Key soybean growth stages and disease considerations

Soybeans face many challenges during the growing season that can impact yield potential. Crops undergo a series of vegetative and reproductive growth stages, each with unique developmental processes and stressors.

The reproductive stages begin with flowering, R1 to R2, and include pod development, R3 to R4; seed development, R5 to R6; and plant maturity, R7 to R8.

Stress during these stages can significantly hinder yield potential. Proper identification of growth stages allows for timely interventions that can mitigate injury and promote optimal growth.

Beginning Bloom — R1: During this stage, the plant has at least one open flower at any node on the main stem. Rapid root and shoot growth occur, making the plant particularly vulnerable to stress.

Full Flower — R2: This stage is characterized by an open flower at one of the top two nodes. Nitrogen fixation and dry matter accumulation peak. Stress management during this phase is crucial as defoliation can reduce yield by up to 6%.

Beginning Pod — R3: At this stage, at least one pod is 3/16-inch long at one of the uppermost nodes. Yield components such as pod number and seed size are established. Effective stress management can help to reduce flower and pod abortion rates.

Full Pod — R4: This is the critical period for determining yield potential, with rapid pod growth and seed development. Stress during this stage can have the most significant impact. Daily water use is high, making this an optimal time for irrigation, if available.

During key growth stages, soybean plants may be susceptible to leaf diseases or nematode infestations.

Disease development depends on several environmental and agronomic factors. Farmers should keep these factors in mind while scouting to properly prevent infection and damage.

As the growing season progresses in Illinois, pay close attention to your local environment, crop conditions and weather patterns. Rainfall patterns and accumulation continue to be localized and can differ drastically from field to field.

Availability of moisture, crop health, growth stage and variety selection play a critical role in disease development and if management is necessary.

Bacterial Diseases: These often result from plant injuries caused by hail, wind, insects, mechanical injuries and animal or human disturbances when the plants are wet. Bacteria enter through wounds or natural openings and can overwinter in residue and seeds. Bacterial blight is more prevalent during the early part of the growing season, but can appear late. Bacterial pustule causes premature defoliation, as well as reduced seed size and set.

Fungal Diseases: Fungal spores can originate from infected crop residue, seed or wind. The right temperature and moisture conditions can promote spore production and transmission. White mold can develop earlier at stem nodes during or after flowering, and symptoms of frogeye leaf spot and sudden death syndrome can appear after flowering throughout the reproductive stages. Use foliar fungicides as needed, paying close attention to specific diseases and application timing.

Viral Diseases: These are typically spread by insect vectors like aphids and bean leaf beetles, but they can also be seedborne. Scouting for these insects and applying insecticides promptly can help prevent viral infections, though some viral diseases may not be fully managed by insecticides alone.

Nematode Infestations: Geography, soil type, previous crops, tillage and wildlife movement all impact nematode pressure. Soybean cyst nematode is common in sandy soils, though it is well distributed throughout most soil types. Conduct regular soil and plant sampling to identify nematode species and infestation levels.

By understanding the various growth stages of soybeans and properly managing associated stressors, farmers can significantly enhance their crop management practices and maintain potential yield.

Always read and follow pesticide label directions. Performance may vary, from location to location and from year to year, as local growing, soil and environmental conditions may vary. Growers should evaluate data from multiple locations and years whenever possible and should consider the impacts of these conditions on their growing environment.

The recommendations in this material are based on trial observations and feedback received from a limited number of growers and growing environments. These recommendations should be considered as one reference point and should not be substituted for the professional opinion of agronomists, entomologists or other relevant experts evaluating specific conditions.

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