UNIVERSITY OF CALIFORNIA COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION CIRCULAR No. 214 SEED TREATMENT FOR THE PREVENTION OF CEREAL SMUTS By W. W. MACKIE* The excessive demand for all cereals has given great stimulus to cereal production. The extensive use of tractors in California has made better cultivation possible, greatly assisting in the maintenance of high yields. Increased yields, however, do not follow better cultiva- tion unless the crops are free from diseases, the most persistent of which are the smuts. Cereal smuts occur every year, causing very considerable damage which may vary according to the weather and the efforts of the farmers to prevent it. During the season of 1918 actual field counts showed the loss from stinking smut of wheat in California to be 5.8 per cent of the total crop, or 461,173 bushels. The estimated loss from wheat smut for the crop of 1917 was 8 per cent. Calculations for smut in wheat losses for the 1919 crop, although not completed, place the average damage at about 2 per cent. This loss does not include plants which have been killed by the attack of smut before maturity. This great reduction in smut losses is apparently due primarily to the campaign waged against smuts since September, 1917, to induce grain growers to practice more universal and better seed treatment methods to overcome smut diseases in cereals. How Smut Grows. — Cereal smuts are caused by fungus parasites which are propagated by means of spores. They are divided into two general classes according to the manner in which they attack the cereal plant, viz., the covered, or kernel smuts which leave the kernel in a more or less solid ball similar in shape to the original seed; and the loose smuts which entirely destroy the head, disintegrating it into a mass of loose black spores which readily fall apart and are blown about by the wind. The attack in the cereal plant of each class of smut differs completely. The kernel smuts (bunt, covered smut of * University of California, and the Office of Cereal Investigations, United States Department of Agriculture cooperating. barley, hidden smut of oats, etc.) are carried on the outside of the seed into the soil at planting time. The only exception is bunt, which may also infect the wheat plant from the soil. In that case the smut spores germinate at the same time as grain seed and by putting forth a thin mycelial thread pierce the young cereal seedling. The parasite then continues to grow within the host until the seed begins to form. At this time the parasite consumes all the starch of the kernel, trans- forming it into a mass of smut spores. This class of spores is destroyed by dipping the seed in fungicidal solutions. The loose smuts of cereals make their attack at the time the grain is in flower. At this time the loose smut spores are being blown about in the air and some find lodgment in the flower of the cereal where they germinate at once and send their threads into the germ of the kernel. When the kernel matures and hardens, the smut becomes dormant and remains in this condition until the seed is planted. When the seed germinates, the smut begins to grow. The smut parasite then con- tinues its growth inside the host plant, maturing just as the grain comes into flower. By its peculiar method of growth the loose smut spore is never exposed on the outside of the seed and consequently cannot be destroyed by dipping the seed in fungicides. It can only be destroyed by heat when dipped in hot water. This method is very difficult to apply on account of the small differences in the degrees of heat which will kill the smut spores and that which will kill the germ of the seed. Fortunately, the damage from loose smuts in wheat, barley, and oats in California is so slight that these smuts are not considered of economic importance. Classification of Cereal Smuts. — Covered Smuts. — Bunt or stinking smut of wheat (Tilletia tritici and T. laevis) ; covered smut of barley (Ustilago hordei) ; hidden smut of oats (Ustilago levis) ; kernel smut of sorghum (Sphacelotheca sorghi) ; ergot of rye (Claviceps purpurea) . Loose Smuts. — Loose smut of wheat (Ustilago tr'itici) ; loose smut of barley (Ustilago nuda) ; loose smut of oats (Ustilago avcivac) ; corn smut (Ustilago zeae). This list of smuts, though formidable in number, may be reduced to three that are causing serious damage to cereal crops in California, namely: stinking smut of wheat, covered smut of barley, and corn smut. Corn smut may be prevented only by eradicating diseased plants at once on their appearance. Seed treatment offers no remedy. Kernel smut of sorghum, Sudan smut, hidden smut of oats and covered smut of barley are controlled by the use of formaldehyde. Stinking smut of wheat, or bunt, is controlled by the same fungicide where no soil infestation from bunt spores occurs. Soil Infestation from Bunt. — Due to the rainless summers of Cali- fornia and other states of the Pacific Coast, the bunt spores are not attacked by the ferments and fungus parasites which may destroy the bunt spores in the humid eastern states. When smutty wheat fields are threshed, the spores are scattered over the soil and may be blown over many miles to be deposited in fields where theretofore the crops may have been free from smut. Summer-fallowed fields are especially liable to smut infection through wind-blown bunt spores, for the soil is not deeply tilled before seeding the crop. Bunt spores require about 14 per cent of moisture in the soil or a moist atmosphere for germina- tion. If the temperature is much above 65° F. the spores do not germinate, although they will survive a much higher temperature for a brief period in midday, provided the temperature is below 65° F. for the remainder of the twenty-four hours. With favorable moisture and temperature conditions bunt spores may germinate, grow and develop