Mid-season soybean scouting

By Dusty Sonnenberg, CCA, Ohio Field Leader, a project of the Ohio Soybean Council and soybean checkoff

Mid-July is an important time to get out into the soybean fields and take a good look at the crop conditions and identify the current stage of development. With the exception of extremely late planted beans or recently double-cropped soybeans, a majority of the fields in the state are in various early reproductive stages (R1-R3).

R2 Soybean Plant

Given the recent weather, some locations may be experiencing drought stress. Meteorologists have used the term “flash drought” to describe these conditions. A sign of soybeans facing drought stress is flipped leaves. This will show the underside of the leaf which has more of a silver-green appearance as it reflects the sunlight. These plants will tend to grow slower and produce smaller leaves compared to those not facing the stress.

Insect pressure is important to monitor at this time of year. Pests such as bean leaf beetle, grasshoppers, and Japanese beetle adults will cause defoliation. At different growth stages defoliation can have more of an impact than others. During the R1-R4 stages defoliation above 15% will result in noticeable yield losses.

Soybean aphids along with two-spotted spider mites (in dry situations) can rapidly multiply and cause yield reduction and create opportunities for pathogens to enter the plant. The economic threshold for Soybean Aphids up to R5 is 250 soybean aphids per plant with an increasing population. Two-spotted Spider Mites will typically be detected at the field edges first. A healthy crop with adequate rainfall will often outgrow the pressure from spider mites.

Diseases to be on the look-out for include Bacterial Leaf Blight and Brown Spot, Downey Mildew, Brown Stem Rot, Sudden Death Syndrome, Pod and Stem Blights, and Frogeye Leaf Spot. Environmental conditions can dictate the frequency of scouting required. The disease triangle of a susceptible host, the pathogen being present in the environment, and correct environmental conditions must all be met for a disease issue to take hold. Many of the diseases are best managed by cultural practices such as crop rotation, planting resistant varieties, and in some cases tillage. Other diseases may require the application of fungicides. Timing of the fungicide application is important when it comes to diseases such as Frogeye Leaf Spot, which is best protected with a fungicide application at R3. Diligent scouting on a regular basis allows timely management decisions to be made.

Nutrient deficiencies are often noticeable at this time. Manganese deficiency symptoms appear as interveinal chlorosis of new upper trifoliates. Nitrogen deficiency appears as pale green or yellow leaves. Phosphorus deficiency will show leaf cupping, stunted growth and some discoloration. Calcium deficiency shows leaf tips the look pinched and leaf bronzing and early leaf drop. Magnesium deficiency shows pale green lower leaves with yellow mottled interveinal tissue. Sulfur deficient plants will be stunted and pale green in color, similar to nitrogen deficiency except the chlorosis may be more apparent on upper leaves.

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