5/24/01 - Cold, Wet Soils and Soybean Emergence

5/24/01

Cold, wet soils and soybean emergence

Dean Reynolds - Extension Educator and Assistant Professor
University of Minnesota, Department of Plant Pathology
Reyno045@umn.edu

Bruce Potter, IPM Specialist, SW Minnesota
bpotter@tc.umn.edu

An inclement weather system parked itself over Minnesota on May 20^thand is expected to remain in place until the 26th. This system is creating cold, wet conditions. Excess soil moisture conditions exist in localized areas. In addition to delaying the completion of planting, this weather will probably have an impact on soybean emergence, resulting in poor stand.

How do you know what caused the problem?

Sorting out the cause of poor soybean stands may be a bit more difficult this spring. As you are probably already aware, soybean seed quality and the germination of many seed lots is poor this spring. Poor late-season growing conditions and low humidity during harvest reduced seed quality. Some of the poor germ also may be due to seed-borne pathogens such as Phomopsis. Seed that is mechanically injured or that produces seedlings with poor vigor is at a greater risk from the stress of slow emergence and soil- borne pathogens. Poor stands may be due to poor planting, poor seed condition, seed- borne disease, soil-borne disease or any combination of these.

Disease is a likely suspect when soybeans die after emergence. Dead seedlings or plants that wilt during hot weather are easily observed. The cause of soybean death is more difficult to determine when it occurs before emergence. In rare cases, it may be impossible to determine unless a sample of the seed lot has been saved. The first step is to look for seed in areas of poor stand to determine whether planting problems may have caused the problem. Disease can be blamed when seed or seedlings rot before and during emergence and when some of the remaining plants show signs of rot or lesions on roots, cotyledons, or hypocotyls.

Pathogens, insect feeding and mechanical injury to meristem tissue (growing point) can cause hypocotyls of soybeans to swell. Tunneling and other signs of feeding can help distinguish disease from insect problems, seed corn maggot and cutworm for example. Several plants may need to be examined in order to make this decision. Finally, timely scouting is important in determining the cause of poor soybean stands. Soybeans, killed at an early growth stage, do not persist for a long time.

The major players in cold soils

Poor emergence can result whenever soybean emergence is delayed. Many genera of seed and soil borne fungi can cause seed rots and damping off. The following discussion highlights the three genera, Phytophthora, Pythium and Fusarium, commonly associated with cool wet conditions. Of the three, Pythium is mainly associated with seed and early seedling soybeans. Infections from Phytophthora and Fusarium can happen just as early in crop development. They also occur later in the season if soil temperature and moisture conditions are favorable. Additionally, the latter two diseases can become season-long chronic problems.

Field identification of causal organism for seed rots and damping is notoriously difficult. For example, it is very difficult to distinguish Phytophthora and Pythium seedling disease based on symptoms. Other identifications can be just as hard. Pathogens often need to be cultured from infected plants for accurate identification of early season diseases. If you need to distinguish the genera causing the problem, it is usually best to submit samples for genus identification to the:

University of Minnesota Plant Disease Clinic
495 Borlaug Hall
1991 Upper Buford Circle
St. Paul, MN 55108

Include some live, or less severely diseased, plants in the sample. Often, plants adjacent to dead plants are better candidates for isolating the causal organism. Do not send specimens late in the week, as they will sit over the weekend. Do not tightly seal plants within plastic bags. Completely decomposed plants are less useful for disease diagnosis. A minimal fee is charged for diagnosis.

Additional information on submitting samples can be obtained by calling the clinic at 612-625-1275 or by emailing gould001@tc.umn.edu. Information about the clinic and sample submission can also be found at: http://www.plpa.agri.umn.edu/extension/plantdiseaseclinic.htm.

The following disease descriptions were adapted from the Minnesota Soybean Field Book (MI-7290-S) and the Compendium of Soybean Diseases (APS Press).

Phytophthora - Seedling Blight, Root and Stem Rot
Phytophthora sojae

Phytophthora can cause stand reductions from seed stage through maturity. Seedling root systems are dark in coloration and above grown portions wilt and die. Hypocotyls may be swollen when meristem tissue is attacked. Taproots of older plants are dark brown. Smaller roots may be rotted or missing. Phytophthora can cause mortality at any stage of soybean development. After plant death, leaves remain attached. A dark brown stem discoloration from soil line upward, a diagnostic character for larger soybeans, can be absent on more tolerant or resistant varieties. Soybeans with chronic infection may only show moderate stunting or early senescence as a symptom.

