UPDATE: Seedling Blights and Seed Decay of Soybean

May 26, 2000
Current weather conditions around the state can account for various seedling disease complexes on soybeans. The more common organisms causing seedling damping-off and blights are Pythium, Phytophthora, Rhizoctonia, and Fusarium spp., which were discussed in detail in "Soybean Seed Treatments for 2000," by Bruce Paulsrud and Wayne Pedersen in the April 7 issue (no. 2) of the Bulletin. However, other seedling blights that are not quite so common are starting to appear in fields. These are seedborne blights such as Anthracnose (caused by Colletotrichum spp.), Phomopsis seed decay (caused by Phomposis longicolla), and purple seed stain (caused by Cercospora kikuchii). All three of these organisms can cause seedling diseases that result in reduced stands. The occurrence of these three diseases is more related to seed-quality issues. All three of these diseases are seedborne and especially problematic on bin-run seed. Typical symptoms growers may see that are caused by these seedling blights can be seen in the photo below.

Seedling blight. (Photo courtesy of Mike Roegge.)

Anthracnose. Symptoms on infected seeds many not be visible, or they may appear as brown staining or small, irregular gray spots with black specks. Seeds may not germinate, or germinated seedlings may damp-off prior to or after emergence. Dark brown, sunken cankers may be seen on the cotyledons and can extend upward on the emerging stem or downward toward the root. Cotyledons may appear water-soaked and withered and fall off during humid weather. This organism has the potential to attack the soybean during all stages of development, and it can grow from infected cotyledons into young stem tissue. In the stem tissue, numerous cankers can develop, leading to death of the young plant. Management options include using high-quality seed that is relatively pathogen free. Infected seed needs to be treated with a recommended fungicide treatment.

Phomopsis. Typically, soybean seeds infected with Phomopsis are shriveled, elongated, and cracked and appear chalky white; however, this is not always the case. Infected seed can look normal and not show symptoms. Seeds that are infected with Phomopsis probably will not germinate, or germination may be slowed. Damping-off of seedlings occurs either before or after emergence; and when severe conditions exist, stand reduction can lead to yield reductions. Soybean seed is more prone to infection by Phomopsis after the plant has reached physiological maturity. Infection of the seed tends to be more likely if harvest is delayed, on early maturing cultivars, or when harvest occurs during warm, humid weather. This pathogen also infests soil and crop debris; therefore, crop rotation with corn can help reduce infection via the field. Use high-quality seed that is relatively pathogen free. When bin-run seed is used and contains greater than 15% infection by this organism, a recommended seed treatment should be applied.

Purple Seed Stain. Purple stain on the seeds is usually very conspicuous and appears as a range of pink to dark purple discolored areas as small as a speck or an irregular blotch, or it may cover the entire surface of the seed. Once again, as with the other two seedling blights, infected seeds may not show symptoms. Infected seeds can result in infected seedlings that have shriveled cotyledons that turn dark purple and drop prematurely. The organism can move from the cotyledons and infect young soybean stems, causing necrotic areas that girdle and kill the young plant. Management of this pathogen is through the use of less susceptible cultivars and the use of high-quality seed.

Another seedling pathogen that may enter the field via infected seed lots is Sclerotinia white mold. More details on this pathogen and the disease it causes will be printed later in the season.--Loretta Ortiz-Ribbing

Author: Loretta Ortiz-Ribbing