Steps to Minimize Losses from Three Important Soybean Diseases

Published: March 8, 2011

When asked which soybean diseases consistently cause losses and which are most difficult to manage both producers and agri-business personnel in Missouri list Phytophthora root rot, soybean cyst nematode and sudden death syndrome. All three of these diseases are caused by pathogens that are present in the soil, all three are found in all soybean producing areas of the state and all three can be difficult to manage. Management options for these three diseases rely primarily on preventative measures since effective rescue treatments are not available. These three soybean diseases along with management options are described below. For additional information and color pictures please see the University of Missouri Extension bulletin IPM1002 Soybean Diseases.

Phytophthora Seedling Blight and Root Rot

Phytophthora seedling blight and root rot is caused by the soil-borne fungus Phytophthora sojae. This soil-inhabiting fungus can cause seed decay, preemergence or postemergence damping-off, seedling blight and root rot as well as mid- to late-season wilt and death of plants. Phytophthora sojae produces structures called oospores, which enable it to survive from year to year in crop residues or in the soil. In the spring, the oospores germinate to produce sporangia. When soils are flooded or saturated, the sporangia release zoospores, which are attracted to the growing soybean root tip, where infection occurs.

Phytophthora seedling blight and root rot is more severe in areas that are low or poorly drained, in compacted areas or in clay or heavy soils, but the disease can appear on plants growing in lighter soils or higher grounder if the soil remains wet after planting. Significant rain after planting favors the development of Phytophthora in all sites. A dry period after planting drastically reduces this disease. Phytophthora may occur at soil temperatures as low as 50 degrees F, but greatest root damage occurs when soil temperatures are 59 degrees F or above.

Numerous races of Phytophthora sojae have been identified based on their ability to overcome specific Rps genes or combinations of Rps genes in soybean varieties. The most recent Missouri survey found Phytophthora sojae in all soybean production areas of the state. When race determinations were done on the Phytophthora sojae isolates recovered from 21 counties throughout the state, fourteen different races were identified with no one race being predominant.

Management options for Phytophthora seedling blight and root rot:

1. Select varieties with either race-specific resistance, tolerance or a combination of race-specific resistance and tolerance, especially for use in fields with a history of Phytophthora. Race-specific varieties contain a single gene or combination of genes (i.e., Rps1c, Rps1d, Rps1k, Rps3a, etc.) that confer resistance to specific races of Phytophthora sojae. Tolerant varieties have a non-race specific, partial resistance and may also be called field-resistant varieties.
2. Plant in good seedbed conditions.
3. Phytophthora is more likely to occur in low, wet areas, poorly drained areas or compacted areas of a field. Tiling to improve drainage and taking steps to reduce or prevent compaction may help minimize disease problems.
4. Avoid the application of high levels of manure or fertilizer (KCl) just before planting.
5. Use an appropriate fungicide seed treatment. Products containing either metalaxyl or mefenoxam as an active ingredient are particularly effective against water mold fungi such as Phytophthora sojae. If high disease pressure is expected, the use of the higher rate of these seed treatment fungicides may be necessary.

Soybean Cyst Nematode (SCN)

The soybean cyst nematode, Heterodera glycines, is a serious problem throughout Missouri and in most soybean producing areas of the United States. Three different surveys for SCN in Missouri have shown that approximately 75% of the surveyed fields have detectable levels of SCN.

Symptoms of SNC range from no obvious symptoms to subtle differences in plant height and vigor or unexpected decreases in yield to severe stunting and discoloration of plants or dead plants. If plants are carefully dug up, females may be evident on the roots. The females appear as tiny (smaller than nitrogen-fixing nodules), whitish to yellow to brownish, lemon-shaped structures on the roots. Symptom expression may be more severe if plants are subjected to other stresses such as moisture stress, nutrient deficiencies, herbicide injury, insect damage or other diseases. The cysts are the bodies of the dead female nematodes. The cysts are actually protective egg cases that contain up to 250 SCN eggs. Eggs in cysts may survive in the soil for extended periods of time even in the absence of soybean crops.

Anything that moves cyst-infested soil can spread SCN, including machinery, animals, migratory birds, people, wind, water and soil peds associated with seed. Once in a field, SCN may take several years to build up to damaging levels. Unfortunately, once SCN is in a field it is likely to be there forever. Management options for soybean cyst nematode:

1. Employ a program of soil sampling to identify problem fields and to determine the extent and severity of the problem within the field. For more detailed information on soil sampling for SCN refer to University of Missouri publication G4450, Soybean Cyst Nematode: Diagnosis and Management or the Plant Nematology Laboratory website http://soilplantlab.missouri.edu/nematode/
2. Select resistant varieties. Most commercial varieties with resistance to SCN have PI88788 as the source of SCN resistance. If PI88788 resistant varieties have been used in the same field for a number of years, that resistance source may not be performing as well as it initially did. If possible rotate to another source of resistance or at least to a different PI88788 variety.
3. Rotate to non-host crops.
4. Maintain good plant vigor.
5. Maintain good weed control.
6. Avoid spreading SCN from infested fields to uninfested fields by working uninfested fields first before moving equipment to infested fields.
7. Although several nematicides are labeled for use on soybeans, economic and environmental concerns limit their use.

Sudden Death Syndrome (SDS)

In Missouri, sudden death syndrome (SDS) has been a problem primarily in river bottom fields in the central and eastern portions of the state. However, the pathogen Fusarium virguliforme (formerly called Fusarium solani f. sp. glycines), appears to be present in soybean-producing areas throughout the state. In years when environmental conditions are favorable for infections and symptom development such as 2008 and 2009 and to a lesser degree in 2010, SDS may be found in most areas of the state.

SDS has been associated with maximum yield potential soybean production, that is, fields with optimum fertility, irrigation and lime applications. Field observations suggest that SDS is more likely to occur and to be more severe with high soil moisture, whether that is supplied by rainfall or irrigation. High soil moisture during vegetative stages of soybean growth seems to be most conducive to disease development. Because early-planted fields have a longer exposure to spring rainfalls than later-planted fields, seedlings in early-planted fields have an increased susceptibility to infection by the SDS pathogen. Later-planted fields in which soybean plants miss early spring rains may have lower levels of root infection and lower levels of SDS throughout the season. The onset of SDS symptoms is associated with wet conditions and below normal temperatures at or near bloom.

Management options for sudden death syndrome:

1. Select varieties that have performed well where SDS has been a problem.
2. Improve drainage in poorly drained fields and avoid compacting soils.
3. Stagger planting dates and delay planting until soils are warm and dry.
4. Rotate crops; avoid continuous soybean cropping.
5. Maintain good crop vigor and avoid crop stress, including soybean cyst nematode.
6. Harvest fields with SDS in a timely fashion.