Seed Treatments for Control of Disease

EARLE W. HANSON, EARL D. HANSING, AND W. T. SCHROEDER.

SEED treatments are used to prevent or reduce losses from diseases caused by organisms associated with seed or present in the soil.

Such organisms are associated with seeds in several ways. They may be mixed with seed in the form of sclerotic, smut balls, nematode galls, and infested plant parts. Pathogens may be present in or on seeds.

Treating infested seeds with chemicals or with heat greatly reduces the incidence of many seedborne pathogens. Seed treatment is used also to protect healthy seed against soilborne organisms, notably Pythium, Fusarium, and Rhizoctonia, which cause seed rots, preemergence damping-off, and seedling blights of many crops.

Some treatments kill organisms mixed with the seed or on its surface. Some destroy pathogens within the seeds. Others kill or retard the activity of soil organisms near the planted seeds.

Mechanical, physical, and chemical methods are used.

The mechanical method is designed to remove infectious materials mixed with seeds. Seeds should be thoroughly cleaned before seeding. Mechanical treatment does not kill pathogens within a seed. It does not remove all organisms from the surfaces of seeds or protect them against soilborne organisms. Mechanically treated seed therefore often requires further treatment.

Physical methods are used primarily to kill pathogens deep in the seeds. Some pathogens, such as those that cause loose smuts of wheat and barley, can be inactivated in no other way.

Physical methods include hot-water and water-soak treatments and ultraviolet, infrared, X-ray, and other kinds of irradiation. Dry heat has been tested. Only the hot-water and the water-soak treatments have been shown to be practical. Physical methods do not protect seeds against soil-borne organisms; they are effective against pathogens on or in seeds.

The hot-water treatment was the most commonly recommended physical method before 195o, but it never has been used extensively because of difficulties in exactly controlling temperature and duration of treatment; there is little margin of safety. Adequate supplies of steam or hot water, accurate thermometers, water tanks or vats, and drying facilities also are required. The method has been used mostly for disinfecting small lots of seed and batches of small-seeded crops that require low seeding rates per acre. Procedures differ with the crop and to some extent with the pathogen.

Various modifications of a water-soak method have been developed since 1950. They are safer and less critical in their requirements than the hot-water treatments. They have been used mostly for controlling loose smuts of wheat and barley but are effective against some other pathogens.

In all water-soak treatments, the seeds must be soaked in water at least 2 hours and subsequently kept under anaerobic or near anaerobic conditions for one or more days. In some instances the seeds are soaked for 64 hours in water at approximately 72 F. and then dried. Sometimes the seeds are soaked for only 2 hours, and then placed in airtight containers at 80 for 48 hours before being dried.

Other effective modifications differ in the temperature used and duration of the treatment. The higher the temperature, the shorter is the time required. Varieties of crops differ considerably in their sensitivity to injury from soaking for long periods. The possibility of injury is reduced by adding 1 percent of common salt or 0.2 percent of Vancide 51 to the water in which the seeds are soaked.

Chemical treatments are the most commonly used method of treating seeds. Many excellent chemicals are available. They may be organic or inorganic, mercurial or nonmercurial, and metallic or nonmetallic.

Organic fungicides are used more than inorganic ones, but the latter are preferred for some purposes. Fungicides may be applied as dusts, liquids, or suspensions. Equipment is available for using them. Recommended dosages vary with the fungicide, the crop, the length of the storage period after treatment, and sometimes with the method of application. The use of excessive amounts of fungicides may injure the seed, waste the chemical, and make the handling and sowing of treated seed disagreeable and even dangerous. The use of less than the recommended amount of fungicide hampers disease control and may cause loss in yield and quality of crop.

Volatile fungicides usually are used at lower dosages than nonvolatile materials and are most effective when the treated seeds are stored for at least a few days before planting.

All dry fungicides present hazards. Special precautions are necessary. All dusts are harmful to people if they are inhaled over long periods or in excessive amounts. Seed fungicides are even more dangerous because all of them are poisonous. The extent of the hazard depends on the amount of dust inhaled, the toxicity of the chemical, the length of the period of exposure, and the person's sensitivity.

When treating large quantities of seeds, particularly over a period of many days, artificial ventilation should be provided to collect and exhaust the dust from the treating room. Workmen should wear clean filter masks over the nose and mouth. A special mask must be used against volatile chemicals. If the fungicides are applied wet, this special equipment may not be necessary, but one must not inhale the chemicals or their fumes. The chemicals should not come in contact with the skin. If a worker does get them on him, he should wash himself promptly with soap and water.

