Larry G. Heatherly - Crop Doctor

Numerous insects and diseases can damage soybeans in the midsouthern US. Soybean pests may cause economic losses throughout the growing season, or only cause damage during a particular stage or at a particular time. In either case, scouting for the presence of a pest and applying control measures (if available) when thresholds have been reached should prevent economic losses.

Scouting generally is most effective when done in time to detect the first appearance of a pest before its level reaches economic threshold. Treatment of some pests when first detected by scouting at an early stage may prevent their being a problem later in the season.

Insects that should be monitored to prevent economic losses in soybeans are shown in the accompanying table. Three-cornered alfalfa hopper and bean leaf beetle populations should be monitored both early and late in the season, while others should be monitored during reproductive development.

Most insects that defoliate soybeans generally appear late in the season or do not cause economic damage prior to bloom. However, periodic scouting during vegetative development should be conducted to determine that defoliation by insects before bloom does not exceed about 35%, at which time an insecticide is justified.

Insect defoliation of soybeans is more critical after R1 (beginning bloom). Thus, more frequent scouting should be started then to ensure that defoliation does not exceed about 20% during reproductive development. Scouting for pod feeding insect damage should begin at or soon after R3 (beginning podset).

Stink bugs are the most important insect pest of soybeans in the midsouth and feed on developing pods and seeds. Scouting for this pest should begin no later than R3 regardless of the time of year, and continue through R7 (beginning maturity) to prevent levels that will damage seed.

Several important diseases (sudden death syndrome, stem canker, phytophthora root rot, charcoal rot, seed and seedling diseases) of soybeans have no curative control. Sudden death syndrome and stem canker can be managed or avoided by using less-susceptible or resistant varieties. Late-season phytophthora root rot can be managed by using resistant varieties. There are no known resistant varieties (only moderately resistant germplasm) or fungicide controls for charcoal rot. Seed and seedling diseases can be managed by using seed treatments.

Prominent diseases of soybeans that have a preventive or curative control are listed in the table. Labeled fungicides are best used when applied prior to or at the earliest appearance of a disease, and the general recommendation is that they should be first applied at early podset. Scouting should be used to accurately determine the reproductive stage for the proper timing of a preventive fungicide application.

Yield increases from use of foliar-applied fungicides are most consistent in fields where soybeans follow soybeans, where soybeans are irrigated, and/or where high yields are expected. Thus, effective scouting is more important in these environments.

Scouting can be valuable in detecting pathogens at any time during the season. Even though thresholds for their treatment may not be reached in a particular season, detecting their presence may justify considering resistant varieties or preventive controls the next year.

Soybean cyst nematode (SCN) is neither an insect nor a pathogen, but is capable of causing considerable yield loss. Scouting for SCN involves collecting soil samples soon after harvest in the fall to determine if SCN is present, and if so, identify the race and density. There is no economical curative control for SCN, but growing resistant varieties based on soil sample results should offset yield losses that will occur if susceptible varieties are planted in infested fields.

(The author appreciates helpful reviews provided by Dr. Jeff Gore and Dr. Billy Moore).

(Successful Farming, online at www.agriculture.com/ag/sfonline 1/12/07)