Fargo, North Dakota
April 29, 2009
Saturated soils due to excessive
rainfall, overland flooding or inadequate drainage can stress
soybeans enough to reduce growth and yield or even kill the
plants. The seriousness of the yield loss depends on the soybean
variety, growth stage, length of saturation and soil type. When
soils become water saturated, the oxygen in the soil pores is
replaced with water, so the movement of oxygen through the soil
is reduced.
Research was conducted at
North Dakota State University to investigate if the
differences in soybean tolerance to wet conditions are linked to
the presence of genes that control Phytophthora root rot in
soybeans.
Phytophthora root and stem rot can be a major cause of soybean
yield loss. The disease is caused by Phytophthora sojae, a
fungallike pathogen commonly found in soybeans grown on the
heavy clay and poor-draining soils of the Red River Valley. Once
the pathogen is present in the soil, it cannot be eradicated.
There are numerous Phytophthora races, but races 3 and 4 are the
most common in North Dakota soils.
Several management strategies can be used to manage
Phytophthora. Genes with major resistance, such as Rps 1k or Rps
6, which NDSU recommends, is used most to manage the disease.
Major gene resistance is very effective, but only against
certain races of the pathogen.
Minor gene resistance (also called partial resistance) can help
manage Phytophthora. Minor gene resistance provides some
resistance (ranging from a low to high level) to all races of
the pathogen. Minor gene resistance is more difficult to
quantify and the availability of it in many varieties is
unknown.
Tolerance to the disease also can be employed as a management
strategy.
Tolerance refers to the ability of the plant to have less yield
loss and other undesirable characteristics when infected.
Although tolerance is effective against all races of the
pathogen, it can be overwhelmed under high disease pressure.
"These three management tools may be best explained in a boxing
analogy," says Sam Markell, NDSU Extension Service pathologist.
"Major gene resistance would be a boxer who is top notch, but
only against certain fighters (loses to left- handers, for
example). Minor gene resistance is an all-around boxer equally
skilled (level of skill is irrelevant) against all types of
opponents. Tolerance would be a relatively poor boxer, but one
who can take a tremendous amount of hits before going down."
Fungicide seed treatments also may help prevent the early season
infection of the disease.
NDSU field studies were conducted in Fargo using different
soybean varieties grown under saturated soil (created with
irrigation) and regular, rain-fed conditions. The saturated
conditions were created for 14 days at the early reproductive
stages (R1to R3). If the yields under saturated conditions
remained relatively high compared with other entries, varieties
were considered "tolerant" to excess moisture.
"However, the average yield of the eight most tolerant varieties
was 83 percent of the yield of the same varieties without
saturated conditions," says Hans Kandel, NDSU Extension Service
agronomist. "The yield of the eight least tolerant varieties was
56 percent of the yield of the same varieties grown without
saturated conditions. The water-tolerant varieties had lower
Phytophthora disease ratings than the least water-tolerant
varieties. This provides an indication that those varieties
tolerant to saturated conditions also had some genetic
resistance (major or minor gene) against Phytophthora."
While varieties tolerant to water-saturated soil conditions
generally had an Rps gene that would defeat Race 3 or 4 of
Phytophthora sojae or had good partial resistance to the
pathogen, varieties that were intolerant to water-saturated soil
conditions generally lacked an effective Rps gene or had less
partial resistance to the pathogen than water-tolerant
varieties.
Although there are other unknown genetic factors involved,
resistance to Phytophthora is an important factor that is
related to soybean's tolerance to water-saturated soil.
In 2008, 40 soybean varieties were tested under wet and natural
rain-fed conditions near Fargo. The average yield of the
varieties under wet soil conditions was 26.3 bushels per acre
compared with a rain-fed soybean average yield of 33.2 bushels
per acre. However, 13 varieties had similar yields under wet and
natural rain-fed conditions. These varieties yielded 31 and 29.8
bushels per acre under wet and rain-fed conditions,
respectively.
"This indicates that selecting varieties that are more tolerant
to wet conditions may provide a strategy to increase soybean
yield potential if wet conditions are expected," Kandel says.
Information on soybean varieties more tolerant to excess
moisture can be found at
http://www.ag.ndsu.edu/pubs/plantsci/rowcrops/a843.pdf. |
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