop into virus-infected seedlings. During the growing season, the disease is transmitted from infected plants to healthy plants by bean leaf beetle feeding. The higher the beetle population, the greater the risk of this disease within a season (see the article in last week's ICM newsletter, Beetles transmit a new soybean virus). If you had disease problems last year, or if you plan to use discolored seeds because that is your only option, planting late is recommended.
Other diseases to watch for this growing season are insect-associated seedling diseases. Some fungal pathogens infect plants through insect wounds. If insect activity is higher than normal this spring, these fungi can secondarily invade plants and cause infections. Seed treatments are not recommended because of a lack of data on these types of diseases. The best way to prevent secondary fungal invasion is to manage insect damage. Furthermore, Rhizoctonia and Phytophthora, two fungi that prefer warmer temperatures, may occur much earlier in river bottom fields where there is sufficient soil moisture for disease development.
This article originally appeared on page 58 of the IC-484 (7) -- May 1, 2000 issue.