Despite a relatively dry growing season, the ISU ISU Plant Disease Clinic still received many soybean samples for disease identification, especially after late August. The clinic also received numerous questions on soybean diseases. Many of the questions indicated a concern about seed quality, including the seeds of the 1997 harvest. The use of pathogen-infected seeds can increase the spread of a seedborne disease, as was discovered by some bin-run seed growers whose fields had severe white mold due to the use of white mold-infected seeds. Growers who want to save soybean seeds for next season are advised to check seed quality before they use them. A few of the diseases that growers may encounter this year are listed below.
Downy mildew is caused by the fungus Peronospora manshurica. The disease was prevalent in late summer, but the damage level was low. The fungus can be seedborne and seeds from infected plants may have fungal colonies on their coats. In Iowa, this disease has not been considered a major production problem and most varieties used in the state are not resistant to this disease. However, seeds infected with downy mildew can be mistaken for white mold-infected seeds, and such a misidentification occurred many times during the 1997 planting season. The pathogen also can survive in crop residues.
Soybean mosaic virus (SMV) also can cause seed quality problems although this disease usually is not a production problem in Iowa. Aphids are the vectors that spread the disease during a growing season. The dry summer this year may have favored the development of this disease. Sometimes, portions of the soybean seed coat have a black discoloration as a result of infection by SMV. However, this discoloration should not be considered as a reliable indicator of SMV because other stresses can cause such mottling. Seed testing is necessary to determine the presence of the infection.
Phomopsis species and Anthracnose species were frequently isolated from premature, dying plants after mid-August of this year. Typical symptoms were bright yellow leaves on the top of the plant followed by death of tissues from the top down. These two fungi can be seedborne. Seeds infested with Phomopsis may be cracked and shriveled and usually have a low germination rate. If these seeds are used, they may result in low emergence or seedling blight. It is recommended to do seed testing if seeds are to be saved from fields where pod and stem blight was severe this year. Seeds from plants infected by Anthracnose may or may not show discoloration. This summer, Anthracnose was more frequently observed in no-till fields compared with the past few years. Both pathogens also survive in crop residues and the use of tillage to bury the residues can reduce disease pressure.
Purple seed stain is caused by Cercospora kikuchii. This fungus causes Cercospora leaf blight during a growing season. Infected seeds have a pink to purple discoloration on their seed coats. Seeds with 50 percent discoloration are considered to be poor quality. The pathogen also survives in crop residues. An Indiana study that is supported by regional soybean check-off dollars has found that this disease is more prevalent in no-till than conventional till fields.
Finally, keep in mind that although the discoloration of seeds is used to identify possible diseases, it may not be reliable because other nonpathogenic factors also can cause discoloration. I have observed discolored seeds from pods having insect wounds. For a reliable identification, seed testing is needed. Furthermore, seed treatments can be effective in controlling some seedborne fungi. The ISU Seed Science Center provides seed testing for a reasonable charge. For more information on seed testing, call 515-294-6821.
This article originally appeared on page 177 of the IC-478(23) -- October 13, 1997 issue.