On-farm seed treatments for disease

Wet and cool weather in early spring always increases the risk of soybean seedling diseases. Since many Iowa soybean fields are still wet and fieldwork is running behind, growers may be thinking about planting these areas in no-till. In these situations, stand reduction from soybean seedling diseases becomes a concern, especially for growers who experienced the disease in the past but did not purchase pre-treated seeds. If this is the case, your best option to manage these diseases would be on-farm treatment of soybean seed.

Stand reduction by damping-off in a soybean field.

On-farm treatment of seed depends on two things: selection of the right chemical, and properly treating the seeds. For help in assessing your level of risk for seedling disease, refer to my article in last year's ICM newsletter on pages 30-31.

Chemicals. Different fungicides are effective on different fungi, so you should select chemicals that can effectively and economically target the fungi that are causing problems in your field. Several fungi can cause seedling diseases. According to an extensive two-year study by the ISU Department of Plant Pathology, major fungal pathogens in Iowa soybean fields are Rhizoctonia solani (which causes 23-27 percent of seedling disease), Fusarium spp. (10-13 percent), Pythium and Phytophthora (cumulative 55-63 percent).

In no-till fields, the fungi that cause damping-off often are Pythium and Phytophthora. Pythium and Phytophthora are closely related fungi and most chemicals have the same efficacy for the two fungi. In a wet season in early planted fields, Pythium should be the major targeted fungus. Phytophthora likes warmer soil and, therefore, causes stand problems in late-planted soybeans. If you do not know what type of fungi may be causing stand reduction in your fields, target Rhizoctonia, Pythium, and Phytophthora because these three fungi account for 90 percent of seedling disease problems in Iowa. Some products contain several different compounds effective to control all three fungi.

Pathologists in different states, including Iowa, have conducted seed treatment experiments for many years. I have found the performance trials of tested chemicals to be generally consistent among states.

The table (below) shows relative efficacy of different chemicals for various seedling diseases. Although the information is from Ohio, recommendations apply to seedling diseases that occur in Iowa. Apron contains an active ingredient, metalaxyl, which is relatively expensive (about $5 per acre) if large doses are used for control.

Methods of treatment.On-farm treatments are known as hopper-box or planter-box treatments. Many chemicals can be used for this purpose, but check with your chemical dealer for products based on the compounds listed in the table. Most chemicals for planter-box treatment are dry products formulated with talc or graphite that adheres the products to the seeds. Liquid products that are fast drying materials also are available.

Good seed coverage is critical to have good control, which often is difficult to achieve when we try to treat seeds in the field at planting time. To ensure good coverage, mix the seeds in a planter box twicethat is, fill the planter box half-full with seeds, add half of required amount of chemical and mix; then add the rest of the seeds and remaining chemical and mix a second time.

Many chemical representatives also have on-farm equipment that can treat bulk quantities of seed. This equipment is convenient and will save you time at planting compared to planter-box treatments. I am aware of one type of equipment for bulk treatment, which costs about $295. It can be mounted directly on a wagon or a truck box to dispense fungicides onto seeds. On-farm bulk-treated seeds also can be stored in a machine shed for some time before use without losing their chemical effectiveness.

Relative efficacy of seed treatments for control of soybean seedling diseases.

Phytophthora Pythium Rhizoctonia Fusarium
Agrosol FL N 1 2 2
Agrosol FL + Apron 4 4 2 2
Agrosol T N N 2 2
Agrosol T + Apron 4 4 2 2
Apron 4 4 N N
Captan N 1 1 2
Captan + Apron 4 4 1 2
Rival 1 1 3 2
Rival + Apron 4 4 3 2
Vitavax 200 N 1 1 1
Vitavax 200 + Apron E 4 4 1 1

4=Excellent, 3=Good, 2=Fair, 1=Poor, N=no activity. Source: Ohio State University.

This article originally appeared on page 39 of the IC-478 (5) -- April 21, 1997 issue.

Updated 04/20/1997 - 1:00pm