Early- to Mid- Season Soybean Problems

Seed Rots and Seedling Blights

There are many pathogens that can attack a young soybean plant from the time of planting through the second or third trifoliate leaf stage.  Some of these organisms reside in the soil and some are borne in or on the seed.  Collectively they are referred to as seed rot and/or seedling blight organisms.  The most important soilborne organisms in Kansas are Pythium, Phytophthora and Rhizoctonia. Important seedborne organisms include Phomopsis (pod and stem blight), Cercospora (purple seed stain), Aspergillus and Fusarium.

Without the use of compound microscopes and other sophisticated laboratory methods, it is usually difficult to determine the exact pathogen causing the problem.

Pathogens such as Pythium and Aspergillus can cause the seed to rot before or immediately after germination.  This often results in skips in the row.

Sometimes plants emerge, but then die within a few days.  If you carefully examine the stem at or just below the soil line, you may notice a constriction of the stem.  This symptom is known as damping-off and is common to many of the pathogens but particularly Pythium, Phytophthora and Rhizoctonia.

If weather conditions are extremely hot at the time of emergence, another condition known as heat canker occasionally occurs.  The symptoms are nearly identical to damping-off. The lack of any pathogens in the tissue when cultured in the laboratory allows us to confirm the diagnosis.

Frequently, plants may emerge normally and then die after a week or two, particularly following heavy rains.  When these plants are dug up and examined, you can often find reddish-brown lesions on the lower stem and root.  This symptom is caused by the Rhizoctonia fungus.  Often times the plants may grow out of infection, but if the lesions girdle the root or stem, the plant usually dies.

Brown Spot (Septoria)

Brown spot is a very common foliar disease of soybeans in Kansas, particularly where rotation is not practiced.

In Kansas, brown spot develops relatively early in the season. The first signs of the disease are the development of many irregular, dark brown spots on both the upper and lower surfaces of the leaves closest to the soil. As the lesions enlarge, they frequently merge together to form irregular shaped blotches.

As the disease progresses, the entire leaf will turn yellow and drop from the plant. In severely affected fields, the entire area between the rows can be covered by dropped leaves.

As the season progresses into the hotter, drier days of summer, new growth will develop unaffected by the disease. Measurable yield loss due to brown spot has never been reported in Kansas.

Bacterial Blight

Bacterial blight is an early- to mid-season disease because it is favored by cool, wet weather.  It is splashed from plant-to-plant during during storms with wind driven rains.  As the summer progresses and longer periods of high heat and less moisture occur, spread of the disease will stop and new foliage will be unaffected.  The disease can be transmitted on the seed, but is worst where no-till is practiced without rotation.  Many varieties have good resistance, but in Kansas the disease is cosmetic and no yield loss occurs.

The first symptom of the disease is small, water-soaked spots.  These soon develop into yellow and eventually reddish-brown to black spots.  The spots are typically surrounded by a bright yellow halo.  The newest growth on the plant is most susceptible. If cool, wet weather persists, the lesions will enlarge and coalesce to form larger, irregularly shaped areas on the leaf.  Eventually, this dead tissue falls out leaving the leave with a tattered appearance similar to minor hail damage.

Bean pod mottle virus

Bean pod mottle virus (BPMV) is one of several viruses that affect soybeans in Kansas, but is by far the most common.  Definitive identification usually requires sophisticated lab testing.  BPMV symptoms are most often confused with soybean mosaic virus symptoms.  Frequently both viruses can be present in the same plant.

Bean pod mottle virus  is transmitted by the bean leaf beetle.   Soybeans infected with BPMV will show a light green to yellow mottling in the newest leaves, especially early in the season when the weather is cool.  These symptoms may disappear with the higher heat of midsummer, and then reappear again in late summer as the weather cools again.  With severe strains of the virus, a puckering of the leaf tissue may be present.  This puckering symptom is also common to soybean mosaic virus, so caution should be used in making a diagnosis when this symptom is present. 

Soybeans infected with BPMV often have fewer pods or the pods present may be smaller and misshapen.

One of the most noticeable symptoms of BPMV is that the stems of the plant often stay green well into the fall, even after the leaves have dropped and the pods are mature.  This can cause serious harvest difficulties in fields with a large percentage of the plants infected.

Lastly, seed infected with BPMV are often smaller in size with a brown or black mottling of the seed coat.  Warning, this seed staining is often common with soybean mosaic virus so be careful.

Stem canker

Stem canker is a disease that is not widespread in Kansas, but usually can be found nearly every year somewhere in the state.  It's main importance is that its symptoms are commonly confused with those of Phytophthora root rot, a much more serious disease in Kansas.

There are two types of stem canker, the northern type and southern type.  The southern type is the one found in Kansas. A key difference between the two is that the northern type is seed borne and can be widely distributed throughout a field.  The southern type is not seed borne and in Kansas, therefore it is usually found in relatively small patches in one or more areas of the field. These patches most often occur in the lower, wetter areas of the field.

The first symptoms appear shortly after flowering. Small, reddish-brown lesions appear, usually near the second or third node (the node is the point where the petiole attaches to the stem) above the soil line.  As the disease progresses, the lesions enlarge and a sunken canker forms.  An older canker may have a reddish brown margin and a lighter colored gray to brown center.  Occasionally these cankers girdle the stem, killing the plant.  The leaves remain attached as with charcoal rot. It is the girdling of the stem and killing of the plant that often confuses this disease with Phytophthora root rot.

Leaf symptoms develop that are very similar to those of sudden death syndrome.  The area between the leaf veins will first turn yellow and then brown, leaving the veins green.  Unlike sudden death syndrome, the leaflets do not drop off the petioles.

Occasionally the small, black fruiting structures of the fungus can be found within the mature canker in the later stages of disease development.

Phytophthora Root Rot

Phytophthora root rot is one of the most serious diseases of soybeans.  In Kansas, losses from Phytophthora rank only behind charcoal rot, soybean cyst nematode and seedling blights.

Phytophthora can attack the plant in the seedling stage of development, but symptoms are indistinguishable from Pythium at that point.

Phytophthora root rot damage most frequently occurs in patches in low, wet areas of the field. in fields with heavy, or poorly drained soils, single plants may die in a random pattern throughout the field. 

As susceptible varieties grow, foliar symptoms may include yellowing between the veins and leaf margins and a yellowing of the upper leaves followed by wilting.  Like charcoal rot and stem canker, leaves remain attached after dying.

Below ground, there is usually a loss of lateral roots and deterioration of the taproot.  A girdling stem lesion usually develops beginning at the soil line.  It may move up the stem as high as 10 nodes before the plant wilts and dies.  The inner tissue of the stem is also discolored. 

In less susceptible varieties, the stem lesion may be a long, narrow, brown lesion that moves up on only one side of the stem.  These plants may not die. 

In more tolerant varieties, only the loss of the lateral roots and the taproot may occur.