Pennsylvania Soybeans: Dealing with Early Season Soilborne Diseases

    Phytophthora root and stem rot in soybean. Photo: Mike Staton, Michigan State University

    With the recent cold air and soil temperatures, germination and emergence of field crops have been delayed in much of the state. Combined with questions about chilling injury in field crops, we should also be concerned about the potential impact of several of our primary soilborne diseases.

    In particular, it is important to be on the lookout for seed rots and seedling diseases caused by PythiumPhytophthoraFusarium, and Rhizoctonia, and while they can all occur in Pennsylvania, they do vary depending on location and soil conditions.

    What are we looking for?

    Low spots, poorly drained areas of the field, as well as compacted soil, can all be indicators of a potential issue due to soybean soilborne diseases early in the season.

    When we find uneven plant stands, we need to look closely at the plants by taking a shovel and carefully removing plants and roots.

    In some situations, it is fairly clear that the emerged soybean plant is diseased. In other situations, we need to look closely at some of the differences in symptomology to make an initial diagnosis.

    What are some of the different symptoms?

    Below, we provide a comparison of different symptoms that can be seen for the primary soilborne diseases. These are helpful to identify a probable cause, although in several cases there are symptoms that can lead to seed decay and damping-off.

    As such, we always recommend that you take a sample and submit this to the Plant Disease Diagnostic Clinic for a more complete diagnosis. Remember that samples submitted from Pennsylvania are free to test.

    Results from such tests are important to understand, (1) what was the primary cause of the problem in the field, which can then be used to (2) develop best management strategies.

    While the conditions of each growing season vary at and around planting, we recognize that spring tends to be wet and favorable for soilborne diseases. Early scouting and identification of these diseases is important.

    Typical symptoms of different soilborne diseases in field crops.

    Pythium spp.

    • Pre-emergent damping-off where seeds fail to germinate and/or the seed is totally disintegrated in the soil.
    • Post-emergent damping-off where there can be lesions and discoloration on the roots, which leads to disintegration and rot. This leads to seedling collapse in severe cases.

    Phytophthora spp.

    • Pre- and post-emergent damping-off: seeds become brown, mushy, and deteriorated. The emerged seedling will collapse due to rotting of the roots.
    • Later in the season there can be a root rot and stem rot, which has symptoms like brown, discolored taproot and secondary roots, with reduced root mass. In the above-ground plant tissue, we can find a dark brown to black discoloration of the stem, which usually begins at the soil line.

    Fusarium spp.

    • Root rot and wilt: infected plants have brown vascular tissue in the roots and stems and can show wilting of the stem tips. It is not common to have external decay or stem lesions above the soil line.
    • Foliar symptoms include scorching of the upper leaves, while the middle and lower canopy may be chlorotic leading to withering and leaf drop from the plant.

    Rhizoctonia solani

    • Pre-emergence symptoms include seed decay and damping off.
    • Post-emergence symptoms can include the appearance of brown to reddish lesions on stems and roots of seedlings just below the soil line. Lesions are sunken, remain firm and dry, and are limited to the outer layer of the tissue

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