Reviewed October 1993
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It is a well established fact that all crop seeds carry fungi on or inside the seed coats. When seeds are put in moist soil, seed and seedling diseases are common. Seed treatment of soybeans with a fungicide before planting will usually help to improve germination and seedling vigor by reducing seed decay, damping off and seedling blights.
Soybean seeds often carry fungus spores of Aspergillus, Penicillium, Rhizopus and other mold species that can cause seed decay. In addition, several fungi may be seedborne. Diaporthe (Phomopsis) pod and stem blight, anthracnose, purple seed stain, downy mildew and other diseases can be introduced into the field by infected seed and can severely affect performance. Harvesting conditions in certain years (very dry) can result in cracks in the seed coats that make the seeds vulnerable to decay and seedling blights.
If less than 80 percent germination is due to disease-producing organisms, seed treatment often improves performance. If germination is poor due to mechanical injury to embryos or other physiological disorders, seed treatment fungicides may not be effective. If germination is more than 90 percent, seed treatment may or may not provide significant additional benefits. Performance of high-germinating and high-vigor seed will depend on the environmental conditions at the time of planting. However, there may be an insurance aspect to consider. In general, the money spent on seed treatment fungicide is one of the "best buys" in a crop production management program.
The value of soybean seed treatment depends on several factors:
Use high-quality seed
Soybean growers should obtain the highest quality seed possible. Purchase certified seed that has the percent germination listed in the certified label along with the variety name. Germination percent of bin-run seed is generally unknown, but such soybeans can be sent in for laboratory test for better assurance.
Soybean seed with greater than 80 percent germination will not always benefit from a fungicide seed treatment. However, under particularly stressful spring conditions-(cold, wet soils, compaction, herbicide injury, etc.) fungicides may provide much needed insurance. Seed with less than 80 percent germination may benefit from seed treatments by increasing stands and possibly increasing yields.
In some years, seed quality may be down. Decisions to use poor quality seed should be based on the nature of the poor quality. Mechanically damaged seed or seed that is dead for other reasons cannot be restored. However, certain seedborne diseases that can reduce germination and seedling performance can be controlled in part by fungicides. Seed treatment fungicides effective against seedborne diseases usually will not increase germination more than 20 percent. If soybeans with 50 percent germination are treated with a fungicide such as captan or the carboxin-thiram combination (Vitavax 200), the grower should expect not more than 70 percent germination in the field.
Germination tests conducted by a competent laboratory are obviously the best. However, a grower can obtain an approximate test by using a home procedure. Place two paper towels in the bottom of a tray, one on top of the other. Wet the towels thoroughly and tilt the tray up on one end so that excess water runs off the tray. Select a random sample of 100 seeds from the seed lot and place them in between the moist paper towels. Put the tray in a plastic bag and tie the end shut to prevent the towels from drying out. Place the tray in a location of diffuse light, not in direct light, such as a north window. The location should be warm enough for good plant growth-good for houseplants. After five days, open the plastic bag and count the number of well germinated seeds with intact roots and shoots. Do not count moldy seed or any abnormal or diseased seedlings. Testing two to four 100-seed replications in the way described will provide an additional level of confidence.
Avoid extremely early planting
Soil temperatures should be consistently above 55 degrees Fahrenheit. Unless the temperature is increasing steadily, planting when the soil is in the low 50s may not be advisable. Emergence occurs very slowly when the soil is cold. Additionally, plants will be more susceptible to seedling infections from Pythium and Phytophthora in cold wet soils. By contrast, seedlings will emerge five to seven days after planting when the soil temperatures reach the middle and upper 60s.
Avoid deep planting
Planting excessively deep will offset benefits of seed treatments. The longer it takes for seedling emergence, the longer the entire plants are exposed to soil-borne fungi. The young seedlings will also be exposed for a longer period to herbicides, with potential injuries that can weaken the plants and make them more susceptible to diseases. If at all possible, do not plant deeper than 1-1/2 to 2 inches. One-inch depth is better.
Avoid soybeans after soybeans
If at all possible, practice a crop rotation scheme in which soybeans are alternated with corn, sorghum or wheat. Sometimes this practice is not followed. With continued plantings of soybeans in the same field, it is becoming more important to treat soybean seed with a fungicide. This measure sometimes increases the original stand by significantly reducing seed and seedling decay by fungi that build up in soils with the same crop for several years in succession. Also, certain root and stem infections, such as Rhizoctonia stem lesions, can be reduced. Young plant health is very important for optimizing soybean yields. Do not try to "bail out" the problem of monoculture with seed treatment fungicides.
