Michigan State University Extension
Michigan Soybean Information - FACT9701
11/16/00
Phytophthora Root and Stem Rot of Soybeans
List of files and visuals associated with this text.
Soybean Facts Winter 1997
Michigan State University Extension
This research and fact sheet funded by Michigan Soybean
Promotion Committee Making Your Checkoff Pay Off
G.R Safir, R C. Kaitany, B.W. Diers and L.P. Hart
Department of Botany and Plant Pathology and Crop and
Soil Sciences
Michigan State University
Control
The Disease
Phytophthora root and stem rot is caused by the fungal
pathogen "Phytophthora sojae". The disease is
economically important because it reduces yield by
killing or stunting plants.
Disease Symptoms
Disease symptoms can appear in all stages of growth.
Stress will often cause plants to express the symptoms.
Disease symptoms include:
(Vis. 1),
(Vis. 2),
(Vis. 3)
* Poor stands resulting from the disease killing emerging
plants or seeds.
* Yellowing of lower leaves followed by yellowing of the
entire plant. The leaves may remain attached to the plant
for a week after plant death.
* Rotted root system with the tap roots dark brown.
* Stem browning that starts at the soil surface and can
reach 10-12 inches above the ground.
Control
* Plant resistant or tolerant varieties which are
generally available from your seed dealer.
* Improve soil drainage by reducing soil compaction, by
improving soil structure or by other means.
* Rotate to other crops which are not hosts.
* If possible, plant soybeans in only those fields which
are well drained.
Disease Cycle
"Phytophthora sojae" survives as spores (called oospores)
in the crop residue of infected plants. The oospores
germinate in wet soil and usually form another structure
called a sporangium. Each sporangiurn release small
swimming spores, called zoospores, which are attracted to
soybean roots. After infection has occurred, new
sporangia and zoospores are formed on the surface of the
roots and initiate further infections.
Conditions that Favor Disease Development
Periods of high soil moisture, rainfall or ponded water
usually followed by a period of dry weather favor the
disease because the zoospores swim through the soil in
free water. The optimum soil temperatures for infection
are 81 to 91 degrees F for seedlings and young plants,
and 77 to 86 degrees F for older plants. Infection can
occur at soil temperatures as low as 50 degrees F, but
the disease development is slower.
Resistance
Some varieties have one or more genes that provide
resistance to specific races of "Phytophthora sojae."
Information on the resistance genes in varieties is
provided in the Michigan Soybean Performance Report which
is published annually by Michigan State University and
the Michigan Soybean Promotion Committee. Seed dealers
would also have this information for the varieties they
sell. There are at least 13 different phytophthora
resistance genes currently used by plant breeders in the
development of soybean varieties. Each gene provides
resistance to a different group of the over 40 races of
Phytophthora sojae that have been isolated and numbered
by researchers. Of these races, 1, 3, 4, 7, 11 and 25
have been found in Michigan. Some varieties that lack
race specific resistance may still have tolerance to the
disease. The tolerant plants will become infected by the
disease, but still yield well. Your seed dealer may have
information of the tolerance level of varieties they
sell.
Through the support of the Michigan Soybean Promotion
Committee, researchers at Michigan State University have
surveyed soybean fields during 1993-1996 to determine
what races of "Phytophthora sojae" are present in the
state. They found races 3, 4, 11 and 25 plus ten new
races. Below are listed the genes that provide resistance
to the races found in Michigan which were identified
either in this recent study or from earlier studies.
The following are the counties where the races of
"Phytophthora sojae" were found in 1993-1996
(Vis. 4):
Race 3 (Ionia), Races 4 and 11 (Eaton), Race 25
(Shiawassee and Monroe), New race A (Berrien, Monroe),
New Race B (Berrien, Ionia), New race C (Saginaw), New
race D (Eaton), New race E (Ingham, Clinton, Monroe), New
race F (Jackson), New race G (Ingham), New race H
(Monroe), New race I (Shiawassee), and New race J
(Ingham). The distribution of the races in more than one
nonadjacent county suggests that several of these races
may be widely distributed in Michigan.
The resistance genes Rpsl-c, Rps1-k, and Rps6 should
provide protection against individual races except New
race F for which there is no known resistance gene.
When doing race identification studies, it should be
noted that finding previously unidentified races present
under field conditions is not uncommon. Researchers in
other states have found similar phenomena. Since such
findings are not uncommon, we have certainly noted their
presence and are monitoring additional reports.
At this time, we are not overly concerned about the
threat of these new races on Michigan's soybean
production.
Race Genes That Provide Resistance
1: Rps1-a, 1-b, 1-c, 1-d, 1-k, 2, 3-a, 3-b, 3-c, 4, 5, 6
3: Rps1-b, 1-c, 1-d, 1-k,2,3-a,3-b,3-c,4, 5, 6
4: Rps1-b, 1-d, 1-k, 2, 3-a, 3-b, 3-c, 4, 5, 6
7: Rps1-b, 1-c, 1-d, 1-k, 3-b
11: Rps1-a, 1-c, 1-d, 1-k, 2, 3-a, 3-b, 3-c, 5
25: Rpsl-d,2,3-a,3-b,3-c,4, 5, 6
New race A: Rps3-c, 4, 5, 6
New race B: Rps1-b, 1-d, 1-k, 3-c, 4, 6
New race C: Rps1-b, 1-c, 1-d, 1-k
New race D: Rps1-b, 1-d, 3-b, 3-c, 6
New race E: Rps1-c
New race F: None
New race G: Rps1-c, 2, 3-b, 4
New race H: Rps1-c, 3-a
New race I: Rps1-b, I-c, 1-d, 1-k,b 2, 3-a, 3-b, 4, 5, 6
New race J: Rps3-b
Copies of the Soybean Research Fact Sheet are available
from:
Michigan Soybean Promotion Committee
http://www.michigansoybean.org
MSUE County Office
http://www.msue.msu.edu/msue/ctyentpg/
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to commercial products or trade names does not imply
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mentioned. This information becomes public property upon
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This file was generated from data base S1 on 11/16/00.
Data base S1 was last revised on 11/16/00.
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