Although relatively few soybeans have been planted yet this year in Illinois, millions of acres will be in the ground very soon. Assuming that all the proper agronomic decisions for a good soybean crop have been made and implemented, seed and seedling diseases are major factors that frequently affect establishment of soybean stands and ultimately yield in many areas of the state. I get a greater volume of questions about this group of soybean problems than any other single disease issue on any crop in Illinois. Not all areas of Illinois seem to have significant problems with seed and seedling diseases, but some areas, such as much of the northeast quarter of the state, seem particularly prone to these problems. This article will briefly describe soybean seed and seedling diseases, their characteristics and the conditions that favor them, and approaches to managing them. Additional articles will appear in future issues of the Bulletin on topics related to soybean seed and seedling diseases.
What should you watch 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 indication of these diseases is reduced germination and/or emergence that results in a thin stand; the second indication is damping-off that kills seedlings after they emerge. These disease 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 because symptoms may be postponed while the infection progresses and kills the plant tissues.
What are some of 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 the main problem. Tan-brown, soft-rot symptoms on roots and stems caused by Pythium and Phytophthora are very similar and cannot be 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.
A new, useful publication with full-color photographs for identification and management of seed and seedling, as well as other, diseases is the Pocket Guide to Soybean Diseases in the Midwestern U.S. It is available from Information Technology and Communication Services (800-345-6087; www.PublicationsPlus.uiuc.edu). In addition, photos and information on soybean seed and seedling diseases can be found on the University of Illinois Field Crop Diseases Web site (http://cropdisease.cropsci.uiuc.edu/).
If a significant problem appears, what should be done this year?
- 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 these plants can be sent to the UIUC Plant Clinic for diagnosis. Proper diagnosis of the major problem in particular fields can help with management because management recommendations can differ for different pathogens (for example, the Pythium and Phytophthora rots). The scouting should include inspections of drainage systems to ensure that nothing is out of order and that drainage is occurring as expected for the area.
- Replanting. A decision to replant or not 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 23rd edition of the Illinois Agronomy Handbook, which is available from Information Technology and Communication Services. 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. These decisions are easier to make when the disease problem has been accurately diagnosed. Too many producers report replanting twice with the same cultivar that has no seed treatments and no resistance to Phytophthora, and fortunately the stand is usually good after the third planting, because by then the weather and soil conditions typically are warm and relatively dry.
Should seed treatments be used? No absolute answer to this question exists. Sometimes seed treatments provide a significant benefit, while at other times they don't provide a clear benefit. Some considerations include planting date, problems with seed and seedling diseases in the past, major types of seed and seedling diseases present, risk tolerance, soil compaction, seeding rates, drainage, 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 have the most benefit during cool and wet conditions.
Not all seed treatments are equally effective against the different seed and seed-rotting pathogens. Two general groups of fungicides exist for control of these pathogens. The systemic compounds Allegiance-FL, Apron-FL, and ApronXL are most effective for control of the "water-loving" pathogens Phytophthora and Pythium. The other group includes Rival, Maxim-4FS, captan, and several other products that protect against Fusarium, Rhizoctonia, and other true fungal pathogens. Azoxystrobin is a new systemic fungicidal seed treatment that recently became available for soybeans in the product SoyGard, for control of Rhizoctonia and perhaps Fusarium.
For full-spectrum control of different pathogens, different combinations of these products are often used. For example, Rival and Allegiance are often applied together, azoxystrobin is combined with metalaxyl, carboxin is combined with PCNB and metalaxyl, and the product ApronMaxx RTA is a mixture of ApronXL and Maxim.
In summary, many different management practices, including seed treatment fungicides, can reduce the damage to soybean crops that often occurs from seed and seedling diseases in Illinois.--Dean Malvick