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.

NDSU Agriculture Communication