Tag Archives: Seedling

Fungal Seedling Disease in Soybean

David Holshouser, Extension Agronomist

Pat Phipps, Extension Plant Pathologist

Rhizoctonia Damping Off and Root Rot.  Rhizoctonia root rot is probably the most common soilborne disease in Virginia soybeans.  Even if other diseases pre-dominate in a diseased plant, rhizoctonia could easily be a component of the problem.

Preemergence symptoms are typical of common seed rots, but are not usually recognized just because these plants never emerge.  More recognizable is the damping off that occurs in the seedling stage.  This will usually occur before the first trifoliate leaf develops.  Infected plants will have a reddish brown lesion on the emerged shoot at the soil line.  This lesion is most visible after the seedling is removed from the soil.

 Resistance to rhizoctonia is not available; variations in variety tolerance have been reported though.  Stresses such as herbicide injury, poor soils, insect damage, and feeding by soybean cyst nematode will increase damage.  Several fungicide seed treatments are effective for this disease.

Fusarium Root Rot.  Fusarium is another common disease in Virginia.  It is one of the diseases that has been implicated in “Essex Syndrome” that we continue to battle in some parts of Virginia.  There are several species of fusarium and each can cause a different plant reaction and/or disease.

 Two of the species, F. oxysporum and F. solani can cause root rot.  The root rot caused by F. oxysporum usually develops on seedlings and young plants during cool weather (<60O soil temperatures).  Older plants are generally less susceptible than younger ones.  Seedlings will emerge very slow and the resulting seedlings are stunted and generally unhealthy.  Symptoms are usually found confined to the roots and lower stems.

F. solani causes preemergence damping-off and root rot.  Damping off after the seedlings emerge is less of a problem, but can occur.  Lesions are generally on the roots and are dark brown to reddish brown to black.  Lesions can also occur on the young stem.

This disease is common in nematode-infested fields.  Soybean cyst, root knot, and sting nematodes will predispose seedlings.  Soybeans growing in soybean cyst nematode-infested fields will frequently develop fusarium symptoms.  This is less likely in root knot infested fields because the injury to the plant from root knot nematode is limited to the root tip.  In contrast, larvae of soybean cyst nematode migrate within the cells and cause more wounding.  In addition, F. oxysporum often interacts with rhizoctonia.

There is some variety resistance to the disease, but this information is not always published in the company literature.  Warm soils that are well-drained are helpful in managing the disease.  Good soil fertility should be maintained and soil compaction avoided.  Fungicide seed treatments provide some, but limited control.

Pythium Damping-Off and Root Rot.  There are many different species of pythium and the dominant species that is present will vary from geographical region to region, usually depending on temperature.  Pythium will cause pre- and postemergence damping-off during the young seedling stages.  It can also cause a root rot in later vegetative stages.  Seedlings may fail to emerge and will have a short, discolored root.  After emergence, symptoms can resemble those of other seedling diseases, especially fusarium and phytophthora.  The disease begins as water-soaked lesions on the young stem or on the cotyledons (seed leaves), and then followed by brown soft rot.

Variety resistance to pythium is not available, but fungicide seed treatments containing metalaxyl or mefenoxam will control the disease.  The best way to avoid the disease is to avoid planting into cool soils (<60oF).

Phytophthora Root Rot.  Of all the seedling disease that you may have heard about, phytophthora is probably the one that you hear and read about most.  It is a serious problem in the Midwest and affects young seedlings and older plants.  Many of our varieties that we grow in Virginia have varying levels of resistance to multiple races of phytophthora.  Yet, most of you have probably never had the disease.  Why is that?

Phytophthora rot is most severe in poorly drained clay soils that are readily flooded.  Most of our soils are sandy in nature, or if a clay, are well-drained.  This doesn’t mean you can’t have the problem just that it is less likely.  Plant loss can occur in lighter soils or on well-drained soils if they are saturated for an extended period of time when the plants are young.

Symptoms are the typical root rot and pre- and postemergence damping off.  The disease is often not diagnosed because it is confused with flooding damage.  Root and stem rots occurring later in the season will occur under similar, saturated conditions.  Tolerant cultivars may escape damage.  Damage does increase with reduced tillage, especially no-till, mainly because those fields absorb more rainfall and can be more easily saturated if the field is poorly drained.  Like most diseases, continuous soybean will increase likelihood of infection and damage.

