Seed Treatments of Soybeans

2003 DATA

By Carl A. Bradley
Extension Plant Pathologist
Dept. Plant Pathology

Soybean seeds may be treated with fungicides to improve stand, protect against seedling infection by some pathogens, and reduce the spread of diseases which may be carried on or in the seeds. Most soybean seed is sold untreated, however. The use of seed treatments may not be necessary if healthy seed is planted under conditions favoring rapid emergence. When planting into conditions less than ideal, such as cool, poorly drained, or no-till/reduced tillage soil, the use of seed treatments will provide a more uniform stand, but not always a yield advantage.

Seed treatments containing mefenoxam or metalaxyl such as Allegiance FL, Apron XL LS, ApronMaxx RTA, Delta-Coat AD, Prevail, Soygard, Stiletto, and Warden RTA are effective against downy mildew and against seedling infection by Phytophthora and Pythium. Season-long management of Phytophthora can be obtained through the use of resistant varieties, however. Products that contain carboxin, PCNB, or fludioxonil, such as ApronMaxx, Delta-Coat AD, Maxim 4FS, or any of the Vitavax products, may provide some protection for seeds and seedlings against Rhizoctonia. Kodiak is a new biological seed treatment that contains spores of the bacteria Bacillus subtilis. It is labeled for suppression of Fusarium and Rhizoctonia infection.
White mold (Sclerotinia stem rot) may be spread through infected seed. It has been shown that fludioxonil, thiram, and captan + PCNB + TBZ will reduce disease spread by seeds. Avoiding "bin-run" seed and planting certified disease-free seed is most important in managing the spread of white mold through seeds, however.

On-Farm Seed Treatment. Since most seed is sold untreated, it may be necessary to use on-farm seed treatment. Some seed treatments are available as hopper-box formulations. It is important that seed be uniformly coated if the seed treatment is to be effective. Check with manufacturers' labels for specific instructions.

Seed Treatment Fungicides and Rhizobium Inoculants. Some seed treatment fungicides have an adverse effect on Rhizobium inoculants. Captan and PCNB severely reduce survival of Rhizobium on treated seed and reduce nodulation compared to inoculated seed with no fungicide. If captan or PCNB treated seed is to be planted, it might be best to use an in-furrow inoculant. Carboxin has a moderate effect on Rhizobium, and could be used if the seed is inoculated immediately before planting. Mefenoxam and metalaxyl have little or no adverse effect on Rhizobium and thiram has no adverse effect.

Refer to this NDSU web site for more specific information and chemical rates on seed treatments for soybean

Evaluation of Fungicide Seed Treatments on Stand Establishment, Root Disease Control, and Yield of Soybean in North Dakota

Carl A. Bradley, Department of Plant Pathology, and Ted Helms, Department of Plant Sciences, North Dakota State University

METHOD

     Soybean fungicide seed treatment trials were conducted during the growing season of 2003 at 7 sites in North Dakota located at Casselton, Grandin, Great Bend, LaMoure, Northwood, Prosper, and Wyndmere.  Fungicide seed treatments included an untreated control, Warden RTA (fludioxonil + mefenoxam, Agriliance) at 5 fl. oz/cwt, SoyGard (azoxystrobin + metalaxyl, Gustafson) at 0.43 oz/cwt, and the biological control seed treatment Yield Shield (Bacillus pumilus, Gustafson) at 0.1 oz/cwt.  Seed treatments were applied to the seeds of soybean cultivar Walsh in a plastic bag and allowed to dry before planting.  Plots were planted at 242,812 seeds/A on May 25 at Casselton, May 29 at Grandin, May 22 at Great Bend, May 27 at LaMoure, May 15 at Northwood, May 27 at Prosper, and May 23 at Wyndmere.  Plots were 4 rows wide (30 in. row spacings) and 17 ft. long.  The experimental design was a randomized complete block with 4 replications.  Stand establishment was measured at each location by counting the number of plants per meter of row in the 2 middle rows.  When plants were at the R1 to R2 stage, 10 plants from each plot were dug and rated for lateral root development and root disease.  Lateral root development was measured on a 0 to 2 scale, where 0 = no lateral roots and 2 = prolific lateral root production.  Root disease was assessed by measuring the total length of lesions on the roots and hypocotyls.  Stand establishment, lateral root development, and root disease data were not collected at the Northwood site.  Diseased roots were surface sterilized with a 10% Clorox solution and placed on media for isolation of the disease-causing pathogens.  Plots were harvested on Oct. 1 at Casselton, Oct. 2 at Grandin, Sept. 25 at Great Bend, Oct. 10 at LaMoure, Sept. 29 at Northwood, and Sept. 30 at Wyndmere by a small-plot combine, and yields were adjusted to 13% moisture.  Plots were not harvested at Prosper.  Data were analyzed with the general linear model procedure using statistical analysis software.  Treatment means were compared using Fisher’s protected least significant difference (LSD) at the P ≤ 0.05.

RESULTS

     Results are presented in Table 1.  Significant differences among treatments occurred for stand at the Grandin site, and for lateral root development at Wyndmere.  At Grandin, seeds treated with SoyGard resulted in significantly fewer plants per acre than the untreated control.  At Wyndmere, all seed treatments resulted in plants with significantly greater lateral root development than the untreated control.  No other significant differences occurred; however, plots that were planted with treated seed resulted in a 3 bu/A benefit compared to plots that were planted with untreated seed (33 vs. 36 bu/A).  The root rot pathogens Rhizoctonia solani and Fusarium solani were the primary pathogens isolated from diseased roots at all locations.

Table 1. Effects of seed treatments on stand, lateral root development, root diseases, and yield at several locations in North Dakota in 2003.

^a Lateral root development was measured on a 0 to 2 scale where 0 = no lateral roots and 2 = prolific lateral root development.

^b Fisher’s protected least significant difference at P ≤ 0.05.

^c NS = Not significant at the P ≤ 0.05 significance level.

^d ND = Not determined.