When soils are cold and wet and when soybean varieties are susceptible or only tolerant to Phytophthora root rot, Metalaxyl seed treatment use is a good option. Metalaxyl (Apron) should be applied in combination with other seed treatment fungicides to control other seedborne diseases including Phomopsis seed rot.
Seed treatment methods
Fungicides labeled for seed treatment are formulated as dusts, wettable powders, flowables, for liquids. Wettable powders are usually applied to seed in a commercial slurry-type treater. Flowables and liquid materials are commonly applied in commercial slurry-type or ready-mix mist-type treaters. Dusts and some other "Ready-to-Use" formulations are available for planter or drill-box treatment.
In general, it is preferable to have the seed treated by a commercial seed-treatment processor because it will provide superior coverage and more accurate rates of application. However, there are times when on-the-farm applications are desirable. When using planter-box type formulations, it is important to get as complete coverage of the seeds as possible by careful stirring. To avoid having treated seed left over, have the majority of the seed treated commercially, then finish out planting with plant box applied fungicide.
Standard seed treatment fungicides
Captan, thiram and the carboxin-thiram (Vitavax 200) and PCNB-terrazole fungicide combinations are the most widely used soybean treatment materials. These materials have varying degrees of activity against seed and seedling diseases. Captan or thiram have little activity against Pythium or Phytophthora or Rhizoctonia damping off. Because these materials act primarily on the seedborne fungi (on the surface of seeds), and on some soil-borne fungi that attack the seed mainly during germination, they cannot be expected to provide protection against diseases in the seedling stage. The carboxin containing formulations have some effect on the soil-borne Phomopsis pod and stem infection. PCNB-terrazole may help to reduce seedling infections caused by Rhizoctonia.
Metalaxyl (sold as Apron) is a relatively new systemic fungicide that has been effective in reducing stand losses from Pythium and Phytophthora damping off. In field situations, complete coverage is necessary for Apron to work well.
Relative effectiveness of common seed treatment fungicides for control of seed and seedling diseases of soybeans are shown in Table 1.
The fungicides listed represent major available combinations. It is not a completely comprehensive list. There may be other comparable formulations by firms other than those noted. The University of Missouri does not endorse any pesticide materials, nor does it imply approval of the products to the exclusion of others which may be equally suitable.
Table 1
General effectiveness of seed treatment fungicides in seed and seedling diseases
| Seed treatment | Some trade names | Phomopsis-Diaporthe seed rot | Pythium seed rot and damping off | Phytophthora damping off | Rhizoctonia damping off | Rhizoctonia seed rot |
|---|---|---|---|---|---|---|
| Captan | Captan 300; Captan Moly | Good | Fair | Not effective | Poor | Fair |
| Captan + TBZ | Captan T; Agrosol Fl | Good | Fair | Not effective | Poor | Fair |
| Thiram | Thiram 50 WP | Fair | Fair | Not effective | Poor | Fair |
| Thiram + TBZ | Agrosol T | Good | Good | Not effective | Fair | Fair |
| Carboxin + thiram | Vitavax 200 | Good | Fair | Not effective | Fair | Fair |
| Carboxin + captan | Vitavax 20-20 | Good | Good | Not effective | Poor | Fair |
| Chloroneb | Chloroneb 65W | Fair | Fair | Not effective | Poor | Fair |
| Carboxin + PCNB | Vitavax-PCNB | Good | Good | Not effective | Poor | Good |
| PCNB + terrazole | Terraclor Super X | Fair | Good | Not effective | Poor | Good |
| Metalaxyl | Apron Fl; Apron 25W | Not effective | Good | Good | Not effective | Not effective |
Table 2
Suggested seed treatment fungicides for commercial treaters to reduce seed decay, seedling blights and other seed and soil-borne soybean diseases
| Fungicide trade names | Formulation | Rates and remarks |
|---|---|---|
| Apron FL (Metalaxyl, Gustafson, Wilbur-Ellis) | Flowable-slurry | Metalaxyl (Apron) is specific for Pythium and Phytophthora damping off. 0.75-1.5 fluid ounces per hundredweight |
