Issue # 4, May 26, 2004

• Cold Soils and Soybean Emergence

• Minimum Stand for Soybean

Cold Soils and Soybean Emergence

    Current weather conditions have not been favorable for soybean emergence.  As a result, several diseases may affect emergence and stand establishment.  Diseases of concern include; Phytophthora root rot, Rhizoctonia root rot, Pythium and Fusarium root rot.  The best defense against these diseases is a good fungicide seed treatment and Phytophthora resist varieties.  Nothing can be done to protect the seed once it's been planted.
Phytophthora root rot: Is a common soil fungus that survives from growing season to season as spores or mycelium in soil or in crop refuse buried in soil.  Soybeans may be infected at any stage of development.  This disease is associated with cool temperatures and high soil moisture.  The best diagnostic symptom is a brown discoloration of the stem and lower branches that extends from below the soil line upward several inches above the soil line.  The tap root is dark brown and the entire root system may be rotted. The disease pattern varies within a field.  Control is to plant resistant varieties.
Rhizoctonia root rot: This disease produces no spores and survives in the soil as sclerotia.  It will grow as a saprophyte on dead plant material in the absence of soybeans.  This disease occurs when conditions are unfavorable for germination and emergence.  Disease symptoms appear as brown, dark brown, or reddish discoloration at the base of seedlings or on roots just below the soil surface.  Use fungicide seed treatments for control.
Pythium: Is a fungi that are found in most cultivated soils and survive in soil or crop refuse as oospores and mycelium.  This disease is found under cool, moist conditions and is associated with seedlings and young plants.  Infected plants develop a "wet rot" which results in pre- and post emergence damping-off.  Cortical tissues may "slip off" when infected seedlings are pulled up.  Control is to use recommended seed treatments.
Fusarium root rot: Is a common soil fungus that survives in the soil as chlamydospores and as mycelium or chlamydospores in crop refuse.  This disease develops on seedlings and young plants under cool, moist conditions throughout a field.  A dry rot of upper roots and lower stems is associated with delayed emergence, and stunted and weak plants.  Under droughty conditions, seedlings may wilt and later develop adventitious roots is moisture is available.  Control is to use recommended seed treatments.

Minimum Stand for Soybean

    Minimum stand for soybean is suggested to be around 75,000 which would be approximately a 50% (based on 150,000 plants/a) stand loss. If one uses the "hula hoop" method of determining stand counts, then one would need a minimum of 1.7 plants per square foot. Even with a 50% stand loss, yield reduction would be somewhere between 10-20%.  This is based on uniform stand reduction.  If large areas of the stand is lost, this will reduce yields further and replanting should be considered.

    The following table is the minimum stand for various crops.

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E-mail: john.swenson@ndsu.edu
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