Late Season Diseases of Soybeans

Sudden Death Syndrome

Identification:

Cut the soybean stem in half to determine if the plant has Sudden Death Syndrome (SDS), because it can be confused with Brown Stem Rot (BSR). As the fungus colonizes the roots it produces a toxin that is translocated to the leaves which causes the typical interveinal leaf symptoms. The fungus overwinters by producing survival structures known as chlamydospores, which can survive in fields for years. Plant infection usually occurs between germination and the V3-V4 growth stage.

Also, SDS can have blue/green fungal growth beneath the soil on the roots when soil moisture is adequate.

Prevention

ILeVO seed treatment helps suppress SDS infection in early growth stages. It’s not a “silver bullet,” but can certainly reduce the disease incidence. ILeVO has nematocidal properties, which aids in the prevention of Soybean Cyst Nematode (SCN). In Ohio, fields with SDS can also have measurable levels of SCN in the same field. Pressures from SCN root feeding further weaken plants, making them more vulnerable to attack from other pathogens like SDS. Fusarium fungal spores have been found on and in SCN cysts, meaning this disease can be transported as cysts move and attack plant roots.

“Cultural” practices to reduce SDS incidence could include: choosingsoybean varieties with a strong SDS and SCN field tolerance, crop rotation, reduce soil compaction or later plantings. Planting early to maximize yield usually means placing seed into cold, wet, no-till soils which compounds disease infections.

Why this year?

May growers experienced good planting conditions, where soils were dry and warm, yet still have severe SDS pressure. Some reasons behind this could be:

  • Continuous soybeans (infected residue from previous soybean crop).
  • SCN pressure, combined with compaction. This was aggravated by multiple heavy rain events that left soils saturated earlier in the growing season allowing for SDS infection.
  • Weak inherent soybean variety resistance. The soybean plants to the right were growing a quarter of an inch apart. One plant is impacted by SDS, the other not. Note the difference in pod numbers.

Brown Stem Rot

Types

Brown Stem Rot is caused by the soilborne fungus Phialophora gregata (P. gregata). There are two distinct types (or genotypes) denoted Type A and Type B. Type A is the more aggressive strain and causes more internal damage and plant defoliation than Type B. P. gregataType A also is associated with higher yield loss.

  1. gregatasurvives in soybean residue, with survival time directly related to the length of time that it takes for soybean residue to decay. Thus, P. gregatasurvives longer when soybean residue is left on the soil surface (e.g. in no till settings) where the rate of residue decay is slow. P. gregatainfects soybean roots early in the growing season. It then moves up into the stems, invading the vascular system (i.e., the water-conducting tissue) and interfering with the movement of water and nutrients.

Influence

Several factors can influence BSR severity. Research from the University of Wisconsin has shown that the incidence and severity of BSR is greatest in soils with low levels of phosphorus and potassium, and a soil pH below 6.3. In addition, P. gregataand soybean cyst nematode (Heterodera glycines) frequently occur in fields together and there is evidence that BSR is more severe in the presence of this nematode. (information courtesy of the University of Wisconsin)

Bacterial Blight

Identification

Brown lesions surrounded by a distinct yellow halo is characteristic of Bacterial Blight disease. The disease overwinters in infected crop residue and on infected seed. The bacteria enters the plant through stomatal openings which are tiny pores used for gas exchange and found mostly on the under surface of the pant, but more easily through a wound in the leaf surface. Hail, sandblasting and insect feeding are more likely to be entry spots.

Yield impact is typically minimal unless infection is severe enough to cause leaf loss.

Keep in mind, Bacterial Pustule and Septoria Brown Spot look similar.

Phomopsis Seed Decay and Pod and Stem Blight

Identification

Phomopsis seed decay and pod and stem blight are two of the diseases that make up the Phomopsis and Diaporthe complex. As with all diseases of crops, when you combine a favorable environment with a susceptible crop host and a source, the complex can manifest itself impacting crop yield.

This complex is seed borne as well as in residue. This causes premature death of plants and reduces seed quality. In 2016, this disease caused significant yield loss up to 35-40% in soybeans plus reduced market value to the farmer from damaged seeds.

Foliar fungicides applied at R2 – R3 in two passes can help reduce the disease.

Information Compiled by: Brian Mitchem, PCT | Sunrise® Research Agronomist

September 6, 2018, 12:09 PM
 

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