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Research Project: Enhancing
Resistance to Root Rot Pathogens of Soybean
Location:
Crop
Production and Pest Control Research
2003
Annual Report
1. What major problem or issue is being resolved and how
are you resolving it?
Disease resistance in soybean to
root rot caused by Phytophthora sojae and Fusarium solani f.sp.
glycines must be improved and available to breeders to minimize
yield losses. The lack of basic knowledge concerning these two
important soybean pathogens is a limiting factor restricting the
development of improved control strategies and the ability to
predict the severity of future outbreaks. Research objectives and
approaches in this project focus on a better understanding of the
genetic basis of host resistance and genetic structure of these
pathogen populations to identify and describe more effective and
efficient use of resistance in soybean production. With the
identification of new races of P. sojae (>18 in the 1990s) and
increased inoculum potential for root rot by both P. sojae and F.
solani in soybean production fields, it is important to identify new
and improved sources of resistance and to continue documenting
distribution and frequency of races and strains to maximize disease
control strategies.
Soybean is the major oilseed crop in the world with an annual
value of nearly 14 billion dollars in the U.S. Cultivars and
germplasm with improved chemical composition and yield potential
must have resistance to major pathogens to make production
profitable. This project will focus on root rot diseases caused by
Phytophthora sojae (Phytophthora root rot) and Fusarium solani f.
sp. glycines (sudden death syndrome). Yield losses attributed to
soybean diseases exceed 14% annually. However, Phytophthora root rot
has the potential to cause much higher yield losses if resistance is
not regularly incorporated into enhanced germplasm. The Rps1-k gene
is currently a widely used form of resistance incorporated in
cultivars, but it is not effective against race 25 and several new
races identified in Indiana and Ohio during the 1990s. Recent
reports by NCR-137 and North Central Soybean Research Program
(NCSRP) committees indicate that more information is needed about
population dynamics of P. sojae races and about host/pathogen
interactions in sudden death syndrome (SDS). Losses from SDS, a
relatively new root rot disease caused by a toxin-producing strain
of F. solani, are not well documented but have increased
dramatically in the 1990s. Initially, SDS damage was more extensive
in river floodplain areas, but now it poses a serious threat to
soybeans throughout the Midwest. The lack of basic knowledge
concerning these two important soybean pathogens is a limiting
factor restricting the development of improved control strategies
and the ability to predict the severity of future outbreaks. A
better understanding of the genetic basis of host resistance and
genetic structure of these pathogen populations is needed and should
lead to a more effective and efficient use of resistance in soybean
production.
2. How serious is the problem? Why does it
matter?
Prior to the 1990s, races of Phytophthora sojae
were controlled by growing soybeans with the Rps1-k gene and little
is known about resistance and epidemiology of SDS caused by Fusarium
solani f. sp. glycines, which is a relatively new disease of
soybeans. Pathology research efforts are underway to verify the
current race situation and population dynamics of P. sojae and F.
solani f. sp. glycines common to Indiana soybean production fields.
Representative races maintained for pathology and breeding efforts
are being utilized to incorporate additional Rps genes (gene
combinations and the recently identified Rps8 gene) into elite
soybean breeding lines to provide improved Phytophthora resistance.
Yield reductions due to Phytophthora are estimated at
$124,000,000 per year for the U.S. in the 1990s. Managing
Phytophthora with genetic resistance currently available would
provide full season disease control and result in significant
economic benefit and reduce the need for fungicide treatment that
only provides partial or temporary control. Yield reductions due to
sudden death syndrome (SDS) are similar to that listed above for
Phytophthora and they continue to increase throughout the Midwest
states. Almost all soybean varieties are highly susceptible to SDS
and yield losses in Indiana were common in central as well as
southwest Indiana in recent years. Identification and incorporation
of genetic resistance is needed and would reduce important losses
caused by this relatively new soybean disease.
3. How does it relate to the National Program(s) and
National Program Component(s) to which it has been
assigned?
National Program 303, Plant Diseases (100%)
This project will contribute to the National Program mission to
reduce crop losses resulting from diseases by defining the genetic,
physiological, and biochemical processes that operate in the host
and pathogen during disease development, determining influence of
crop production practices on disease vulnerability, determining
critical processes in pathogenesis that can be interrupted or
prevented, and by identifying and manipulating new and existing
sources of resistance to root diseases of soybean. Although bridging
slightly into a couple of other components, the project will make
primary contributions in Component 4 - Pathogen Biology, Genetics,
Population Biology, Spread and Relationship with Hosts and Vectors.
New and important biotypes of major soybean pathogens regularly
evolve and overcome once-effective management tactics. The ongoing
soybean disease research is needed to devise effective management
strategies to keep up with the changing disease situation.
