By Berlin Nelson, Professor, Dept. Plant Pathology
The fungus Rhizoctonia solani causes pre- and post-emergence
damping-off and root rot of young and adult plants. When soil populations
of Rhizoctonia are high, pre- and post-emergence damping-off
can reduce stands by 50 percent or greater. Generally, Rhizoctonia
on soybeans is a seedling disease, but damage has been observed
on older plants. The pathogen survives in the soil and is common
in this region. The higher the amount of the pathogen in the soil,
the greater the potential damage to the plants.
Symptoms consist of seed decay and brown to reddish lesions on seedling
stems and roots just below the soil line. These reddish brown lesions
may become sunken and girdle the stems and kill the plant. Plants
may often appear stunted and unthrifty throughout the season or,
less commonly, will die. Often the stand will appear uneven because
of stunted plants. Disease is often found in patches in fields.
On older plants, the pathogen causes a reddish brown dry cortical
root rot that may extend into the base of the stem. Root rot can
greatly reduce nodulation. Foliar symptoms may include yellowing
or wilting of leaves. Plants infected with Rhizoctonia may
not be able to tolerate stresses such drought or hail that occur
later in the season. Infected plants may wilt on hot dry days. Observations
in North Dakota suggest that Rhizoctonia root rot is more severe
in plants showing iron chlorosis. Damage from Rhizoctonia
is commonly observed in areas when there is a long history of soybean
production with close rotations or during weather conditions not
favorable for seed germination and rapid growth of seedlings. Damage
is usually more common in cool, wet springs when conditions are
not favorable for plant emergence.
Stand reduction due to Rhizoctonia
Lesions caused by AG-4 on lower stems.
Healthy plant on the right.
Lesions caused by AG2-2 on older stems
Stunting of soybean (on right) in soil infested with Rhizoctonia.
Healthy plants on the left.
There are various anastomosis groups (AG) of R. solani. AG-4
and AG-5 are most common on soybeans but AG-2-2 and AG-3 are occasionally
found. AG-2-2 can be highly pathogenic, especially at high temperatures
and is more likely found on adult plants. AG-3, generally found
on potatoes, is weakly pathogenic on soybeans. AG-4 and AG-2-2 are
also common on sugarbeet. Because R. solani has a wide host
range that includes many broadleaf crops grown in this region, crop
rotation practices may affect severity of disease.
Crop rotation to non-susceptible hosts such as small grains will
reduce populations of Rhizoctonia in the soil. Avoid close
rotations with sugarbeets if there is evidence of Rhizoctonia
in the field. Close rotations with dry beans may also increase incidence
of disease. Protective seed treatments and good seedbed preparation can reduce damping-off. Seed
treatments with bacteria (biological fungicide) designed to colonize
the root systems has shown some control of root rot. Cultivating
soil to hill up around stems promotes lateral root growth and may
lessen the effect of root rot on older plants. Cultural practices
to reduce stresses on the plant will help reduce the damage from