Treating Seeds To Prevent Diseases

R. W. Leukel.

Sometimes chemicals are applied to seeds, bulbs, corms, tubers, and roots to prevent their decay .after planting and to control seed-borne and soil-borne plant diseases.

To be satisfactory, a seed treatment has to be effective yet reasonably safe from seed injury in case of overdosage; economical, readily available, easily applied, and chemically stable; and not overly poisonous or disagreeable to operators or corrosive to metal.

FUNGICIDES may be classified as seed disinfestants, disinfectants, or protectants, according to the location of the organisms to be combated.

Disinfestants inactivate organisms, such as bunt spores, that are borne on the surface of the seed.

Disinfectants are effective against those located deeper within the seed. Protestants protect the seeds from attack by organisms that are present in the soil.

Practically all effective seed-treatment materials are disinfestants. Many are also disinfectants and protectants. The formaldehyde and hot-water treatments, however, are disinfestants and disinfectants but are not seed protectants. In fact, seeds that have been treated with formaldehyde or hot water frequently are attacked by soil-borne fungi more severely than are untreated seeds and therefore should be treated also with a protestant before planting.

BASED ON COMPOSITION, fungicides may be organic or inorganic, mercurial or nonmercurial, and metallic or nonmetallic. There are organic mercurials (Ceresan) and inorganic mercurials (calomel); there are non-mercurial metallic organics (Fermate) and nonmetallic organics (Spergon); there are metallic inorganics (copper carbonate) and nonmetallic inorganics (sulfur). Fungicidal seed treatments may be dry or wet according to the form in which the fungicide is applied to the seed.

In a dry treatment, the fungicide is applied in dust form, usually in a mechanical mixer at rates ranging from 1/2 to 4 ounces or more to the bushel.

Wet treatments once meant soaking the seed in a water solution of the fungicide for a certain period, after which the seed was allowed to drain and dry. Wet treatments now are applied mostly by the slurry method or the "quick-wet" method.

In the slurry method, the fungicide is applied to the seed as a soup-like water suspension, which is mixed with the seed in a special slurry treater. The seed requires no drying but may be bagged immediately for sowing or storage.

In the "quick-wet" method, a concentrated solution of a volatile fungicide is applied to the seed and thoroughly mixed with it. The dosage may range from 1/2 to 5 fluid ounces to a bushel. As in the slurry treatment, that adds less than 1 percent of moisture to the seed. The well-known formaldehyde spray treatment of oats is essentially a "quick-wet" treatment. So also is the method recommended for applying Panogen, Mercuran, Setrete, and several other materials.

Inorganic mercurials used for treating seed are limited practically to mercuric chloride, mercurous chloride (calomel), and mercuric oxide.

Mercuric chloride, as a 1 to 1,000 solution, may be used for treating potato seed pieces, sweetpotatoes, and rhubarb roots for planting. It is also used for seed of crucifers (plants of the mustard or cabbage family), celery, cucumber, pepper, tomato, watermelon, and certain other vegetables. Most seeds are more or less susceptible to injury by mercuric chloride.

Calomel is used on seeds of crucifers, celery, and onion. Mercuric oxide may be used as a dip treatment for sweet-potatoes (1 pound to 30 gallons of water) .

ORGANIC MERCURIALS are more numerous and more widely used than the inorganics just mentioned. They are used on seed of small grains, legumes, grasses, cotton, beets, flax, sorghum, and some other field crops, and also on certain corms, bulbs, tubers, and roots and the seeds of some vegetables.

Ceresan, 2 percent ethyl mercury chloride, introduced in 1926, was the first organic mercurial widely used in the United States. It is applied at 2 ounces a bushel. It was followed and largely replaced in 1933 by New Improved Ceresan, 5 percent ethyl mercury phosphate, which is applied at 1/2 ounce per bushel. Both were used mostly on small grains, flax, cotton, peas, hemp, and sugar beets.

Ceresan M, 7.7 percent ethyl mercury p-toluene sulfonanilide, appeared in 1948. It largely replaced the two previous Ceresans because of several advantages over them, including its application as a slurry.

Leytosan and Agrox, 7.2 percent and 6.8 percent phenyl mercury urea, respectively, are applied to small grains, peas, rice, and sorghum at 1/2 ounce to the bushel and to flax at 1 1/2 ounces. They may be applied in dust or slurry form.

Mercuran, 3.5 percent mercury as methoxy ethyl mercuric acetate, is used at the rate of 1/2 ounce per bushel on small grains. It may be applied as a dust, in concentrated solution by the "quick-wet" method, or in a more dilute solution with a slurry machine.

Panogen, 2.2 percent methyl mercury dicyan diamide, is a concentrated liquid applied at 3/4 fluid ounce per bushel to small grains, 1 1/2 fluid ounces to flax, and 4 fluid ounces per 100 pounds of segregated beet seed. It is applied in a special Panogen treater, but can be successfully applied in a slurry treater if diluted with water.

