Damping Off and Seed Decay
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
More Information
Soybean seed health (pdf)
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