Insure Rapid Seedling Emergence and Survival

Once the seed is in the ground with good soil-to-seed contact (insuring a good environment for germination), we must then turn our attention to getting the growing plant out of the ground as quickly as possible and insuring that it survives.  The longer the seed takes to germinate and emerge, the greater likelihood of poor stands and unhealthy plants.

Soil Moisture and Seedbed Condition.  Always try to plant into moist soils.  However, I recognize that this is easier said than done, especially when several hundred acres of soybean needs to be planted and it’s getting closer and closer to wheat harvest or you need to side-dress the corn, or many other things need doing.  Still, seed are expensive and replanting is even more expensive.

If the soil is completely dry, then the seed will usually wait on moisture.  Unfortunately, during dry times, the field is rarely completely dry.  If there is enough moisture to swell the seed but not get the seedling out of the ground, then we have a problem.  Seed need to imbibe over half of their weight in water to germinate.  The critical seed moisture content for germination is 20%.  Germination is not affected if the seed has imbibed water for 6 hours (seed is swollen, but seed coat not broken), then it dehydrates to 10% moisture.  If seed has imbibed water for 12-24 hours (seed coat is broken, no radical), then germination may be reduced 35-40%.  If the radical has emerged and seed drops to 10% moisture, few if any seedlings will survive.

Surface soil compaction will physically hinder emergence and also result in less soil moisture because of less pore space.  Row spacing will affect seedling emergence under crusted or compacted soil conditions because seed are closer together within the row; therefore, will exert more upward force, per foot of row, to emerge.

Soil Temperature.  The ideal temperature for soybean germination is 77OF and the optimum range is 68 to 86OF.  The maximum is 94OF, where temperature will actually inhibit germination.  Planting early (mid-April to mid-May) may delay emergence due to cool soils.  Planting in July increases risk of high soil temperatures, but soil moisture during this time of year is usually the bigger problem.  I did see high temperature inhibition in some of our double-crop plantings in 2010.  Regardless, if the soil temperature is not in the optimum range, emergence will be delayed.  With this delay comes a greater likelihood of the seedling being attacked by disease or insects.

Planting Depth.  Soybean seed depth should be between ¾ to 1¼ inches.  If topsoil moisture is lacking, use the deeper placement, but never plant deeper than 1½ inches.  If topsoil is adequate shallower seed placement may speed up emergence, but probably won’t make much difference under warm soil conditions.   Just make sure that the seed is in contact with moist soil.  Placement depth is controlled by the gauge wheels.  Ideally, these should be adjacent to the disk opener.  If controlled by the seed firming/closing wheel, then uniformity of seed depth could be erratic.  The deeper you plant, the longer it takes for the seedling to emerge.  The seedling physically has further to go to emerge, plus the soil may be cooler at deeper depths.  Germination and emergence rate is controlled by temperature.  So, when soils are warm, deeper planting depth has less detrimental effect on emergence.  If soils are hot, deeper placement may actually be beneficial.  Finally, uniform time of emergence of seedlings should be a goal even though soybean compensate well.

Protecting the Seed and Seedling.  Certain environments such as cold soils, non-rotated land, poor quality seed, or a field history of insect pests or disease favor specific pests.  Again, the greater time required for emergence, the greater probability that the seed will become infected with soil-borne disease.  Therefore, rapid emergence is the best defense against seedling disease.  No-tillage/high residues, early planting, planting into cold soils, and deep seed placement are all factors that may delay emergence; therefore, your management will need to be adjusted when one or more of these conditions are present.  Seed treatments will protect the seed and seedling if emergence is delayed, but should only be used if other methods to decrease emergence time have been exhausted.

To review, here’s a checklist for rapid seedling emergence:

  • Know the germination and vigor of your seed.
  • Insure good soil-to-seed contact
  • Plant soybeans ¾ to 1 ½ inches deep and into sufficient soil moisture.
  • Plant into warm soils (68 to 86OF).
  • Consider fungicide seed treatments if planting early into cool soils.