Early Season Soilborne Diseases of Soybean
Cool and wet conditions during soybean planting and early development heighten the risk of key soilborne diseases. Scouting for seed rots and seedling diseases caused by Pythium, Phytophthora, Fusarium, and Rhizoctonia is critical. While all are present in Pennsylvania, their prevalence varies by location and soil type.
What to look for?
Low spots, poorly drained areas of the field, and compacted soil can all indicate 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 (Figures 1 and 2). In some situations, it is fairly clear that the emerged soybean plant is diseased, as shown in Figure 3. In other situations, we need to look closely at some of the differences in symptomology to make an initial diagnosis.
Right: Figure 3. Dead soybean plant showing symptoms indicative of a soilborne disease. Photo: Alyssa Collins, Penn State
What are some of the different symptoms?
Below, we provide critical things to examine when comparing different soilborne diseases. These help identify a probable cause, although symptoms can lead to seed decay and damping-off in several cases. As such, we recommend you take a sample and submit it to the Plant Disease Clinic at Penn State for a more complete diagnosis. Remember that samples submitted from Pennsylvania are free to test.
Results from such tests are essential to understand (1) what the primary cause of the problem in the field was, 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 identifying these diseases is critical since calls later in the season to determine why stands look poor often do not yield a precise diagnosis.
Typical symptoms of different soilborne diseases in field crops.
Pythium spp.
- Pre-emergent damping-off occurs when seeds fail to germinate/or are disintegrated in the soil.
- Post-emergent damping-off can cause lesions and discoloration of the roots, which leads to disintegration and rot. In severe cases, this can lead to seedling collapse.
Phytophthora spp.
- Pre- and post-emergent damping-off: Seeds become brown, mushy, and deteriorated. The emerged seedling will collapse due to the rotting of the roots.
- Later in the season, root rot and stem rot can occur. These have 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. External decay or stem lesions above the soil line are not common.
- 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 the stems and roots of seedlings just below the soil line. The lesions are sunken, remain firm and dry, and are limited to the outer layer of the tissue (Figure 4).
