Soybean Seed and Seedling Diseases
Soybean Disease Information Note 2
Stephen Koenning, Extension Plant Pathologist
Jan Ferguson, Extension Seed Specialist, Crop Science
E. James Dunphy, Extension Crop Scientist

Seedling diseases in soybean are a minor, yet chronic problem in North Carolina. Our mild coastal climate usually results in warmer soil temperatures earlier in the spring than occurs in states further inland. As a result, seedling diseases are generally of minor importance in this state. Nevertheless, because of the wide range of planting dates and soil types encountered, seedling diseases cause problems for some growers on a yearly basis. Seedling diseases can cause reduced stands and limit crop growth, thus getting the crop off to a slow start. These factors may lower yields and make management of other pest problems more difficult.

Environmental Factors

Seedling diseases tend to be more severe in poorly drained soils. They can, however, present problems in any soil type, especially when high rainfall or cold weather follow planting. Any factor that delays germination and seedling emergence such as poor seed quality, inadequate seedbed preparation, compaction, planting too deep, nematode infestations, and high rates of some herbicides can contribute to the incidence and severity of seedling diseases.

Distinguishing between Diseases and Herbicide Injury

Most soybean fields receive one or more herbicides, and most of the available soybean herbicides have the potential to injure soybeans. Some can generate visual symptoms that are similar to disease symptoms. Soybean tends to be more seriously injured by broadleaf herbicides than grass herbicides. Pre-emergence herbicides whose label suggests planting soybean below a certain depth probably have the greatest potential to injure soybeans if the herbicide gets into the soybean root zone.

Distinguishing between herbicide injury and seedling diseases is frequently difficult. The pattern of where symptoms do and do not appear may give the best indication of whether it is a disease problem or herbicide injury. Symptoms of seedling diseases usually occur in irregular patterns which may correspond to changes in soil type. A disease is more likely to be severe on one plant and show no symptoms on adjacent plants than is herbicide injury. The pattern of herbicide injury symptoms typically appear to be equipment related, although they may be modified by differences in soil type. Weed control is often excellent in the affected areas, and seldom will one plant be seriously affected and the adjacent plants appear unaffected. Finding an area in the field the sprayer may have missed, or overlapped in a previous pass, can be very useful in diagnosing herbicide injury.

Soilborne Diseases

Soybean seedling diseases are caused by fungi that reside in the seed or the soil. Pythium spp., Phytophthora sojae, and Rhizoctonia solani are the soil fungi most commonly associated with seedling diseases. Pythium and Rhizoctonia are found in all agricultural soils. These fungi will commonly cause a root rot and either lesions on the stem or soft watery stem tissue. See the figure at the top of this note.

Sore shin (a red-to-brown lesion at or above the soil line), damping off and root rot can be caused by Rhizoctonia solani. Plants frequently recover from the root rot phase if optimal conditions for soybean growth occur soon after emergence. This fungus affects many plant species and is found in all soils. Other common soil inhabitants are various species of Pythium. Pythium rot can occur at any stage of plant development, but is primarily a seedling disease favored by high moisture. A variety of symptoms are associated with this disease: a seed rot, a wet rot (soft watery stem tissue), baldhead (retarded development of the growing point), swelling of the stem below the cotyledons (which can be confused with herbicide injury), a root rot and/or wilt and death of seedlings. Diseases caused by Pythium spp. and Rhizoctonia may be suppressed, but not eliminated by rotation with grass crops such as corn or sorghum. Although rotation is not an especially effective tactic for managing these diseases, the reduction in other soybean pests which occurs with rotation generally results in a lower incidence of these seedling diseases. Seed-treatment fungicides containing thiram or PCNB are reasonably effective against Rhizoctonia solani, whereas seed-treatments containing mefanoxam (Ridomil Gold, Apron) are necessary if Pythium is the problem.

The soil fungus responsible for Phytophthora rot of soybean, Phytophthora sojae, is similar to Pythium spp. It is most commonly a problem in heavy, poorly drained soils or in low spots in the field. Diseases caused by this organism range from seed rot to stem and root rot, which may result in plant death. Rotation with non-leguminous crops (two or more years may be required), resistant or tolerant varieties and fungicide (mefanoxam) treatments can be used to manage this pest.

Seedborne Diseases

Several fungi that cause seed decay and seedling diseases are seedborne. Phomopsis seed decay, frogeye leaf spot (SOY003), anthracnose, purple seed stain, and downy mildew can cause seed rots, reduce emergence and prevent adequate stand establishment. These problems are usually more severe in cool wet soils. The best tactic for avoiding these diseases is the use of high quality disease-free seed.