in soil solutions for a considerable period, perhaps six weeks or longer, without a wheat host plant. When seed is sown in such infested soil, the wheat seedling is liable to attack until after it emerges from the soil. With the emergence of the growing point of the seedling into the air, it becomes immune from attack because the smut fungus thread appears not to be able to attack the seedling after it appears above ground. It is because of the antagonism of the blue- stone which remains on the wheat seed, that this fungicide gives a certain amount of immunity from attack until the growing seedling passes out of the soil solution charged with the bluestone. If the seedling is close to the soil surface or if the soil moisture and tem- perature are not favorable to the growth of the smut mycelium, the wheat seedling may escape attack. Formaldehyde is less effective against soil infection because the formaldehyde evaporates rapidly from the seed. Soil infestation by bunt spores accounts for many appar- ent failures of seed treatment both by bluestone and formaldehyde, to eradicate stinking smut in wheat. Both of these fungicides are effective against seed borne bunt spores and spores of other covered smuts, especially covered smut of barley which appears to be incapable of causing soil infestation. Soil infestation from bunt spores appears to be confined to the areas away from the coast where the air usually carries only a small amount of moisture in the summer. The interior valleys are subject to this trouble, but soil infestation is not always certain, and there is evidence that some areas may not be affected. Variation in the occurrence of the first and second rains of the season may cause great variation in soil infestation. Early rains may germinate smut spores in the soil which may die out later for lack of moisture or host plants. Soils may be moist enough to sprout the wheat while too dry to germinate bunt spores. In this way wheat seedlings emerge from the soil free from attack. Late sown wheat, when the seed is carefully treated, usually escapes with little or no smut, even in soils where soil infestation has been found in the preceding fall. Very late sown wheat planted when the spring weather is too warm for the germina- tion of bunt spores may escape smut attack even when smutty seed is sown. Late sowing of wheat, however, as a rule, results in poor yields. The best results are secured by sowing early with seed which has been treated to destroy the smut spores. Fungicides Used to Control Smut. — Bluestone solutions have been used to destroy smut spores in wheat seed for nearly a century. Tn California bluestone formulas have given the best results because of the liability of soil infestation from bunt spores and because of the practice of seeding in dry soil for which these formulas are well adapted. Formula for Bluestone-Lime Treatment 1. Dissolve 1 pound of bluestone (copper sulphate) in 4 gallons of water which must be held in a wooden trough or tub. 2. Dip the seed contained in half sacks or butts for 3 to 5 minutes in the bluestone solution. Shake out the air as the sacks soak. 3. Drain the sacks for 15 minutes and then dip in a lime solution prepared by slaking 1 pound of quick-lime in 8 gallons of water. If the lime solution becomes blue, replenish with a new solution. 4. Dry thoroughly before storing. When barley is treated with this solution or when the lime dip is not used after the bluestone treatment, use 1 pound of bluestone to 5 gallons of water. Wash seed in water before dipping in bluestone where the lime dip is omitted. Instructions. — Wash out smut balls. If the seed wheat contains smut balls they should be washed out before dipping the seed in the bluestone. If the wheat is poured slowly into a receptacle containing water and stirred, the smut balls may be skimmed off as they rise to the surface together with chaff, etc. A smut-cleaning machine is sold which performs this task automatically. Washing out smut balls should not be neglected, for no bluestone or formaldehyde solution will penetrate the smut balls or smut masses sufficiently to destroy the smut spores without severe injury to the seed germ. With the washing out of the smut balls the water tends to disperse and wet the remaining spores. After this washing the immersion of the seed in the bluestone solution gives better results in destroying the smut spores which are softened by the water and thus rendered more susceptible to the effects of the fungicide. Also the seed wheat, being saturated with water, excludes excessive quantities of bluestone which might otherwise enter the seeds through broken or injured seed-coats, causing death or delayed germination of the seed germ. Smut balls left in the seed wheat are a source of reinfection be- cause, on account of their oily constitution, they are not readily pene- trated and destroyed by the fungicidal solutions. Frequently the balls are broken by the drill or broadcaster in the operation of seeding. When not broken the smut balls are seeded into the soil where they may send out mycelial threads from live spores germinating within the interior of the ball. As each smut ball contains about four million spores a very few balls are capable of infecting a large number of seeds. A bushel of wheat contains from 600,000 to 1,000,000 kernels. Seed wheat which is very smutty should not be used for seed as it is fre- quently impossible to destroy all the smut spores on a seed which is visibly darkened with the smut spores. Such a seed may contain thousands of spores on its surface. Masses of spores grouped in the brush end and the creases of the seed hold small bubbles of air which do not permit the fungicide to reach the spores. These spores may remain viable even in strong solutions, later to become a source of smut infection. How to Test the Strength of the Bluestone