Phytophthora survives long periods without soybeans on infected crop debris. Under wet, flooded conditions motile “zoospores” are produced that are attracted to and swim toward soybean roots. This fungus exists in most, if not all fields, with a history of soybeans. Wet conditions and moderate soil temperatures favor infection. Poorly drained soils or compacted soils increase disease severity. Sites that are normally well drained but experience prolonged wet conditions sometimes experience severe disease.

Resistant and field tolerant varieties are available. Several sources of single gene resistance (e.g. Rps1a, Rps1c, Rps1K, Rps6) are available in soybean varieties to combat most races of Phytophthora. To be effective, these Rps genes must be matched to the form(s) or race(s) of the fungus predominate in a field. Historically, these individual Rps genes have been overcome within a field due to race shifts of the fungus. Overcoming a single resistance gene is likely when the gene is used exclusively over several soybean crops. The loss of effectiveness of the 1k gene in some Minnesota fields is the latest example of this constant battle between plant breeders and fungi.

Field tolerance is not race specific like the Rps genes. Tolerance is best described as the ability to maintain yield when infected by the fungus but it may not provide enough protection under some environmental conditions. Field tolerance scores are not standardized between seed companies. When using tolerance, and especially when using Rps genes, it is important to document successes and failures and make adjustments when needed.

Fungicide seed treatments containing metalaxyl or mefanoxam provide early season control of this fungus as well as Pythium. Infection, however, can still occur when conditions favorable for infection occur later in the season.

Pythium - Seed Decay, Seedling Blight and Root Rot
Pythium species

Pythium usually attacks soybeans early. Seed rots and death before emergence are typical for these fungi. Younger seedlings are most susceptible because soybeans become more resistant to this fungus as they age. Symptoms are indistinguishable from early season Phytophthora in the field. Roots and hypocotyls are often completely decayed and infected tissue often has a watery appearance. These species along with Phytophthora are often called "Water Molds." They survive in soil and in plant residue. Cool (50°F to 60°F), wet soils favor release of "swimming" spores and rapid disease development. Unlike Phytophthora, there are no resistant varieties although there may be tolerance differences. Fungicide seed treatments can provide early season control.

Fusarium - Root Rot
Fusarium oxysporum, Fusarium solani & other F. species

Soybeans in soils below 58°F, especially when flooded, are prone to infection by this fungus. Infected seedlings may damp off. Infections of older plants cause stunting. Leaf necrosis resembling brown stem rot and leaf senescence can occur. Severely infected plants can be killed. Infection is often limited to the lower tap and lateral roots, which may be completely rotted. In contrast to the “water molds”, white to salmon colored Fusarium mycelium can sometimes be observed on infected roots. With the removal of excess soil moisture, new roots often develop from the upper taproot. Although the resulting shallow root system is prone to failure in dry soils, severely rotted root systems often recover to produce harvestable yield. Fusarium can affect and darken the soybean vascular system. This increases late season plant wilt under moisture limiting conditions. In addition to being primary pathogens, Fusarium spp. often colonizes root systems weakened from other causes. It is often isolated from soybean root systems that may have been killed by other fungi (e.g. Phytophthora). Stress/injury from soybean cyst or other nematodes, and DNA herbicides can predispose plants to Fusarium infection. These widespread soil-borne fungi survive long periods as chlamydospores and as mycelium in plant residues.

Sudden Death Syndrome (SDS) is caused by a specific strain of Fusarium solani. It is commonly reported in Iowa and Illinois and probably exists at some level in Minnesota. Although symptoms resembling SDS are often observed in SW Minnesota, the isolates cultured have not been like those reported in Iowa and Illinois. Unlike SDS, the symptomatic plants observed here, often recover.

Fusarium resistant soybean varieties are not available, although there may be differences in tolerance. Several fungicide seed treatments containing captan, especially in combination with PCNB + TBZ (Rival) can reduce early season damage.

Row cultivation of soybeans infected with root rots can sometimes encourage partial recovery of diseased root systems.

Fungicidal seed treatments are recommended when planting into fields with a history of serious fungal root rot problems, when planting into cold soils, planting into no or minimal till situations or when resistance/tolerance has not been completely effective.

Fungicidal seed treatments are often recommended when seed born pathogens are reducing germination. This spring, however, mechanical damage to seed was one of the primary causes behind poor germination. Application of seed treatments involves an additional handling operation. Concern over further damage to already poor seed coats caused some producers to reconsider using seed treatments this spring.

URL: http://www.soybeans.umn.edu/resources/news/news_5-24-01.htm

Last Modified 8/9/05 by:
Sarah Jameson-Jones
Web Designer/Developer
james039@umn.edu