The most common of these seedborne diseases is phomopsis seed decay. Seed infected with Phomopsis spp. typically have greatly reduced seed germination in a cold test compared to a standard germination test. This seed decay, like most other seedborne diseases, is a result of delayed soybean harvest and humid conditions during seed development. Pod and stem blight caused by Phomopsis spp. is present in nearly all fields at soybean maturity. If soybean harvest is late and warm humid conditions persist, infection of the seed will occur. Seed from these plants may not germinate and may produce seedlings lower in vigor. The other seedborne fungi mentioned cause similar problems, but are less common. Late-season scouting of soybean fields should include identification of foliar, pod, and stem diseases if soybeans are grown for seed. Seed should not be saved if anthracnose or frogeye leaf spot are present in the field. Your County Agent can forward plant samples to the Plant Disease and Insect Clinic if you are uncertain as to exactly which diseases are present. Seed saved for planting next year should be tested for germination. Send seed lots to: NCDA-Seed Laboratory, P.O. Box 27647, Raleigh, NC 27611. If seed of dubious quality must be planted, the use of a fungicide-seed treatment may prove beneficial. Seed treatments containing carboxin and/or captan are most effective against these fungi with the exception of downey mildew, which can be controlled with metalaxyl.

When to Replant

Soybean can yield well over a relatively wide range of plant population densities. Unless populations are so high that lodging becomes a problem or so low that the canopy does not close, yields will not be affected markedly. Target population densities are 6-8 plants per foot of row with 36-inch row spacing and 2 per foot of row in 7-inch rows for May-planted soybean. A higher population is desirable in June with 9-11 plants per foot of row and 2.5-3 plants per foot of row with 36- and 7-inch rows, respectively.

If the remaining plants are healthy, and fairly uniformly distributed, stands of half the levels in the previous paragraph are probably adequate. The added productivity of the replanted crop would have to be enough to cover the additional twenty to thirty dollars per acre cost of replanting. The later planting date lessens the likelihood that this will happen. The greatest uncertainty comes in guessing the current and future health of the remaining plants. When evaluating an uneven stand of very young soybean plants, it is difficult not to view the remaining stand pessimistically. Probably more fields are replanted which did not need to be than fields not replanted which should have been.

The herbicides used on the field must be considered when a decision to replant is made. Do not apply more pre-emergence herbicide with the second planting. Scout the field periodically to determine if a post-emergence treatment is necessary. Tillage can be used to dilute the concentration of the broadcast herbicide if it is labeled for preplant incorporation and is suspected of being part of the problem. If the broadcast herbicide is not labelled for preplant incorporation (e.g., linuron and linuron tank mixes), or does not appear to have been leached deep enough into the seedbed to be causing injury to the first crop, additional tillage should not be performed before replanting. If a soil-applied herbicide was banded over the original planting, and the decision is made to replant between the original rows (and thus away from the banded herbicide) a banded herbicide could also be used over the replanted rows as well, provided the sum of the two banded applications do not exceed the labelled per-acre rates for that herbicide.

Management of Seed and Seedling Diseases

1. Plant high quality disease-free seed. Professionally grown seed is worth the added cost.
2. Rotations with a grass crop such as corn or grain sorghum are generally beneficial in reducing population densities of soilborne pathogens.
3. Soils which tend to be poorly drained and/or cool should be planted later in the season when conditions for soybean germination and growth are optimal.
4. Consider resistant or tolerant varieties for fields where Phytophthora rot is frequently a problem.
5. Seed treatments may be beneficial when planting in cool, wet soils or when seed is of marginal quality.
6. In-furrow fungicide treatments may be considered when a persistent problem with Phytophthora sojae or Pythium spp. occurs and other options are unacceptable.
7. When soybean are grown for seed, inspect the crop for diseases and harvest in a timely manner.

Other Resources

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Soybean Disease Atlas
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For assistance with a specific problem, contact your local North Carolina Cooperative Extension Service personnel.

Published by North Carolina Cooperative Extension Service

Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age, or disability. North Carolina State University at Raleigh, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.

Recommendations of specific chemicals are based upon information on the manufacturer's label and performance in a limited number of trials. Because environmental conditions and methods of application by growers may vary widely, performance of the chemical will not always conform to the safety and pest control standards indicated by experimental data.

Recommendations for the use of chemicals are included in this publication as a convenience to the reader. The use of brand names and any mention or listing of commercial products or services in this publication does not imply endorsement by the North Carolina Cooperative Extension Service nor discrimination against similar products or services not mentioned. Individuals who use chemicals are responsible for ensuring that the intended use complies with current regulations and conforms to the product label. Be sure to obtain current information about usage and examine a current product label before applying any chemical. For assistance, contact your county North Carolina Cooperative Extension Service agent.

Last update to information: May 2000
Last checked by author: May 2000
Web page last updated on May 2000 by A.V. Lemay.