Soybean Seedling Diseases

Questions on Seedling Diseases

1. What are seedling diseases? The term is used to cover seed rot, pre- and postemergence damping-off, and root rot. Most soybean seedling diseases are caused by soilborne fungi, mainly Pythium spp. and Rhizoctonia solani, or by seedborne fungi such as the Phomopsis/Diaporthe complex. Pythium spp. and Rhizoctonia solani may cause all four stages of seedling disease symptoms, while the Phomopsis/Diaporthe complex is primarily responsible for seed rot and preemergence damping-off.
2. What is the connection between poor quality seed and seedling disease? Poor quality seed resulting from seed coat contamination by Phomopsis/Diaporthe and other fungi generally means lower germination and vigor. Thus, there is a greater chance of seed rot and a longer period between germination and seedling establishment. The longer this period, the greater the chance for disease loss.
3. How does weather affect seedling diseases? Cool (less than 68 °F), wet, poorly drained soils slow germination and the plant-growth processes and favor many of the fungi that cause seedling disease.
4. How prevalent are the fungi that cause seedling disease? Continued soybean cropping has increased populations of the soilborne fungi Pythium spp. and R. solani. Contamination of seed by Phomopsis/Diaporthe is most serious when harvest delays the previous season are accompanied by warm, wet weather, resulting in a high proportion of infected seeds. Do not use such seeds for planting purposes.
5. How can you recognize seedling diseases? The damage these fungi cause at times may be confused with herbicide damage, effects of low pH, or other seedling disorders. The following should help in recognizing seedling disease.
   • Pythium causes seed rot or kills young seedlings before or shortly after emergence. The infected areas of the root are soft but rapidly turn brown, and tissues slough off to leave a "wire root" appearance.
   • Rhizoctonia on roots frequently kills young soybean plants. Look for sunken reddish-brown areas on the main and upper roots near the soil line, which resemble "sore-shin" on cotton. The infected area is firm, in contrast to Pythium.
   • Fusarium occasionally causes a root rot problem on seedlings. Affected seedlings are stunted and weak. The lower part of the taproot and lateral root system may be destroyed.
   • Suspect Phomopsis/Diaporthe seedling disease problems when moldy, decayed, partially germinated seeds are found in the seed-drill area.
6. How does a fungicide seed treatment work? A seed treatment will not make good seed out of bad seed. Seed that is not going to germinate will not do so even if treated. A fungicide can provide a zone of protection around the seed. It can help prevent seed decay for a short time in unfavorable weather conditions.

   As favorable germination conditions return and germination occurs, the young seedling has a good chance of emergence. However, even treated seed under prolonged unfavorable conditions may not produce a uniform stand.

7. Which type of treatment should I plan to use-a "liquid hopper box" or a "dry hopper box?" The liquid treatment generally gives better coverage and fungicide adhesion to seed coats. The dry treatment performs satisfactorily if the fungicide is layered with the seeds in the hopper box or a mechanical mixer is used to distribute the fungicide evenly. Generally, dry treatments are less expensive than are liquid treatments.
8. What about treatments containing a mixture of fungicides, molybdenum, and inoculant? There is no interaction between the fungicide and molybdenum; however, research indicates fungicides packaged with rhizobium inoculant can kill the nitrogen-fixing bacteria. If you use an inoculant, it should be packaged separately, applied last, and applied immediately before planting.
9. Can you use a dry- or liquid-hopper-box treatment in a grain drill? Yes, but be extra careful to insure good fungicide-seed mixing.
10. Can you use hopper-box treatments in air planters or plateless planters with monitor lenses? The dry treatments generally cause problems. Liquid treatments work if applied in advance and seeds are allowed to dry before planting. Consider using pretreated seeds.

Criteria for Soybean Seed Treatments with Fungicides

1. Seed germination is less than 80 percent;
2. You observed stand problems from seedling disease in a particular field in the past;
3. You expect wet soils for prolonged periods when planting early (before May 15) or when planting late (when soil temperatures are high), or
4. Wheat stubble or residue is present in the seedbed.

Warning: Treated seeds cannot be used for oil, food, or other-than-planting purposes. Read and follow label instructions before using any pesticide product. An effort has been made to make the list of seed treatment fungicides as complete as possible. Use of trade names in this publication does not imply endorsement of the products named or criticism of similar ones not mentioned.

By Joseph A. Fox, Ph.D., Extension Plant Pathologist; Gabe Sclumbato, Ph.D., Mississippi Agriculture, Forestry, and Experiment Station Plant Pathologist; J. Frank Killebrew, Ph.D., Extension Plant Pathologist; and Harry Fulton, Pesticide Registration, Bureau of Plant Industry

Mississippi State University does not discriminate on the basis of race, color, religion, national origin, sex, age, disability, or veteran status.

Information Sheet 1167
Extension Service of Mississippi State University, cooperating with U.S. Department of Agriculture. Published in furtherance of Acts of Congress, May 8 and June 30, 1914. Ronald A. Brown, Director

Copyright by Mississippi State University. All rights reserved.

This document may be copied and distributed for nonprofit educational purposes provided that credit is given to the Mississippi State University Extension Service.

Visit: DAFVM || USDA
Search our Site || Need more information about this subject?
Last Modified: Friday, 17-Jan-03 10:57:04
URL: http://msucares.com/pubs/infosheets/is1167.htm
Mississippi State University is an equal opportunity institution.
Recommendations on this web site do not endorse any commercial products or trade names.