Treatment is recommended for all seed corn to prevent or reduce seed decay and seedling blights. The germination and early growth of the seedling is a critical period in the life cycle of corn. Soilborne organisms may invade and kill the embryo before germination, or when invasion occurs later, the seedlings may be destroyed before or after emergence. Seedlings surviving fungal infections at germination are often less vigorous.
Corn seedling diseases are more prevalent in cold, wet soil than when soil temperatures are above 55 degrees F. Therefore, early planted corn needs the added protection of a good seed treatment fungicide. Seedling blights tend to be more severe in fields with poor seed bed preparation such as in no-tillage or reduced-tillage fields. Corn seed is generally treated with a fungicide or fungicide-insecticide combination by the seed producer or seed processor.
Corn smut, leaf blights, stalk and ear rots, and virus diseases are not controlled by seed treatment fungicides.
Seed treatment of small grains is recommended to control smut diseases and to reduce seed decay and seedling blights. The smut diseases include stinking smut (common bunt) of wheat, loose smut of wheat and spelt, and the loose and covered smuts of oats and barley.
All of the wheat varieties planted in Ohio are susceptible to stinking smut (common bunt). Wheat and oat varieties have resistance to some, but not all, races of loose smut (wheat) and loose and covered smuts (oats). The only sure method of smut control is by using an effective seed treatment fungicide. Spelt is extremely susceptible to loose smut and all seed should be treated with an effective fungicide.
Seedling blight phases of head scab (Gibberella zeae) and glume blotch (Stagonospora nodorum) contribute to poor stands of small grains in Ohio. Both seed infecting fungi contribute to poor quality seed by causing lightweight, shriveled grain. Since these seed-borne diseases are common, all small grain seeds should be treated to control seedling blight when infected seed are planted.
Seed treatment for soybean is recommended in three disease situations; 1) when poor quality seed is used for planting (germination below 80 percent), 2) when early season control of Phytophthora damping-off is needed, and 3) when planting early into cool, wet soils, especially in reduced tillage.
The major cause of poor-quality soybean seed in Ohio is Phomopsis seed rot. Infected seed may be visibly moldy, yet others appear healthy. Purchasing certified seed with the percent germination, the variety name, and the seed treatment material used listed on the label is one way to ensure quality seed is planted. Germination percentage of bin-run beans is generally unknown unless they are tested. Seed lots with greater than 80 percent germination generally do not benefit from seed treatment when planted at recommended seeding rates in warm soil. Those with less than 80 percent germination may benefit from seed treatments by increased stand and, possibly, increased yield. Decisions to use poor-quality seed should be based on the fact that any seed treatment fungicide effective against seed-borne diseases will not increase germination more than 20 percent. If bin-run beans of 50 percent germination are treated with a fungicide, the grower should not expect over 70 percent germination in the field. Planting rates can be increased to compensate for seed of low germination.
A number of fungicides are effective in controlling Phomopsis seed rot. These materials have varying degrees of activity against seed and seedling diseases.
Phytophthora damping-off is a serious disease of soybean seedlings in the more heavy, poorly-drained soils in Ohio. The increase in no-tillage and reduced tillage has increased the incidence of Phytophthora damping off in the state. No-till soils remain wetter longer in the spring and less precipitation is required to saturate them compared with plowed soil during the early stages of crop development. Extra moisture in the soil pore spaces favors germination of Phytophthora. It is recommended that seed treatments be used even when planting Phytophthora resistant varieties to ensure good stand establishment.
Rhizoctonia seedling blight has caused considerable stand losses in Ohio over the last decade. Damage from Rhizoctonia appears to be a stress related in that factors that adversely affect germination and establishment of seedlings predisposes the plants to infection. Stress from planting in dry soil, herbicide injury reduced tillage fields and shallow planting appears to increase the potential for damage by Rhizoctonia. Adequate seed bed preparation, soil fertility, and soil moisture for rapid emergence lessens the potential for problems from Rhizoctonia.
Sclerotinia stem rot may be introduced into new fields by sclerotia-contaminated seed lots or infested seed. Seed should be well cleaned to remove sclerotia and soybean seed should be treated with a fungicide to eradicate the fungus on infested seed.
Although a number of fungicides are registered for use as seed treatments of alfalfa, clover, etc., these crops usually do not respond to seed treatments under field conditions. Some increases in stand may result but these increases have not been reflected in increased yield. The one notable exception to this is the use of fungicide seed treatment on alfalfa for control of seedling damping-off caused by Phytophthora. Significant improvement in stand establishment of alfalfa has been noted in research plots using treated seed. In areas where Phytophthora root rot has caused a serious problem in reducing alfalfa stands or preventing establishment of alfalfa, a variety with resistance to Phytophthora that has been treated with a Phytophthora specific seed treatment is highly recommended.
Rhizobium and Bradyrhizobium are bacteria which can fix nitrogen from the air or collect nitrogen from the soil water solution and incorporate it into compounds that plants can use. The nodules that form on soybean roots are the sites where the bacteria reside. Rhizobia are present in Ohio soils, but the commercial strains and some newer strains identified by the USDA are much more efficient at fixing nitrogen. The bacteria may be applied to the seed or placed in the seed furrow at planting.
Insecticide and fungicide seed treatments can have a negative impact on inoculants applied to soybean seed. Several factors affect the impact of seed treatments on seed applied inoculants including: toxicity of material, toxicity of carrier or formulation, length of time inoculant is in contact with seed treatment, and formulation of inoculant. Insecticide seed treatments tend to be the most toxic of all seed treatments to inoculants. The fungicides themselves are not necessarily the problem, but the formulation or carriers may inhibit Rhizobial growth and colonization. Using fungicides in combination with inoculants can be successful if some precautions are taken. If a specific material is highly toxic to inoculants, then placing the inoculant directly in the furrow and not on the seed is recommended. Treating the seed first and allowing the seed treatment material to dry followed by using humus preparation of inoculant may be successful. Liquid inoculants and fungicides should not be mixed and applied simultaneously. Producers should minimize the time the inoculants are in contact with the seed treatment fungicides prior to planting.