Soybeans Update 8/22/2018

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The 2018 growing season has been anything but normal. Soybeans were planted into soils ranging from wet to extremely dry. We had packing rains early on some fields and a lack of rain on others, both causing poor germination and replants where required. Then, some areas suffered through extended periods of dry weather as soybeans bloomed (R1). In the past 3 weeks, most areas have received above normal rainfall and high humidity, triggering soybeans to grow at a fast pace.

Soybeans will show every stress event in the leaves, and this year is no different. I have been called to look at cupping in plants, which looks odd from the road. Cupping can be caused by several things. What I am seeing the most right now is rapid growth cupping and cupping caused by the soil being saturated.

Rapid growth (physiological) Cupping Too wet
soybean plants growing rapid soybean plant growing and had too much water
soybean plant growing with dicamba cupping new growth on soybean plant with leaves cupping

Almost always, Dicamba cupping will be in the newest growth that was there when sprayed. Notice the distinct “upward” cupping of the leaves, with cupping going all around the leaf margin. When leaf is flattened, the leaf looks abnormal. Rapid growth and suffocation is also in the new leaves, but leaves appear to look normal, except for cupping.

The good news is that soybeans very often easily outgrow physiological cupping and we normally will dry out in a few days after heavy rainfall. You may see a permanent “layer” in the soybean canopy that has a few leaves cupped, but normal growth usually resumes in a short amount of time, and yield is not affected. Physiological cupped soybeans can often be a sign of acceptable to excellent growing conditions, and cupped leaves are generally not a concern, once we eliminate the obvious causes of damage.

When dealing with leaf cupping, eliminate the obvious, check patterns, look for gradients, check soil types, and you can always take a soil and tissue sample, like the following:

Soil Report from NCDA&CS Agronomic DivisionPlant Tissue Report from NCDA&CS Agronomic Division

Always take a soil sample with tissue. Both samples were curling and one tissue sample was light green. In this case, I took two tissue and two soil samples. Nothing in the sample would be a cause for leaf curling. Light green sample low in N, but could be related to the N:S ratio. Other nutrients were in the recommended range. pH is a concern as it bordering low in both samples.

Other issues

Soybean (Physiological) scorch

soybean plant with scorched leaves scorched soybean plant leaves

Physiological Scorch as shown above. Regardless of the cause, this symptom is indicative of a problem with the vascular system once soybean has shifted to the reproductive phase. In this case, soils were soaked and roots were suffocating. Spraying a fungicide will not impact this or most any other soil disease at this time and should be avoided.

Kudzu Bug

Nymphs Fungus Beauveria bassiana
nymphs in soybean plant fungus on soybean plant

Kudzu bug has been a problem in the past. The threshold is 15 nymphs per 15 sweeps. Always do 60-75 sweeps (4-5 stops) in a field in different areas. The fungus Beauveria bassiana was also present in the field and kills kudzu bug. 

 Most of the groups IV’s and V’s have reached R6 and some have reached R7.

By R6, seed growth is rapid. Leaves on the lowest nodes will start to yellow. Depending on weather senescence and leaf loss can be rapid. Soybeans at R6 are still vulnerable to stink bug damage, but in most cases, are not attractive to Podworm moths. The threshold is 5 stinkbugs/15 sweeps.

R7 begins when 1 normal pod on the main stem is mature in color (brown or tan). Dry matter is peaking in seeds. Green is disappearing, and both seeds and pods appear yellow. The seeds are 60% moisture at physical maturity. Stress has little effect unless pods are shattered or fall to the ground. At this point. stinkbugs cannot so much damage.

However, Group VI’s and higher are still in bloom (R2) and early VI’s may be producing pods at the Bottom (R3) of the plant.

At the R2 stage, the plant is beginning full bloom. The appearance of flowers begins to slow. Nitrogen (N) fixation increases rapidly and nodulation is at maximum. Flowers at most nodes throughout the plant. The plant normally sets more blooms than it can fill and most nodes will have multiple flowers. As stresses occur over the next 2 weeks many of these will abort instead of developing viable pods.

When 1 pod on 1 of the 4 upper nodes reaches 3/16″ long, the plant is at R3. Typically, 60% to 75% of flowers abort, but stress can increase the loss. Temperature or moisture stress can limit pod numbers, beans per pod or bean size. Because of its long flowering period, the plant can compensate some for losses, but its ability to make up ground dwindles as it approaches R5.

While plants are blooming, they are attractive to adult podworm moths. Although moth flights decline through August, we can see an in small increase in numbers in late August. If you have been able to afford a spray, you may not have to for pod worms. Again, R2-R6 soybeans are susceptible to Stinkbug damage. The threshold is 5 stinkbugs/15 sweeps.