Soybeans -- Late season conditions to watch for

Thursday, August 13, 2015

Thanks to the rain, all crops are developing well. Soybeans have pollinated for the most part, and pods are developing. There are several potential problems to watch for at this time of the growing season.

Late-season damage from insects can reduce yield potential. Stress during R4 (full pod) to R6 (full seed) growth stages can cause more reduction in yield potential than at any other growth stage. Early detection, identification and population assessment is critical information needed to make an accurate economic decision on whether to apply an insecticide treatment. Following are some insects to watch for:

1. Bean leaf beetle -- About a quarter of an inch long, green, yellow, tan or red but always have a distinctive black triangle behind the head. Cause defoliation, clip pods and pod feeding.

2.Grasshoppers -- Leaf feeding as well as aggressive pod and seed feeding. Damage is generally more likely during a hot dry summer and in weedy fields and border rows.

3. Japanese beetle -- one-third to two-thirds of an inch long, green metallic or greenish bronze and have several prominent white spots around edge of wings. Majority of feeding occurs during reproductive stages. Should be winding down now.

4. Soybean podworm -- also known as corn earworm, larvae feed on foliage while older ones feed on flowers and pods.

5. Spotted cucumber beetle also known as southern corn rootworm beetle -- about 0.4 inches long, yellow to light green with 12 black spots on wing covers. Primarily feed on foliage.

6. Two-spotted spider mite -- an insect that is usually found during extended dry spells.

Diseases can also cause significant yield reduction beginning at pod (R3) through full seed (R6). Some of the diseases to watch for in this area are:

1. Bacterial leaf blight -- symptoms include small angular, water soaked spots which turn yellow, then brown surrounded by a yellow halo as the tissue dies. Disease is favored by cool, wet weather and favors the spread of bacteria by wind, rain and mechanical means. Commonly observed in mid to lower canopy.

2. Cercospora leaf blight -- caused by a fungus that initial symptoms include mottled purple-to-orange discoloration of the upper most leaves. Later, the leaves have a leathery appearance highlighted with bronzing. The same fungus causes purple seed stain.

3. Downey mildew -- fungus that appears as small, yellow-green areas on the upper leaf surface. These areas enlarge and become grayish to dark brown with yellow-green margins.

4. Frogeye leafspot -- appears as round, brown to gray lesions on the leaves which are surrounded by a thin, dark reddish-colored ring.

5. Septoria brown spot -- lesions are angular to irregular and dark brown and caused by a fungus. Leaf tissue surrounding lesions becomes yellow. Symptoms first appear on lower leaves during warm, wet conditions and then progresses to the upper leaves.

6. Sudden death syndrome -- usually appears mid to late season. Symptoms begin as yellow spots scattered on the leaves. The spots become necrotic and merge leaving the leaf veins green.