Soybean root diseases and wet soils
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EmailEditor’s note: This article is from the archives of the MSU Crop Advisory Team
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Recent
heavy rainfalls have delayed soybean planting, and put more pressure on
growers to plant when conditions are less than ideal, but it’s a good
idea to consider the effect of planting into wet soils on soybean seeds
from a perspective of seed and seedling diseases.
During wet years, fields of newly-planted soybeans and soybean seedlings
can be subject to significant losses from root-decaying fungi such as Pythium, Phytophthora, Rhizoctonia and Fusarium.
Although each pathogen has an ideal temperature range for germination of
spores and development of disease, one thing they all have in common is
a liking for wet soils. Soils may be soggy due to poor drainage
resulting from heavy clay, or compaction, or saturated from heavy
rainfall. Soils may remain wet for extended periods of time, especially
in low areas of fields where water collects. Fields that are under
no-till or reduced tillage, tend to stay wet longer and warm up more
slowly than fields under conventional tillage.
Pythium spp. are common soil-borne pathogens that can infect seeds,
seedlings and roots of soybeans at cool soil temperatures (40-59oF). Pythium
can cause delayed emergence and poor stands, resulting in significant
plant losses. Pythium belongs to a class of fungi called oomycetes,
whose dead spots on roots where infection is mild, to dead root tips and
loss of the taproot. Severely infected seedlings are stunted, chlorotic
and may die. Symptoms caused by another oomycete, Phytophthora sojae, are similar. P. sojae
is most severe in flooded or poorly drained soils, but its development
is favored by warmer soil temperatures (68-77°F). A row of dead plants
may occur in a low area or in patches in the field. Plants pull up
easily from the ground, because there are almost no roots attached.
Rhizoctonia root and stem rot also tends to occur in patches, and is
more common on seedlings and young plants. A reddish-brown lesion
develops on the root and may extend upward to the stem. Plants look
chlorotic due to lack of roots, but the chlorosis is sometimes mistaken
for nitrogen deficiency. Wet soils, high organic matter and temperatures
of 77 to 84°F are optimum for development of the disease, although
symptoms become much more visible when plants are under drought stress
later in the season.
Several species of Fusarium cause root rots that produce
sunken, light to dark brown lesions and loss of lateral roots. The
damage may go undetected during scouting, and the disease may only be
evident when a loss of yield is noted. In a limited survey of soybean
root and stem disease in Michigan last year, the most commonly isolated
fungus from field samples collected was F. oxysporum, but we have not yet determined how significant a role it may be playing in affecting soybean yield.
Regardless of the disease, good drainage is important for management.
Take what measures you can to improve drainage. If your field has a
history of seedling disease, fungicidal seed treatments may be helpful.
Consult the field crops guide for a list of seed treatments and the
fungi they are labeled to control. A few seed treatments are labeled for
Phytophthora, but look carefully at the label for the rate to use. Use
Phytophthora- resistant varieties if this disease has been a problem in
the past. Try to delay planting into wet fields until planting
conditions improve.
