Many common root, stem and pod diseases can also cause infected plants to wilt, turn brown or even die prematurely.
Pod and Stem Blight
This soybean blight disease may be more common when harvesting is delayed during wet weather. Symptoms may be confused with anthracnose because both diseases can occur together on plants late in the season. Signs of infection can appear in mid-season and on plants nearing maturity.
Key signs of soybean pod and stem blight are many small, black, raised dots arranged in rows on infected stems, pods and fallen petioles late in the season. Upper portions of infected plants may turn yellow and die. Infected seeds are cracked, shriveled and dull and may have a gray mold. Infected seeds can have low viability and decompose at harvest.
Sclerotinia Stem Rot (White Mold)
Sclerotinia stem rot, also called white mold, is particularly a problem in the upper Midwest. Early symptoms may be the wilting of leaves in the upper canopy.
Leaves may look gray-green, off-color or wilted. Cankers may appear on the stems at the nodes. White mold can cause extensive yield losses in fields that otherwise have high yield potential. It is prone to develop in moderate temperatures (less than 82 degrees F) in the canopy and under frequent rainfall, especially as the plants begin to flower and set pods.
Sudden Death Syndrome (SDS)
This soilborne fungus (Fusarium virguliforme) generally appears midsummer, after blooming. Infection actually occurs at the seedling stage, but visual symptoms above ground are late in the season. SDS is identified by small, scattered yellow blotches or spots that begin in the lower leaves and progress upward with maturity. The spots enlarge, merge and turn the tissues brown between the veins. Leaves may curl up or drop prematurely, but the petioles remain firmly attached. Fields with severe foliar symptoms will have a tan to brown cast. Next, flowers abort and pods fail to fill seed. One characteristic of SDS is that the interior, or pith, of the stem remains white. SDS can occur as a disease complex with soybean cyst nematode (SCN), often resulting in early and more severe symptoms.
Fusarium fungi over season in crop residue or soil. The disease is most severe in cool, wet growing conditions and on well-managed soybeans that have high yield potential. The disease is a serious threat from the time the seed starts to germinate and throughout the growing season A soybean field may look healthy one day and startlingly bad the next. With the above ground symptoms appearing so far into the growing season, replanting is not an option.
Rhizoctonia Root Rot
While disease caused by the fungus Rhizoctonia can cause seedling blight, it also causes root rot in more mature soybean plants. At this later growth stage, lower leaves may begin to turn yellow. Plants may appear stunted and less vigorous. Infected plants, when dug up from the soil may reveal poorly developed root systems and lateral roots that are discolored or rotted. The stem may have a brick-red discoloration at the soil line that extends in either direction. Severely infected soybean plants that are stressed by hot, dry conditions may die. If cool, wet conditions occur after a Rhizoctonia infection, a flush of secondary roots above the stem may be visible. Rhizoctonia root rot symptoms often mimic those of charcoal rot.
Also known as dry weather rot, charcoal rot is caused by a widespread soilborne fungus that attacks the roots and bottom section of the plant. Charcoal rot typically develops in hot, dry weather in combination with nutrient-deficient soils or unfavorable growing conditions. In advanced stages, leaves turn yellow, wilt and wither, but remain attached. After the flowering stage, the taproot and lower stem appear silver or light gray. In later stages of the disease, black specks appear underneath the “bark” that is peeled away from the roots and stem base – hence the name charcoal rot. Seedling infestations of charcoal rot may mimic those of Rhizoctonia, producing reddish lesions on the stem of the seedling. Where charcoal rot is present, these lesions can be scraped off with light pressure.
Phytophthora Root Rot
Plants infected early in the season with the fungus Phytophthora may appear as leaf blight, not showing symptoms until later in the season. As plants enter the flowering stage and begin to set pods, symptoms of late-season stem and root rot may develop. Infected older plants show reduced vigor or gradually die over the season. The stems show brown discoloration, which extends upward from below the soil line.
Typically, this fungal disease develops as a stem and pod disease on soybean plants during later reproductive growth. Anthracnose may also cause tip blight in early pod-filling stages. Irregularly shaped brown lesions may develop on pods, petioles and stems. The fungus also produces small, black, fruiting bodies, or acervuli, that erupt in infected tissue. Anthracnose typically develops in warm, wet weather. If wet weather continues through harvest, the symptoms on stems will be more severe.
Brown Stem Rot
This disease is caused by the fungus Cadophora (Phialophora) gregats, which survives in soil and infested crop residue. Foliage symptoms usually develop as soybean plants are beginning to set pods. Leaf symptoms, which may be similar to those of sudden death syndrome, appear as light green to yellow blotches in the interveinal leaf tissue. The vascular tissues and pith of the soybean stem is also affected with a brown discoloration; this can be seen when the stems are split open. Later in the season, this brown discoloration may appear almost continuous within the stem.