Originally wet treatments involved soaking the seeds in a water solution of a fungicide for a prescribed time, after which the seeds were removed and dried before they could be used or stored. Farmers never liked the method, because it took time, much work, and extra space in the granary for drying. Seeds that were not dried properly were hurt.

Today wet treatments are applied mostly by the slurry method or by quick-wet procedures, in which no drying is necessary because the treatments add less than 1 percent of moisture to the seeds.

In the slurry method, the seeds are completely coated with a thick suspension of the chemical in water. The suspension is applied by a special machine, a slurry treater. Because this method eliminates flying dust during treating, it is safer and less disagreeable for the workmen. More accurate and uniform dosages of chemical can be applied by it to most kinds of seeds.

In the quick-wet method, a concentrated solution of a volatile fungicide is added to the seeds and thoroughly mixed with them. Panogen 75 and Ceresan 75 are examples of fungicides applied in this way. The use of volatile liquid fungicides has increased greatly since 1950, especially for treating small grains, sorghum, cotton, flax, and rice.

Pelleting is another method of applying chemicals. It is used mostly as a protectant against soil organisms and as a repellant against birds and rodents. It has been particularly valuable for treating seeds of pine and other conifers but is used to some extent for treating seeds of other crops, notably onion for the control of smut.

Still other methods are used. For instance, the best method of destroying the pathogen that causes bacterial canker of tomato is to ferment the seeds and pulp at about 70 for 72 hours before extraction. Storage of seed for one or more seasons destroys some pathogens for example, the fungus that causes late blight of celery and the virus causing tobacco mosaic in tomato.

It is outside our scope to discuss seed treatment for the control of insects, but we should point out that insecticides are applied to the seeds of many crops and that the efficiency of the fungicide may be affected by the insecticide used.

Insecticides also increase the need for fungicide treatment, since they tend to predispose seeds and young seedlings to attack by soil fungi. Compatible fungicides and insecticides such as captan-dieldrin and thiramdieldrin are available.

Some manufacturers package insecticide-fungicide mixtures, and the combination is applied as a single treatment. Others market the insecticide and fungicide separately, and the materials are applied separately as a dual treatment.

All seeds containing chemical dusts should be placed in closely woven bags (10 ounce or heavier) to reduce sifting of the chemical during handling and shipment. This is particularly desirable when chemicals are applied at high rates to smooth-surface seeds. If the seed is to be offered for sale, it must be labeled to indicate that it has been treated and is unfit for food or feed.

The ideal fungicide would be highly effective in disease control; harmless to the seed, even at higher than recommended dosages; economical to use; easy to apply; relatively nontoxic to people; noncorrosive to machinery; adapted to use in planting equipment, so as not to interfere materially with uniform seed flow; stable for relatively long periods; and relatively harmless to animals that may consume the treated seeds. No chemical in use in 1961 met all these requirements.

The choice of treatment depends on the crop, the nature of the disease problem, the condition of the seeds, the relative cost and availability of acceptable fungicides, the availability of treating equipment, and the weather conditions expected after seeding.

Crops differ in their responses to seed treatment. Some benefit more than others. Some are more sensitive to injury than others. That is true also of different lots of seed of a species. It is essential to know which diseases are to be controlled and whether the pathogen is located in or on the seeds or in the soil.

Only seeds of high quality should be planted. They should be thoroughly cleaned, cured, and dried before treating. Cracked, damaged, and old seeds sometimes benefit more from treatment than good seeds, but treatment should not be expected to substitute for good seeds. Even good seeds may benefit from a protectant.

When soil and weather conditions after seeding are unfavorable for rapid germination and development of seedlings, treatment of seeds will often mean the difference between a good and a poor stand. When growing conditions are favorable for the host, there may be no apparent benefit from treatment. Since the weather cannot be foreseen several weeks in advance and treating costs are low, however, many crops are treated every year as insurance against losses. Corn and sorghum are examples of such crops.

The treatments have been standardized to a remarkable degree, and recommendations for controlling diseases in specific crops can be made that are applicable over wide areas. Nevertheless, a grower will do well to consult his agricultural experiment station for the best fungicides for his situation.

CORN is extensively treated. Almost all of the seed of hybrid corn is treated chemically to prevent seed rots and seedling diseases.