Mid- to Late-Season Soybean Problems

Charcoal Rot

Charcoal rot is the most serious disease of soybeans in Kansas.  It is favored by hot, droughty conditions.  Statewide yield losses have approached 20% in the worst years.  In an average year, losses range from 5-10%, although individual fields can have 100% loss from charcoal rot.  Losses from charcoal rot and drought occur simultaneously and are difficult to separate.  In general, plants infected with charcoal rot will die about two weeks earlier than plants affected by drought alone.

Because of the shallow soils, fields in southeast Kansas generally have the most problems with charcoal rot, however, charcoal rot is common in all of the major soybean production areas except the irrigated acreages in western Kansas.

Soybean Cyst Nematode

Nearly all of the major soybean producing counties in Kansas are infested with soybean cyst nematode (SCN).  Nationwide it is the number one soybean disease problem, although in Kansas losses are well behind those caused by charcoal rot.

The most commonly observed symptom associated with SCN is reduced yield.  Yield loss may occur for several years before visible symptoms appear.  The first noticeable symptoms are roughly circular spots in the field in which soybean plants may show signs of stunting, yellowing, or nutrient deficiency.  Roots have fewer feeder toots and nitrogen-fixing nodules.

In heavy infestations, symptoms are most evident by the time plants are 6 to 12 inches tall.  In lighter infestations, symptoms may not appear until the beginning of pod formation.

The glossy, enlarged bodies of female cyst nematodes can often be seen attached to roots.  Mature cysts are more difficult to see because they are only the size of a pinhead and their color changes from white to a golden yellow to dark brown as it matures.  They also dislodge easily form the roots unless the soil is carefully broken away.  Early in the season (about one month after emergence), white females are most common, but as the season progresses, all stages of cyst colors are present.  Attached cysts may be easier to find on roots near the perimeter of a severely affected area, rather than on the most severely damaged plants.  Do not rely upon visual inspection of attached cysts for diagnosis.  The only sure way to identify SCN damage is by a soil test.

Sudden Death Syndrome (SDS)

Symptoms of SDS generally begin around the time of pod fill.  An unusual aspect of the disease is that it generally occurs only in high yield environments. Thus, in Kansas, most SDS is found in irrigated fields. Also, SCN can serve as a predisposition agent for SDS.  Therefore, the majority of SDS infected fields in Kansas are also infested with SCN.

The first noticeable symptom is the development of scattered chlorotic spots between the veins of the leaves.  These spots gradually enlarge and coalesce into chlorotic streaks that eventually turn brown.  The veins remain green, however.  Severely affected leaflets will drop to the ground leaving the petioles still attached.  In severe cases, the affected area of the field will have a brown cast to it. A gray to brown discoloration is present when the taproot and lower stem are split open longitudinally.

SDS symptoms are often confused with stem canker or brown stem rot.  Brown stem rot fortunately is not common in Kansas.  Plants infected with the stem canker organism will have one to several cankers on the lower stem that are not found on plants with SDS.

Pod and Stem Blight (Phomopsis stem blight)

Pod and stem blight is a fungal disease associated with wet weather during the time from pod fill through harvest. While it has only minor effects on yield, it can greatly reduce seed quality and is a particular problem for seed producers.  Soybean seeds infected with the Phomopsis fungus, will  be more susceptible to seed rot and seedling blight the following spring if the seed is saved for planting.

No leaf symptoms are present with this disease.  Following plant maturation, small black structures known as pycnidia form on the entire plant. Dead stems may be covered with speck-sized pycnidia, usually arranged linearly, or the pycnidia may be limited to small patches, generally near the nodes.  Pycnidia can also be found scattered on dry, poorly developed pods.  Not all infected pods produce pycnidia, but mature pods with pycnidia always contain infected seed.

Seeds severely infected by the Phomopsis fungus are shriveled, elongated, and cracked with a brown or sometimes white, chalky appearance.  Infected seeds may also show no symptoms at all.

Purple Seed Stain

Like pod and stem blight, yield is little affected by this disease.  The primary concern is the loss of seed quality.

In Kansas, foliar symptoms of purple seed stain are not common.  When present, the upper leaves exposed to the sun develop light purple areas.

Purple seed stain on seeds is easily identifiable.  Discoloration of the seed varies from pink or pale purple to a deep purple.  The discoloration can range from scattered specks to large irregular blotches that may cover nearly the entire surface of the seed coat. One caution in identification is that fields heavily infested with black nightshade may produce seed stained with the juice of the nightshade berries, and this can look very similar to purple seed stain symptoms.

Heavily infected seeds may produce diseased seedlings if planted so purple seed stain infected seed should not be saved for planting.