Soybean seed treatments

Every year, growers consider the use of seed treatment to prevent stand reduction from soybean seedling diseases, especially those growers who do not purchase pretreated seed. When making a decision for seed treatment, the first step is to assess the risk of seedling diseases. If a field has a history of seedling diseases, the risk that disease will occur again is high. If good records on a field are not available, risk can be assessed by analyzing three factors that are associated with seedling diseases: (1) poor quality seed, (2) early planting in cold and wet soil, and (3) tillage practices. Below, I will discuss these factors as they relate to seed treatment.

Soybean damping-off.

Seed quality. Use of poor quality seed increases the risk of seedling diseases and one may need to treat seeds when the quality is low. Low quality is characterized by low germination rate and a high level of fungal infection as indicated by seed-testing results. Seed treatment is a must if fungal infection is detected.

Early planting. Early planting generally increases the possibility of achieving maximum yield. In some fields, however, disease risk may increase because soybeans are planted in soils that are cold and wet. According to data recorded at Ames, the average soil temperature at a 1-inch depth at 7 a.m. is around 50° F before the second week of May and rises to above 60° F after the end of the third week of May. Cool (below 60° F) and wet soils are conducive to fungal seed rot or the death of seedlings caused by Pythium and Fusarium. A study at Iowa State University showed that most problematic Pythium species in Iowa prefer cool soil temperatures (below 60° F) for infection.

When soil temperature increases, the risk of Pythium damping-off should not be a concern. Damping-off caused by Phytophthora and Rhizoctonia are the problems of late-planted soybeans because the two fungi are the most active when soil temperatures are between 70-80° F. Stand reduction in warm soils in late spring is a good indication of damping-off by these two fungi. Table 1 gives the optimum soil temperatures for four common seedling diseases in Iowa.

Tillage. A three-year study by soybean pathologists in the north central region showed that the risk of seedling disease generally is higher in no-till fields compared with conventional till fields. Many pathogens are concentrated on aboveground parts of plants and reproduce on crop debris; thus, the residue that remains in untilled fields increases pathogen inoculum. Furthermore, large amounts of residue also reduce soil temperature and increase soil moisture, which favors Pythium seedling blight. Increased soil moisture favors Phytophthora root rot. However, this does not mean that seed should be treated whenever planted in no-till fields. The level of risk due to the use of no-till varies from farm to farm and from field to field, depending on several factors, one of which is soil-type.

Target the right fungi. Different chemicals are effective on different fungi. Only the use of the correct chemicals can prevent damping-off. Table 2 lists some chemicals and their control efficacies for major seedling pathogens in Iowa. Generally speaking, Apron is effective for control of Pythium and Phytophthora, and Rival is very good for control of Rhizoctonia. If you do not know what fungi cause disease problems in your fields, use a combination of the above two chemicals to target Pythium, Phytophthora, and Rhizoctonia because these three fungi cause more than 85 percent of the damping-off problems in Iowa. Keep in mind that Phytophthora damping-off also can be prevented by planting resistant varieties.

Seed treatment and white mold. White mold-infected seed may not be a concern this spring, except for northeastern Iowa, because most Iowa areas had very low levels of white mold in the 1997 growing season, and consequently, low infection rate. If you saved seeds from white mold-infested fields and want to treat the seeds to reduce risk, consider to use Rival or Vitavax. Studies under controlled conditions (not fields) by Iowa State University show that these fungicides are effective in preventing white mold growth from infected seeds. More information on white mold-infected seed can be found in the February 1998 ICM article, Questions on white mold control.

Methods of treatment. Because most commercial seeds are untreated, on-farm seed treatments are the only option to growers who want to use treated seed to reduce the risk of seedling diseases. Good seed coverage is critical and the quality of on-farm treatment varies with the equipment used. Simple equipment for on-farm seed treatment is available at affordable prices. This equipment can be mounted directly and conveniently on a wagon or a truck box to dispense fungicides onto seeds during planting, an improvement compared with planter-box treatments. Some chemical representatives may have on-farm equipment that can treat large quantities of seed.

Table 1. Optimum temperatures for development of seedling diseases by different fungi commonly found in Iowa.

Phytophthora Pythium Rhizoctonia Fusarium
Temperature (F) 72-80 50-60 70-80 approx. 60

Table 2. Relative efficacy of seed treatments for control of four soybean seedling diseases.

Treatment Phytophthora Pythium Rhizoctonia Fusarium
Agrosol FL + Apron E E F F
Agrosol T N N F F
Agrosol T + Apron E E F F
Apron E E N N
Captan + Apron E E P F
Rival P P G F
Rival + Apron E E G F
Vitavax 200 N P F P
Vitavax 200 + Apron E E F P

E, excellent; G, good; F, fair; P, poor; N, no activity. Modified from an article by P. Lipps, Seed Treatments for Agronomic Crops, Bulletin 639, The Ohio State University.

This article originally appeared on pages 41-42 of the IC-480 (5) -- April 13, 1998 issue.

Updated 04/12/1998 - 1:00pm