Documentation of new races or biotypes of major soybean pathogens
and identification of host resistance permit enhancement of soybean
germplasm and contributes to the development of soybean cultivars
that minimize yield losses causes by soybean disease pathogens.
National Program 301, Plant, microbial, and insect germplasm
conservation and development is not officially identified but is
directly related. Germplasm enhancement relative to disease
resistance is addressed by ongoing cooperative research with
breeders and by the coordination of the USDA Uniform Soybean Tests
Northern Region assigned to this project by the USDA-ARS National
Program Staff.
4. What were the most significant accomplishments this
past year?
A. Single Most Significant Accomplishment
during FY 2003: Soybean yield losses due to races of Phytophthora
sojae that are not controlled by the Rps1-k gene have increased in
recent years. Plant and soil samples from Indiana soybean fields
were evaluated and compared to previous years data to identify races
of the genetically diverse pathogen. Results of 2002 research still
identify ten predominant races and verify a need for gene
combinations to control Phytophthora root rot; and Rps virulence
data for the isolates suggest Rps gene combinations (1-k or 1-c +
3-a) or the new Rps8 gene will control races identified in soybean
fields, whereas disease control using Rps1-k or 1-c resistance genes
would be less than 50%. This type of research provides new insights
about genetic diversity and distribution of P. sojae that will
provide clues to key factors influencing change in this important
pathogen and it is impacting development of resistant soybean
cultivars which should result in significant economic benefit.
B. Other Significant Accomplishments: It is assumed long-term
usage of soybeans with specific Rps resistance genes favor
development of genetically diverse Phytophthora sojae populations.
Population dynamics data for P. sojae races were further verified
where resistant (Rps1-k) vs. susceptible (rps) soybean cultivars had
been planted for more than 10-years. Thirteen races were identified.
The race diversity was similar whether a resistant or susceptible
cultivar was grown during the previous decade. However, P. sojae had
5-times the population density in field plots where susceptible
cultivars had been grown compared to field plots where the resistant
soybeans were planted. This new information that will impact
management strategies used to control this devastating soybean
disease.
C. Significant Activities that Support Special Target
Populations: None.
D. Progress Report: Influence of tillage and rotation on SDS of
soybean: Studies documenting the role of Fusarium solani f.sp.
glycines root infection and SDS yield losses in soybean with tillage
(no-till vs. conventional tillage) and crop rotation were continued
in 2003. The limited research reported in southern states has
indicated that SDS is more likely to occur in soybeans with minimum
tillage than with conventional tillage. Indiana data collected from
2002 does not agree with this statement. Root colonization of
soybean plants by F. solani and SDS yield reductions were
consistently higher in conventional tillage plots than in the
no-till plots. The root infection results also indicated that
soybeans with continuous cropping are more likely to develop SDS
than plants in the corn-soybean rotation.
Evaluation of soybean germplasm and pathogens to minimize disease
losses: Studies were established in 2003 to continue verifying and
updating the frequency of dominant races or biotypes of P. sojae and
F. solani in soybean production fields. This information is needed
to document changes in the pathogens and to develop control
strategies. Management and culture of the P. sojae germplasm
collection were continued in 2003. In addition, the Lead Scientist
will continue to identify and provide selected P. sojae races for
germplasm enhancement. Virginia State University and USDA-ARS
approved the soybean cultivar 'Asmara' for release in 2003. This
jointly developed 'vegetable' soybean also has excellent mature seed
agronomic qualities as wells as resistance to a number of important
fungal pathogens. Mature seeds can be used for several soyfood
products.
Soybean disease information for part of an Online- and CD-Rom
Publication was completed that includes more than 20 different
fungal diseases, five viral diseases, and three bacterial diseases.
The diagnostic symptom of each disease is illustrated and
information relative to management and control are described. The
information describing symptoms, disease cycle, and control outlined
in this Extension Publication will provide crop specialists and
soybean growers current insight into disease diagnosis and
management practices that will reduce disease losses.
5. Describe the major accomplishments over the life of
the project, including their predicted or actual
impact.
This is a new project based on a Project Plan
certified by the OSQR 02/12/03, replacing 3602-21220-007-00D
entitled Enhancing Soybean Seed Composition and Pest Resistance.
Research objectives of the new project address the primary needs
to achieve improved disease resistance in soybean. The four major
objectives are below: 1. Identify and describe predominant strains
of root rot pathogens and assess the effectiveness of soybean
resistance genes for incorporation into improved germplasm. 2.
Identify and determine genetic variability of new and established
Phytophthora sojae races currently maintained by this project for
germplasm enhancement and race identification of new field isolates.
3. Determine the impact of infection at different stages of host
development on severity of SDS to reveal potentially vulnerable
phases in the disease cycle. 4. Determine the influence of crop
management practices on root colonization of Fusarium solani f. sp.
glycines and population dynamics of P. sojae races in soybean field
plots.
