Emergence Problems

Early detection and diagnosis of soybean emergence problems is important to achieving high yields in Minnesota. Early detection and diagnosis enables you to minimize yield losses by taking prompt corrective action such as rotary hoeing or replanting if necessary. Soybean emergence ranges from six days under ideal conditions to 15 days under more challenging soil conditions. Conditions that can lead to delayed or uneven emergence include:

• Cold soil temperatures
• Excess soil moisture
• Inadequate soil moisture
• Soil crusting
• Improper seeding depth or uniformity
• Poor seed-to-soil contact
• Insect feeding and disease infestations

If slow or uneven emergence occurs, dig up the plants and inspect them for signs of disease or insect damage to the root, hypocotyl or cotyledons.
Seed and seedling diseases of soybean are common and significant problems. They can decrease plant populations that result in replanting and production losses. Several different pathogens can cause these diseases, and the most common tend to be Fusarium, Rhizoctonia, Phytophthora, and Pythium. They can kill and rot seeds before germination or cause seedling death. They are most common when soil is very wet in the first few weeks after planting and in heavy, poorly-drained soils.

General conditions that promote seed and seedling disease diseases include wet, poorly-drained, and compacted soils. However, the different pathogens have different optimal conditions. For example, Phytophthora and Rhizoctonia are favored by wet & warm soils or moist & warm soils respectively, whereas Pythium is typically favored by wet soils for under both cool and warm soils depending on the species of Pythium present in the field. Seed and seedling diseases may be enhanced by slow germination and growth of soybeans, poor quality seed, and plant stress.

Pythium can attack and rot seeds and seedlings prior to emergence and can cause post-emergence damping-off under wet conditions. The characteristic symptom of most Pythium infections is soft, brownish-colored, rotting tissue. Thus, Pythium causes symptoms similar to Phytophthora in seedlings, and can only be distinguished by laboratory examination. Although Pythium causes most damage to seeds and seedlings, roots of established plants can be rotted and plants may be stunted.

Fusarium is also a common pathogen that can damage seeds and seedlings. It causes light to dark brown lesions on roots that may spread over much of the root system and may appear shrunken. Fusarium may attack the tap root and promote adventitious root growth near the soil surface and may degrade lateral roots.

Phytophthora can attack and rot seeds prior to emergence, and can cause pre- and post-emergence damping off. It produces tan-brown, soft, rotted tissue. At the primary leaf stage (V1), infected stems appear bruised and soft, secondary roots are rotted, the leaves turn yellow, and plants frequently wilt and die.

Rhizoctonia can damage seeds and plants prior to or after emergence. In seedlings and older plants, a firm, rusty-brown decay or sunken lesion on the root or on the lower stem is a characteristic symptom. The infections can be superficial and cause no noticeable damage, or they can girdle the stem and stunt or kill plants.

Disease Management

Seed and seedling diseases of soybean are difficult to manage, and the different pathogens and diseases require different management approaches. Thus, accurate diagnosis can help in managing them.

In general, these diseases can be reduced by planting good-quality seed in well-drained, non-compacted fields. Delaying planting until soils are >55°F and relatively dry to allow for rapid emergence and growth can be beneficial. Crop rotation and tillage may be of some benefit. Genetic resistance in soybean varieties is only clearly available for managing Phytophthora infections.

Fungicidal seed treatments may reduce seed and seedling diseases. Seed treatments with two or more active ingredients should typically be used. For example, products containing mefenoxam (ApronXL®) or metalayl (Allegiance®) can be effective against Pythium and Phytophthora, and products containing fludioxonil (Maxim®) or a strobilurin product (azoxystrobin, trifloxystrobin, or pyraclostrobin) may help to reduce damage from true fungi such as Fusarium and Rhizoctonia.

Other Emergence Problems in Soybeans

If no insect feeding or disease symptoms or lesions are present, determine if the surface of the soil has developed a crust. If a crust exists and significant rainfall greater than 0.5 inches is not predicted, consider using a rotary hoe to break up the crust. To prevent damage to emerging seedlings, avoid rotary hoeing when the plants are in the “crook” stage and for three days after this brittle stage occurs.

Large soybean seed is more likely to experience emergence problems in crusted soils than small soybean seed due to the larger cotyledons. Soybeans planted in 30-inch rows are more likely to emerge from crusted soils than beans planted in narrow rows as the closer seed spacing within the row enables the emerging seedlings to crack the crust. Seedlings that have lost only one cotyledon when emerging from crusted soils will produce normal yields. When both cotyledons are pulled off during emergence, yield losses will range between 2 and 7 percent.
If insects, diseases and crusting are not the problem, determine if the planting depth is correct (1 to 1.5 inches deep) and uniform, and check to see that soil is firmed around the seeds. If serious planting problems are found and the stand is not adequate, the field may need to be replanted.

If no planting problems are detected and the seeds or seedlings look healthy, inadequate soil moisture is the likely cause of the delayed emergence. Wait until a rain occurs and recheck the field.

When deciding if replanting is warranted, always compare the yield potential of the existing stand to that of the replanted stand and account for all replanting costs.

Impact of later soybean planting in southern Minnesota

Since early-May plantings usually result in maximum yields, lower yields should be expected for later plantings. University of Minnesota Extension research indicates that on average planting soybeans in Minnesota by May 20th results in a 6% yield loss compared to a planting date of May1st and planting by May 25th or May 30th resulted in a 9% and 13% yield loss respectively. Another consideration for later planting is the later maturity of replanted fields.

From Dave Nicolai, University of Minnesota Extension Educator, Crops, Farmington, MN