Water standing in soybean field

Editor’s note: The following was written by Emmanuel Byamukama, South Dakota State University associate professor and Extension plant pathologist, and Connie Strunk, Extension plant pathology field specialist, for the university website. 


If the forecast holds true, it looks like it is going to be another year of excessive soil moisture and possible flooding come this spring.

The increased level of soil moisture has implications with regards to plant stand establishment as well as root rot and nematode infestations. Managing root rots and nematodes requires taking action before planting since no in-season treatments can be done to control them.

Fungicide/nematicide seed treatments provide protection against seed-borne and soil-borne pathogens and nematodes, which may interfere with seed germination or may infect the developing roots soon after germination. Seed treatments enhance plant vigor in the presence of soil/seed-borne pathogens.

For fields in low-lying areas with poor drainage or with a history of root rots, a seed treatment may be needed this year. The decision to use a given active ingredient should depend on the pathogen to be managed.

Pythium and Phytophthora (water-mold pathogens) are managed with three common active ingredients: metalaxyl, mefonoxam and oxanthiopiprolin. Common nematicides used to manage SCN are: Clariva, Ilevo, Poncho Votivo, Avicata Complete, Nemastrike and Aveo EZ.

Other seed and root rot pathogens can be managed with several products available on the market.

It is important to note seed treatments do not compensate for poor seed quality. Even with seed treatments, cracked, shriveled or poorly stored seed may still not germinate well. Use clean, disease-free seed to reduce inoculum on the seed.

Safety while treating and handling treated seed is important. Follow safety guidelines on the product label. Always use personal protection equipment recommended on the pesticide label and on the treated seed bag tag. Follow the label directions to know how to deal with spilled or leftover seed.

One of the effective tools in plant disease management is variety/hybrid selection. Varieties/hybrids vary in their susceptibility to plant diseases. For instance, resistance to Phytophthora is available in soybean cultivars, as well as resistance to SCN.

A soybean variety with resistance to SCN may not require nematicide seed treatment provided the SCN counts in the field are not very high (under 6,000 eggs/cc of soil).

Resistance genes in varieties can sometimes fail to manage the pathogen due to persistent use of the same resistance gene. It is important to rotate within varieties with different resistance genes to avoid pathogen resistance from developing. For instance, soybean varieties with PI88788 resistance source should be rotated with Peking and other sources.

Phytophthora resistance genes found to be more durable are Rps 2, 3a, 3b, 3c, 4, 5, 6, and those with stacked genes.

Time of planting can influence the level of certain root rots in crops. Planting too early when the soil is cool (below 55 degrees) and wet or slow to warm-up increases the chances of seed rots and root rots caused by Pythium spp and Fusarium spp. It is recommended to delay planting to when soils are warmer and not too wet to reduce the risk of root rots developing.

Flooding can move soil within and between fields, spreading soil-borne pathogens. It is therefore beneficial to carry out soil tests for various pathogens in subsequent seasons, especially for the soybean cyst nematode, which is the No. 1 pest of soybean. Testing the soil for various pathogens may aid decision making such as seed treatment and variety selection.

Growers are encouraged to test their soils for SCN in order to deploy resistant varieties if SCN is found in their fields.