6. What do you expect to accomplish, year by year, over
the next 3 years?
The following are anticipated
accomplishments of the new Project Plan. FY 2004: We will conduct
further studies to verify and update the frequency of dominant races
or biotypes of Phytophthora sojae and Fusarium solani in Indiana
soybean production fields. This information is needed to document
changes in the pathogens and to develop control strategies. In
addition, we will continue to identify and provide selected P. sojae
races (i.e. races 1, 3, 4, 7, 25, and 28) that can be utilized in
the 'Soybean Pathogen Germplasm' initiative currently being
coordinated by Dr. Dan Phillips, University of Georgia, and Dr.
Peter Bretting, USDA-ARS, NPS. Virulence evaluations of isolates in
the culture collections are critical to germplasm enhancement and
have to be documented regularly. Evaluations will be expanded to
include additional sources of soybean PR-differentials. Seed of
several PR-differentials is being increased in 2002 and 2003 to
enhance race verification and for distribution to soybean
researchers. The P. sojae race collection maintained for use in the
pathology and breeding programs supported by this project are also
needed for germplasm enhancement by numerous seed companies.
Selected isolates and races with referenced virulence are also
needed and used regularly for critical reference in identifying new
and wild biotypes of the pathogens. Preliminary disease reactions of
the selected Plant Introductions increased in 2001 and 2002 merit
additional evaluation and elite lines will be used in adding disease
resistance to elite breeding lines. The selected germplasm has a
high potential for resistance to most races of Phytophthora sojae
and/or Fusarium solani. Data from the SDS field evaluations will
provide important information on the value of resistance for this
relatively new disease of soybean and also document the correlation
of field and greenhouse data involving SDS. Studies to identify
potentially new sources of Phytophthora resistance initiated in 2001
using seed of 36 selected Plant Introduction are being increased and
evaluated in 2003. This germplasm appears to have new (i.e. Rps8)
and possibly other unique P. sojae resistance traits based on
preliminary data and will be evaluated to determine the genetic
control. Crosses performed with several of the Plant Introduction
were established in collaboration with the Purdue soybean breeder,
Dr. Alan Leroy. In additional collaborative efforts, soybean
germplasm with enhanced seed composition and specialty germplasm for
the vegetable market will be evaluated for Phytophthora resistance.
Dr. Leroy established crosses to incorporate Rps gene combination
(Rps1-k + 3-a and Rps1-c + 3a) in advanced breeding lines with
potential for the vegetable market and in several advanced lines
with enhanced seed composition that Dr. J. R. Wilcox, former
USDA-ARS scientist, developed. FY 2005: We will continue our
physiological and pathological studies of new and established P.
sojae races common to Indiana. Germplasm enhancement studies with
general and unique molecular characteristics of P. sojae races will
receive more emphasis and particularly for new races that appear to
be widespread in occurrence and have virulence patterns somewhat
similar to established races. Isolates of P. sojae initially
selected for DNA fingerprinting will consist of eight races
(1,3,4,7,25,33,43, and 44). Amplified fragment length polymorphism
(AFLP) techniques will be used to determine unique, race-specific
markers and to establish genetic relationship among specific races.
Studies designed to evaluate the role of post-herbicide weed control
treatments on the sudden death syndrome disease reaction of soybean
lines should be finalized based on two years of field data. The
information will contribute to our understanding of the role of
herbicide and disease interactions in soybean. Specifically, the
information will provide data relative to root colonization by F.
solani f.sp. glycines, methods of assessment, and new insight about
the role of the widely used 'round-up-ready' gene in soybean disease
development. FY 2006: We plan to conduct further studies to confirm
the roles of the two forms of F. solani that infect soybean roots.
Two forms of F. solani are regularly isolated from soybean roots.
Little is known about the root rot damage caused by the two forms
individually or the role of root rot in yield reduction when foliage
symptoms caused by the toxins produced by F. solani f. sp. glycines
do not occur. Additional studies evaluating root colonization and
foliage symptoms of SDS in soybeans are needed. These studies in
combination with evaluation of resistance to SDS in diverse
germplasm and advanced breeding lines will aid in identifying
epidemiology aspects of the disease. The epidemiology information
will contribute to our understanding of the genetic control of
resistance.
Review Publications
Schweitzer, L.E., Abney,
T.S, Shaner, G.E. Soybean diseases. In Soybean Growth, Development
and Diagnostics ed. L.E. Schweitzer, Purdue University Cooperative
Extension AY-4. Available from:
http://www.agcom.purdue.edu/AgCom/pubs/menu.htm. and Soybean Growth,
Development and Diagnostics. CD-ROM. West Lafayette, IN: Purdue
University. 2003.
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