Setrete, 7 percent phenyl mercury ammonium acetate, is a concentrated liquid that may be applied as such at 1/2 ounce per bushel, or it may be diluted 1 to 9 with water and applied in a slurry treater.

Mersolite, 5 percent phenyl mercury acetate, is used as a dip treatment (1 pound to 800 gallons) for narcissus corms to combat basal rot.

Merthiolate, sodium ethyl mercury thiosalicylate, is used to prevent corm rot and yellows in gladiolus.

Sanoseed, 7.9 percent ethanol mercury chloride, and Corona P. D. 7, 5 percent mercury in a mercury bromine-phenol compound, are used as dip treatments for seed potatoes.

Semesan, 30 percent hydroxy mercuric chlorophenol (19 percent Hg), is an excellent mercurial used as a wet soak treatment for bulbs, tubers, and corms and as a dust treatment for seeds of flowers and vegetables.

Semesan Bel, a mixture of 2 percent hydroxy inercurichlorophenol and 12 percent hydroxymercurinitrophenol, is used as a dip treatment for seed potatoes.

Puratized N-5 E, 10 percent phenyl mercury triethanol ammonium lactate, is used for treating seed potatoes and lily bulbs.

L-224, an experimental mercuryzinc-chromate material, is an excellent treatment for seed corn.

Aagrano, 3.5 percent ethoxy propyl mercury bromide, is effective against cereal diseases, especially when it is applied in slurry form.

Semenon, 2 percent isopropyl methyl mercury acetate, gave excellent results in controlling diseases of small grains and sorghum. Both Aagrano and Semenon are European products. They were not available in the United States in 1953.

NONMERCURIAL organic fungicides have increased greatly in number since 1945. Generally they are less effective than the mercurials, but as a rule they are less injurious to seeds and less dangerous to persons using them. The organic sulfurs and quinones are prominent ingredients in these compounds and often are combined with phenol, chlorine, bromine, quinoline, zinc, iron, copper, sodium, or other materials.

Spergon, 98 percent chloranil (tetrachloro-p-benzoquinone), was among the first nonmetallic organics to be widely used for treating seed, especially peas and beans. It is used for vegetable seeds, corn, sorghum, peanuts, alfalfa, clover, soybeans, and some other crops. It may be applied as a dust or as a slurry.

Arasan, 50 percent thiram (tetramethylthiuram disulfide), still another early organic fungicide, is used for the same crops as Spergon. Both will also control bunt in wheat, but are not recommended for treating oats or barley.

Arasan SFX, 75 percent thiram, is the wettable form of Arasan for treating seeds by the slurry method. Tersan, also a wettable form of thiram, is used for the control of diseases of turf and lawn grass.

Phygon (formerly Phygon XL) consists of 50 percent 2,3-dichloro-1,4- napthoquinone and 50 percent tale. It is an effective seed treatment for corn, peanuts, rice, sorghum, and most vegetables. It controls bunt in wheat, but is not recommended for other small grains.

Zerlate, 70 percent ziram (zinc di-methyl dithiocarbamate), is effective as a prebedding dip for controlling black rot in sweetpotatoes. It is similar to Zincate, Methasan, Zimate, and Karbam, as all contain ziram as the active ingredient.

Fermate, 70 percent ferbam (ferric dimethyl dithiocarbamate), like Zerlate, is used as a prebedding dip for sweetpotatoes. Both materials are used also as foliage dusts or sprays.

Dow 9 B, 50 percent zinc trichlorophenate, has been used to treat gladiolus bulbs and seed of cotton, corn, and sorghum.

Dithane Z-78, 65 percent zinc ethylene bisdithiocarbamate, has shown promise as a disinfectant and chemotherapeutic fungicide.

Mycon, 7.7 percent methyl arsenisulfide, in extensive field tests, has been found effective in controlling those seed-borne diseases of wheat, oats, and barley that are amenable to control by fungicides.

SeedOx, 50 percent 2,4,5-trichlorophenyl acetate, has been used to treat cottonseed. Mycotox is similar to Seed-ox. Neither is effective as a seed treatment for small grains.

Anticarie, 40 percent hexachlorobenzene, is effective as a seed treatment for the control of bunt in wheat. When applied to the soil it also prevents infection due to bunt spores in the soil. It is not recommended for treating seeds of other cereals.

Pentachloronitrobenzene (50 percent) controlled covered kernel smut in kafir and a 20-percent product controlled bunt in wheat. In Europe this chemical is reported as having controlled infection from soil-borne spores of both common bunt and dwarf bunt when it was applied to the soil at planting time at the rate of about 50 pounds an acre.