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Issue No. 11, Article 7/June 3, 2005

Watch for Soybean Seed and Seedling Diseases

In most areas of Illinois this spring, seed and seedling diseases have not been a significant concern because of widespread dry soil conditions. There may not be major problems and subsequent need for replanting this season, but don't ignore the possibility of these problems. Assuming that proper agronomic decisions for a good soybean crop have been made and implemented, seed and seedling diseases are major factors that can affect establishment of soybean stands and ultimately yield in many areas of the state. Not all areas of Illinois frequently have significant problems with seed and seedling diseases of soybean, but some areas, such as much of the northeast quarter of the state, seem particularly prone to them. This article will briefly describe soybean seed and seedling diseases, their characteristics and the conditions that favor them, and approaches to managing them.

What are the major pathogens involved and what symptoms do they cause?

Prior to observing reduced emergence and plant death, the plants often show symptoms that are somewhat characteristic for different pathogens. The major pathogens that cause seed and seedling diseases are the fungal or fungal-like pathogens Pythium, Phytophthora, Rhizoctonia, and Fusarium. They all survive and persist in soil. Any of these pathogens, or a combination of them, can cause seed rot, and it is difficult to determine which is or are the main problem. Tan-brown, soft-rot symptoms on roots and stems caused by Pythium and Phytophthora are very similar and cannot be readily differentiated without laboratory testing. Reddish to dark brown, often sunken, lesions caused by Rhizoctonia are easier to recognize. The symptoms associated with Fusarium damping-off and root rot are light to dark brown lesions. Guidance on identifying and managing seed and seedling diseases, as well as other soybean diseases, is available in the Pocket Guide to Soybean Diseases in the Midwestern U.S. and the Field Crop Scouting Manual, both published by University of Illinois Extension (800-345-6087). Photos and information on soybean seed and seedling diseases can also be found on the University of Illinois Field Crop Diseases Web site.

What should you watch out for and when?

Seed and seedling diseases can begin soon after the seed is in the ground and continue up to and beyond the V2 stage. The first clue to the presence of a disease is reduced germination and/or emergence that results in a thin stand, and the second clue is damping-off that kills seedlings after they emerge. These problems are usually associated with cool and wet soil conditions, although major problems may not be noted until a week or more after the cool and wet conditions occurred.

If a significant problem appears, what should be done?

Scouting and diagnosis. Because these diseases often progress quickly, it is important to scout fields to determine if and when the problem begins. Timely scouting will also enable collection of plants before they become completely rotted and will allow for diagnosis by the UIUC Plant Clinic. Proper diagnosis of the major problem in particular fields can help, because management recommendations differ for different pathogens (for example, management differs for the Pythium and Rhizoctonia rots). The scouting should include inspections of drainage systems to ensure drainage is occurring as expected for the area.

Replanting. Whether or not to replant is based on several factors, including the magnitude of the stand reduction and the planting date. Additional information on replanting decisions can be found in the Illinois Agronomy Handbook (23rd ed.; 800-345-6087). If the decision is made to replant because of disease loss, you may want to consider using fungicidal seed treatments and perhaps a different cultivar with improved resistance to Phytophthora.

Seed treatments. Sometimes fungicidal seed treatments provide a significant benefit, but at other times the benefit is not clear. Considerations include planting date, tillage, past problems with seed and seedling diseases, major types of seed and seedling diseases present, risk tolerance, soil compaction, seeding rates, and tillage. Each of these is affected by the weather. Warm and dry conditions usually result in fewer seed and seedling disease problems, and fungicides generally provide most benefit during cool and wet conditions.

Not all seed treatments are equally effective against the different seed and seed-rotting pathogens. There are two general groups of fungicides for control of these pathogens. The compounds Allegiance-FL and ApronXL are most effective for control of the "water-loving" fungal-like pathogens Phytophthora and Pythium. The other group includes Rival, Maxim, azoxystrobin, captan, and several other products that protect against Fusarium, Rhizoctonia, Colletotrichum, and other true fungal pathogens. For full-spectrum control of seed and seedling pathogens, combinations of these products are often used. For example, ApronMaxx RTA is a mixture of ApronXL and Maxim, Rival and Allegiance are often applied together, and azoxystrobin is combined with metalaxyl in SoyGard. In summary, many different management practices, including seed treatment fungicides, can reduce the damage to soybean crops that often occurs because of seed and seedling diseases in Illinois.

As noted last year, we have limited information on which seed and seedling pathogens of soybean are commonly causing problems in Illinois. We initiated a project last year to get more information about these pathogens in the state, and we are requesting your help again. We would like samples of soybeans with seedling diseases from Illinois to determine which pathogens are causing problems. If you visit fields or get calls regarding soybean disease at the seedling or stand-establishment stage, please collect plants or have them collected (10 plants per field, along with exact location, soybean variety, and seed treatment if any). Send or bring them to me at Department of Crop Sciences, N533 Turner Hall, 1102 S. Goodwin Avenue, University of Illinois, Urbana, IL 61801.--Dean Malvick

Author:
Dean Malvick

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