Seed and Seedling Diseases
[Soilborne Diseases] [Seedborne Diseases]
[When to Replant] [Management] [Back to Soybean Disease Notes]
diseases in soybean are a minor, yet chronic problem in North Carolina.
Our mild coastal climate usually results in warmer soil temperatures earlier
in the spring than occurs in states further inland. As a result, seedling
diseases are generally of minor importance in this state. Nevertheless,
because of the wide range of planting dates and soil types encountered,
seedling diseases cause problems for some growers on a yearly basis. Seedling
diseases can cause reduced stands and limit crop growth, thus getting
the crop off to a slow start. These factors may lower yields and make
management of other pest problems more difficult.
diseases tend to be more severe in poorly drained soils. They can, however,
present problems in any soil type, especially when high rainfall or cold
weather follow planting. Any factor that delays germination and seedling
emergence such as poor seed quality, inadequate seedbed preparation, compaction,
planting too deep, nematode infestations, and high rates of some herbicides
can contribute to the incidence and severity of seedling diseases.
Most soybean fields receive one or more herbicides, and most of the available soybean herbicides have the potential to injure soybeans. Some can generate visual symptoms that are similar to disease symptoms. Soybean tends to be more seriously injured by broadleaf herbicides than grass herbicides. Pre-emergence herbicides whose label suggests planting soybean below a certain depth probably have the greatest potential to injure soybeans if the herbicide gets into the soybean root zone.
between herbicide injury and seedling diseases is frequently difficult.
The pattern of where symptoms do and do not appear may give the best indication
of whether it is a disease problem or herbicide injury. Symptoms of seedling
diseases usually occur in irregular patterns which may correspond to changes
in soil type. A disease is more likely to be severe on one plant and show
no symptoms on adjacent plants than is herbicide injury. The pattern of
herbicide injury symptoms typically appear to be equipment related, although
they may be modified by differences in soil type. Weed control is often
excellent in the affected areas, and seldom will one plant be seriously
affected and the adjacent plants appear unaffected. Finding an area in
the field the sprayer may have missed, or overlapped in a previous pass,
can be very useful in diagnosing herbicide injury.
Soybean seedling diseases are caused by fungi that reside in the seed or the soil. Pythium spp., Phytophthora sojae, and Rhizoctonia solani are the soil fungi most commonly associated with seedling diseases. Pythium and Rhizoctonia are found in all agricultural soils. These fungi will commonly cause a root rot and either lesions on the stem or soft watery stem tissue. See the figure at the top of this note.
Sore shin (a red-to-brown lesion at or above the soil line), damping off
and root rot can be caused by Rhizoctonia solani. Plants frequently
recover from the root rot phase if optimal conditions for soybean growth
occur soon after emergence. This fungus affects many plant species and
is found in all soils. Other common soil inhabitants are various species
of Pythium. Pythium rot can occur at any stage of plant development,
but is primarily a seedling disease favored by high moisture. A variety
of symptoms are associated with this disease: a seed rot, a wet rot (soft
watery stem tissue), baldhead (retarded development of the growing point),
swelling of the stem below the cotyledons (which can be confused with
herbicide injury), a root rot and/or wilt and death of seedlings. Diseases
caused by Pythium spp. and Rhizoctonia may be suppressed,
but not eliminated by rotation with grass crops such as corn or sorghum.
Although rotation is not an especially effective tactic for managing these
diseases, the reduction in other soybean pests which occurs with rotation
generally results in a lower incidence of these seedling diseases. Seed-treatment
fungicides containing thiram or PCNB are reasonably effective against
Rhizoctonia solani, whereas seed-treatments containing mefanoxam
(Ridomil Gold, Apron) are necessary if Pythium is the problem.
fungus responsible for Phytophthora rot of soybean, Phytophthora sojae,
is similar to Pythium spp. It is most commonly a problem in heavy,
poorly drained soils or in low spots in the field. Diseases caused by
this organism range from seed rot to stem and root rot, which may result
in plant death. Rotation with non-leguminous crops (two or more years
may be required), resistant or tolerant varieties and fungicide (mefanoxam)
treatments can be used to manage this pest.
Several fungi that cause seed decay and seedling diseases are seedborne. Phomopsis seed decay, frogeye leaf spot (SOY003), anthracnose, purple seed stain, and downy mildew can cause seed rots, reduce emergence and prevent adequate stand establishment. These problems are usually more severe in cool wet soils. The best tactic for avoiding these diseases is the use of high quality disease-free seed.
common of these seedborne diseases is phomopsis seed decay. Seed infected
with Phomopsis spp. typically have greatly reduced seed germination
in a cold test compared to a standard germination test. This seed decay,
like most other seedborne diseases, is a result of delayed soybean harvest
and humid conditions during seed development. Pod and stem blight caused
by Phomopsis spp. is present in nearly all fields at soybean maturity.
If soybean harvest is late and warm humid conditions persist, infection
of the seed will occur. Seed from these plants may not germinate and may
produce seedlings lower in vigor. The other seedborne fungi mentioned
cause similar problems, but are less common. Late-season scouting of soybean
fields should include identification of foliar, pod, and stem diseases
if soybeans are grown for seed. Seed should not be saved if anthracnose
or frogeye leaf spot are present in the field. Your County Agent can forward
plant samples to the Plant Disease and Insect Clinic if you are uncertain
as to exactly which diseases are present. Seed saved for planting next
year should be tested for germination. Send seed lots to: NCDA-Seed Laboratory,
P.O. Box 27647, Raleigh, NC 27611. If seed of dubious quality must be
planted, the use of a fungicide-seed treatment may prove beneficial. Seed
treatments containing carboxin and/or captan are most effective against
these fungi with the exception of downey mildew, which can be controlled
Soybean can yield well over a relatively wide range of plant population densities. Unless populations are so high that lodging becomes a problem or so low that the canopy does not close, yields will not be affected markedly. Target population densities are 6-8 plants per foot of row with 36-inch row spacing and 2 per foot of row in 7-inch rows for May-planted soybean. A higher population is desirable in June with 9-11 plants per foot of row and 2.5-3 plants per foot of row with 36- and 7-inch rows, respectively.
If the remaining plants are healthy, and fairly uniformly distributed, stands of half the levels in the previous paragraph are probably adequate. The added productivity of the replanted crop would have to be enough to cover the additional twenty to thirty dollars per acre cost of replanting. The later planting date lessens the likelihood that this will happen. The greatest uncertainty comes in guessing the current and future health of the remaining plants. When evaluating an uneven stand of very young soybean plants, it is difficult not to view the remaining stand pessimistically. Probably more fields are replanted which did not need to be than fields not replanted which should have been.
used on the field must be considered when a decision to replant is made.
Do not apply more pre-emergence herbicide with the second planting. Scout
the field periodically to determine if a post-emergence treatment is necessary.
Tillage can be used to dilute the concentration of the broadcast herbicide
if it is labeled for preplant incorporation and is suspected of being
part of the problem. If the broadcast herbicide is not labelled for preplant
incorporation (e.g., linuron and linuron tank mixes), or does not appear
to have been leached deep enough into the seedbed to be causing injury
to the first crop, additional tillage should not be performed before replanting.
If a soil-applied herbicide was banded over the original planting, and
the decision is made to replant between the original rows (and thus away
from the banded herbicide) a banded herbicide could also be used over
the replanted rows as well, provided the sum of the two banded applications
do not exceed the labelled per-acre rates for that herbicide.
For assistance with a specific problem, contact your local North Carolina Cooperative Extension Service personnel.