By Berlin Nelson, Professor, Dept. Plant Pathology
Phytophthora root rot is a major disease of soybean, especially
in areas where soybeans have been cultivated for many years. The
disease is caused by the fungus Phytophthora sojae. Yield
losses can be substantial; entire fields have been destroyed. The
disease is common in the Red River Valley, expecially in the southern
portion. The pathogen survives in soil as spores called oospores
which are produced in infected plants. When there is high soil moisture
the spores germinate and infect the roots. Infection and disease
development can occur at any stage of plant development, but are
most commonly observed and damaging at the seedling or young plant
stage. Disease is most common in heavy, compacted clay soils and
fields subject to flooding. Flooding rains, especially near planting,
favor disease development. Reduced tillage, especially no-till,
is reported to increase damage. The pathogen does not naturally
infect other crops grown in this region. Only three Lupinis
spp. and soybeans are natural hosts.
RACES AND VIRULENCE PHENOTYPE
There are more than 40 races of P. sojae. A race is a specific
form of the pathogen that attacks certain resistance genes in the
soybean. Races of the fungus are identified by the resistance genes
they can defeat (defeat means cause disease on). In 2002 a race
survey was conducted in North Dakota. The results are shown in the
pie chart below. The most prevalent races in this area are races
3 and 4 (about 80% of the isolates). Races 1, 5, 8, 21, 25,
28, 41, 43, and 44 are also found, but at low frequency. Three new
races were also identified. In the early 1990s, races 9, 14 and
34 were found, but they did not appear in the 2002 survey. Studies
with single zoospores of isolates from North Dakota have revealed
there is great genetic diversity within the population of P.
sojae. There are strains of the fungus that can defeat most
of the major resistance genes (14 Rps genes) for control of P.
sojae. The bar chart below indicates the perecentage of isolates
collected in 2002 that can defeat each of 7 resistance genes. The
Rps genes 1a, 1c, 1k and 6 are the genes commonly found in cultivars
adapted to North Dakota. The genes 1k and 6, for example, are still
valuable genes becasue they protect plants from most races. But
there are races that will attack and kill plants with those genes.
As soybeans continue to be a widely planted crop there will be selection
pressure for the development of new strains of the pathogen and
we will continue to see more evidence of plants with the 1k and
6 genes becoming diseased.
The term race is now being supplemented by a new term, virulence
phenotype, to designate the virulence of the fungus stain on the
various host resistance genes. Most races are identified based on
the differential reaction on 8 resistance genes (Rps genes 1a, 1b,
1c, 1d, 1k, 3a, 6 and 7). Because there appear to be so many races
and the numbering system has no biological meaning, designating
the strains by the genes in the host that they defeat has practical
use. For example, race three is now designated with the virulence
phenotype of 1a, 7. This means that of those 8 resistance genes,
race three defeats the resistance genes 1a and 7. Race 4 has a virulence
phenotype of 1a, 1c, and 7. So a cultivar with Rps 1c or 1a would
not be resistant to race 4, because race 4 defeats those genes.
Races 3 and 4, however, can be controlled with a cultivar with Rps
6 or Rps 1k because those races can not defeat those resistance
Races of Phytophthora sojae in North Dakota
in 2002. These results are based on isolates of the pathogen
recovered from soil in eastern North Dakota.
The symptoms are seed rot and pre- and post-emergence damping-off
and wilting of plants. These are common in flooded soils and are
often misidentified as water damage. On older plants, leaves may
become yellow and plants will wilt with wilted leaves remaining
on the plant. The lateral and tap roots are rotted and destroyed.
A dark brown discoloration that can turn into a girdling lesion
often appears on the lower portion of the stem. Disease is usually
patchy in the field, often occurring in low or flooded areas. Symptoms
can appear at any time during the year when wet soil conditions
Phytophthora root rot: Wilting soybean showing the brown lesion
on the lower part of the stem.
A field with about 50% dead or wilting plants. Many plants
died right after emergence.
Management. Planting resistant cultivars
is the best method to control Phytophthora root rot.
Choose a resistant cultivar that contains a gene for control of
races 4 and 3, since those are the most prevalent races. The genes
Rps lk and Rps 6 will control races 3 and 4. Rps 1c will not control
race four. The gene Rps la, which is found in some cultivars, will
not control races 3 and 4. If you are using a cultivar resistant
to races 3 and 4, and you observe Phytophthora root rot, it indicates
that another race is appearing in the field.
Some cultivars are reported to have tolerance to Phytophthora root
rot. These cultivars may not be as susceptible under low to moderate
disease pressure, but can be severely damaged under high disease
pressure. Crop rotation is not an effective method to reduce disease
because the oospores are very long lived in soil. Metalaxyl and
mefenoxam seed treatments will protect seedlings but not older plants.
See the section on seed treatments
The value of one resistance gene: Soybean plants inoculated
with Phytophthora. The ones on the right have the resistance
gene Rps 6 while the ones on the left have no resistance gene.