Many soilborne fungi infect planted soybean seeds and seedlings, causing seedling blight and seed decay. If stand reduction is severe, it's important to determine if a fungal pathogen is involved before the decision to replant is made.
In early-planted soybeans, Pythium is the most common fungus responsible for damping off because this fungus prefers cold soil temperatures. Pythium can cause seed rot (pre-emergence damping-off) or infect young seedlings after emergence-called seedling blight or post-emergence damping off (Figure 1). The infected plants often have a rotted appearance, and are easily pulled from the soil because of rotted roots.
When soils warm up, the Phytophthora fungus becomes more active. Phytophthora can be a common and aggressive root rot pathogen that infects in warm (greater than 65°F) and wet soils. Therefore this seedling disease is more likely to occur in soybeans planted in late May and early June. Phytophthora diseases are most common in fields or parts of fields with poor drainage, but they can occur in well-drained fields that are saturated for 7-14 days due to excessive rain or irrigation.
Rhizoctonia fungi are also active when soils are warm. Unlike Phytophthora damping-off, stem discoloration by Rhizoctonia are usually localized brown-to-reddish brown lesions that are limited to the cortical layer (Figure 2). The infected stems remain firm and dry. Rhizoctonia can continue to attack plants and cause root rot until plants into mid-season.
Several species of Fusarium, including the causal agent of Sudden Death Syndrome, are pathogenic on soybean seedlings, as well as Macrophomina phaseolina, the causal pathogen of charcoal rot. Seeds infested with Phomopsis (pod and stem blight) often have a reduced germination rate due to seed rot or seedling blight.
It's important to identify the cause of stand reduction before the replant decision is made.
If stand reduction is severe, it is important to determine if a fungal disease is involved before the decision to replant is made. If early-planted soybeans emerge before a frost, the seedling injury may be frost injury. Frost-injured seedlings are only damaged in the upper portions of the plants - there will not be any root rot symptoms. This distinguishes frost injury from fungal injury. If not killed, frost-injured plants will re-grow.
If fungal damping-off is the cause of stand reduction, seed treatment may be needed for replanting. Identification of the pathogens involved is important because different fungicides are effective in controlling different fungi.
Management of damping off and seed decay
If the stand reduction is due to Phytophthora sojae, resistant varieties are available. Many soybean varieties marketed in Iowa have race-specific resistance, which means a complete resistance to a specific race of P. sojae. Soybean varieties are also generally rated for their level of partial resistance to Phytophthora (also called field resistance or tolerance ). However, partial resistance is most effective against Phytophthora root and stem rot in later growth stages and less effective in the seedling stage.
If damping-off is caused by Phytophthora or Pythium, seed treatments with a fungicide containing mefenoxam (trade name Apron) will be effective when a resistant variety is not available, such as in the case of specialty soybeans. However, mefenoxam fungicides will not be effective against other damping-off fungi. Some formulations have a combination of active ingredients and therefore are effective against a spectrum of fungi. In all cases, good coverage of the fungicide on the seed coat is critical.
Read more about fungicide seed treatments
Soybean seed health (pdf)