Early defoliation and death of soybeans in patches has been reported recently in fields across Minnesota. Although numerous soybean fields have started to mature and have suddenly turned yellow in the past week or so, it is obvious in many areas that the yellowing and plant death have been accelerated well beyond what would be typical. Several factors are undoubtedly playing a part, but disease is often a key component.

Many different diseases are occurring at some place in Minnesota, including Phytophthora rot in scattered areas, white mold in the southeast, soybean cyst nematode, pod and stem blight, brown stem rot (BSR), sudden death syndrome (SDS), and charcoal rot. This article will focus on SDS, BSR, and charcoal rot. Charcoal rot seems to be unusually common this year due to the hot and dry conditions experienced across much of Minnesota, and this disease is often not considered when diagnosing late season disease problems.

Figure 1. Sudden death syndrome symptoms include diffuse yellow spots/blotches and brown dead areas develop between veins on leaves. [click to enlarge] Dean Malvick

Figure 2. SDS Infection in the lower stem can be seen as a brown-gray discoloration that develops just under the epidermis and into the vascular tissues. [click to enlarge] Dean Malvick

Sudden death syndrome (SDS) has been reported in Minnesota much less frequently in 2007 than in 2006, probably because the dry weather conditions in many parts of MN have not favored development of SDS. Regardless, this fungal disease has been confirmed in several parts of southern MN ranging from Dakota and Houston Counties in the east to Swift County in the west, as well as in scattered points in between these points across southern MN. The most obvious symptoms of SDS are on leaves where diffuse yellow spots/blotches and brown dead areas develop between veins.

The leaves curl and fall off prematurely as the disease progresses, leaving petioles behind. The leaf symptoms can look very similar to symptoms caused by brown stem rot (BSR), however, the pith in the stem remains white in plants infected with SDS while the pith in the lower stem becomes brown in plants infected with BSR. Infection in the lower stem can be seen as a brown-gray discoloration that develops just under the epidermis and into the vascular tissues. SDS also causes root rot and results in death of nitrogen-fixing nodules, and in moist soil conditions a blue fungal growth sometimes develops on the surface of infected roots.

Figure 3. If stems are split late in the season, healthy stems have white pith tissue, while pith in BSRinfected plants tissue is brown. [click to enlarge] Dean Malvick

Figure 4. BSR infected leaves may turn brown and dry while remaining attached to the stem, and plants may mature and drop their leaves prematurely. [click to enlarge] Dean Malvick

Brown Stem Rot (BSR) is widespread across MN and is occurring in many fields this season. The BSR pathogen (a fungus) infects through the roots and stem early in the growing season, but symptoms typically do not appear until 10 to 30 days before harvest. Either just stem or both leaf and stem symptoms may develop. If stems are split late in the season, healthy stems have white pith tissue, while pith in infected plants tissue is brown. The browning may only occur at the nodes or lower stem. On the leaves, in some but not all cases brown and yellow discoloration develops between the leaf veins.

Leaves may turn brown and dry while remaining attached to the stem, and plants may mature and drop their leaves prematurely. The BSR leaf symptoms can be confused with SDS symptoms, but these two diseases can be most easily differentiated by the internal stem symptoms. Both SDS and BSR can occur in the same plant. 

Charcoal rot is also known as dry weather wilt. Charcoal rot is typically more common across southern, dry areas of the Midwest, but it occurs in Iowa and North Dakota and is more widespread in Minnesota than is often recognized. It can probably occur almost anywhere in Minnesota where plants are stressed, especially due to hot and dry weather. The most seriously affected areas are often in the drier or sandy parts of fields.

Figure 5. Plants infected with charcoal rot will have tiny gray/black specks called microsclerotia that appear similar to scattered bits of charcoal dust. [click to enlarge] NSRL

Figure 6. If the root and lower stem of plants infected with charcoal rot are cut diagonally, wavy gray to black streaks and tiny black specks are often seen. [click to enlarge] Dean Malvick

Look for several characteristics for diagnosis. Leaves of severely infected plants prematurely turn yellow and brown, especially on the tops of plants, then wilt and often stay attached to the plant. The initial yellowing is usually more uniform across the leaf than that caused by SDS or BSR. A more clear diagnosis is based on the appearance of the root and lower stem. Scrape off the surface of the root and lower stem with a fingernail or knife and look on and under the epidermis. Plants infected with charcoal rot will have tiny gray/black specks called microsclerotia that appear similar to scattered bits of charcoal dust. They are difficult to see without the help of a hand lens. If the root and lower stem are cut diagonally, wavy gray to black streaks and tiny black specks are often seen. Reports have suggested the pathogen that causes charcoal rot (Macrophomina phaseolina) infects early in the season and then causes most damage when dry, hot weather puts stress on soybeans.

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