| Captan 30 DD/colorant (Gustafson) | Flowable-slurry | Captan provides broad spectrum control of seed and seedling fungi. 2 fluid ounces per hundredweight |
| Captan 300 (Gustafson) | Flowable-slurry | 2 fluid ounces per hundredweight |
| Captan 400 or 400 D (Gustafson) | Flowable-slurry | 1-1/2 -- 2-1/2 fluid ounces per hundredweight |
| Captan 4000 (Wilbur-Ellis) | Flowable-slurry | 2-1/2 fluid ounces per hundredweight |
| Captan 65 Sprills (Wilbur-Ellis) | Slurry | 2 ounces per hundredweight |
| Captan 75 percent (Gustafson) | Slurry | 1-2/3 ounces per hundredweight |
| Captan + TBZ (Captan T-Gustafson, Agrosol Flowable-Wilbur-Ellis) | Flowable-slurry | 1.5 fluid ounces per bushel; 2.5 fluid ounces per hundredweight |
| Chloroneb 64 W | Slurry | Use Chloroneb as seed overcoat with Captan or Thiram for added Rhizoctonia and Pythium control. 306 cc per hundredweight |
| Chloroneb + Metalaxyl (Nu-Flow AD-Wilbur-Ellis) | Slurry | 5.5 - 7.0 fluid ounces per hundredweight |
| Thiram - 50 WP or 425 (Gustafson, Loveland) | Slurry | Thiram provides broad spectrum control of seed and seedling diseases. 3.3 ounces per hundredweight |
| Thiram - TBZ (Agrosol T-Wilbur-Ellis) | Slurry | 6.6 fluid ounces per hundredweight |
| Vitavax - PCNB (Carboxin + PCNB, Gustafson) | Flowable-slurry | Vitavax in combination with PCNB provides broad spectrum control of seed and seedling diseases plus Rhizoctonia control. 4 fluid ounces per hundredweight |
| Vitavax - 200 or RTU Vitavax-Thiram (Carboxin + Thiram, Gustafson) | Flowable-slurry | A very popular combination for broad spectrum control. 4 fluid ounces per hundredweight |
Note
Apron seed treatment should be used in combination with other protectant
or systemic fungicides for broader spectrum control.
Table 3
Suggested seed treatment fungicides for on-the-farm treatments to reduce seed decay, seedling blights and other seed and soil-borne diseases of soybeans
| Fungicide trade names | Formulations | Rates and remarks |
|---|---|---|
| Apron Dry Seed Protectant (Gustafson) | Dry powder for planter box application | For Pythium damping off and early season Phytophthora. 3 to 4 ounces per hundredweight. |
| Apron + Captan (Gustafson) | Dry powder | Wider spectrum control than Apron alone-includes seed and seedling protection. 4 ounces per hundredweight. |
| Apron + Terraclor (Gustafson) | Dry powder | Controls seed and seedling diseases and Pythium, Phytophthora, Rhizoctonia, 4 ounces per bushel. |
| Captan-Moly (Gustafson) | Dry powder | Controls seed and seedling blights -- Moly (molybdenum) provides aid in nitrogen fixation. 3.8 ounces per bushel. |
| Captan-TBZ-Moly (Wilbur-Ellis) | Dry powder | Broad spectrum control of seed and seedling diseases. 2 ounces per bushel. |
| Captan-Vitavax (Enhance-Gustafson)(NuGro-Wilbur-Ellis) | Dry powder | Seed and seedling diseases. 3 ounces per bushel. |
| Captan-Diazon-Lindane (Agrox D-L Plus-Wilbur-Ellis) | Dry powder | Broad spectrum seed and seedling control plus insect control. 3.6 ounces per hundredweight. |
| Carboxin (Vitavax) + Diazinon + Lindane (Germate Plus-Gustafson) | Dry powder | Used on seed previously treated with Captan or Thiram; adds insect control. 2 ounces per bushel. |
| Thiram + Molybdenum (Moly-T-Gustafson) | Dry powder | Protects against seed and soil-borne diseases plus benefit for nitrogen fixing bacteria. |
| Thiram-Moly-Inoculant (Triple-Nictin L, Gustafson) | Liquid -- Ready to use | Protects against seed and seedling blight plus Molybdenum for nitrogen fixation. 4 fluid ounces per bushel. |
| Terraclor Super X (20 percent PCNB + Terrazole, Gustafson) | Dry powder | Broad spectrum control of seed and seedling diseases -- especially Rhizoctonia. 2 to 4 ounces per bushel. |
| Vitavax Pour On (Carboxin + Thiram, Gustafson) | Pour on flowable | Broad spectrum control of seed and seedling diseases; good for Rhizoctonia control. 6 fluid ounces per bushel. |
| Thiram + Moly (Yield Shield-Gustafson, Gustafson) | Pour on | Broad spectrum seed and seedling disease control plus Molybdemun for nitrogen fixation and microelements (Co, Fe). 4 fluid ounces per bushel. |
| Thiram + TBZ + Moly (Agrosol Pour-On-Wilbur-Ellis) | Flowable -- pour on | Broad spectrum seed and seedling disease control. 4 fluid ounces per bushel. |
G4441, reviewed October 1993