Solution. — The solution of bluestone should be kept at the prescribed strength. If it is stronger than required, the danger of injury to the seed germ is increased, and if much weaker, the solution fails to destroy all the smut spores. The use of the brix sugar hydrometer is recommended in determin- ing the strength of the bluestone solution. This hydrometer can be furnished by any large chemical supply house for a small price. As the density of the solution changes with the temperature, the simplest method of securing the correct reading with the hydrometer consists in floating the hydrometer in a cylindrical vessel containing the standard solution carefully prepared by weighing the bluestone and accurately measuring the water. When the bluestone is dissolved the hydrometer will give a certain reading on the scale at the surface of the solution. The bluestone solution in the wooden trough should read exactly the same. If the hydrometer sinks into the bluestone solution submerging the measured reading, the solution is too weak and should be strengthened by dissolving into it more bluestone by suspending a grain sack containing bluestone in one end of the trough. If the reading rises into the air the solution is too strong and should be diluted with water until the correct reading reaches the surface. Dipping Bluestoned Seed in Lime Solution. — Fifteen minutes is given as the usual length of time for draining the dipped seed. As long as the seed is wet the fungicide is still active. After draining about 15 minutes the bluestoned wheat should now be dipped into the lime solution in order to check the injurious effect of the bluestone on the germ of the seed. This the lime does effectively for all but seeds with badly injured seed-coats. Strong bluestone solutions sufficient to kill the spores (1 pound of bluestone to 4 to 5 gallons of water) have been found, by testing hundreds of samples collected on farms over the state, to kill on the average 60 per cent of the seed. This loss is almost entirely eliminated by the lime dip. In addition the lime causes the seedling to make a more rapid start which is plainly evident by contrast where bluestone alone is used. Where no lime is used the rootlets are delayed long after the plumule has made a good growth. The rootlets should naturally develop first. This the lime assists the roots to do. Lime coated over the seed tends to delay drying. This may lead to heating of the seed if stored before it is thoroughly dried. When dipping operations are carried on in cold, damp weather, care should be taken to keep the butts or sacks of seed separated or spaced in open, well ventilated sheds. Dry the Lime-Treated Seed. — To avoid the possibility of injury due to slow drying the seed wheat should be cleaned and dipped soon after harvest when the weather is dry and warm. At this time the seed can be rapidly and thoroughly dried and then placed in the granary awaiting the seeding time. Tests show no more deterioration for seed wheat treated with the bluestone-lime dip after having been kept for two years, than for untreated seed from the same lot of wheat or barley. Bluestone Injury Caused by Broken Seed Coats. — Seed injury from bluestone may be traced usually to injury of the seed coat. When grain is threshed with, the cylinder running too close much injury to the seed coat results. When seed wheat is recleaned with powerfully driven machinery it is frequently badly injured, especially over the germ where the seed coat is softer and much wrinkled and therefore more easily torn. Through these ruptures the bluestone pene- trates readily and injury to the germ results. In ordinary threshing and harvesting the majority of the wheat kernels are scratched or slightly injured. These injuries need cause no concern when lime is used after the bluestone. A small hand lens will readily show the ruptures which may occur over the germ. Severe recleaning may be detected when the hairs which occur on the brush end of the kernel have been scoured away. Wheat seed with perfect seed coats has shown no injury when soaked for six hours in a concentrated solution of bluestone. The bluestone-lime dip has been in use in Australia for many years, where it has established its superiority over the formaldehyde solution or bluestone when used without the lime dip. Sprinkling Bluestone Solutions on, Seed. — Bluestone in solutions of standard strength may be sprinkled on loose seed wheat if the wheat is constantly stirred to thoroughly wet all the seed surface. This process appears to give less injury to germination than the dip- ping, but greater care is required in the mixing of the seed and solu- tion in order to thoroughly wet all the seed. An auger elevator, such as used in flour mills, has, in one instance, been operated successfully to mix seed wheat with a strong solution of bluestone which was steadily dripping on the moving wheat. Treatment with Formaldehyde Solution Formula: Formaldehyde (U. S. P. 37 per cent), 1 pint to 40 gallons of water; soak the seed in this solution for 10 minutes; after removing the seed from the solution cover it with canvas or sacks for 2 hours, ventilate, thoroughly dry and sow within 48 hours. The seed may be dipped in half filled sacks or butts tied at the end or passed loose through a formaldehyde dipping machine. This practice may be varied with equally good results by wetting with a sprinkling can the pile of seed, using the same strength of solution. The seed should be constantly shovelled over while being sprinkled until the whole mass is thoroughly wetted. The pile should then be covered for at least two hours. Dipping the butts or half-filled sacks in tubs or troughs as is done with the bluestone solution may be successfully practiced. A number of hand and power driven machines for cleaning out smut balls and trash, and for dipping the seed in formaldehyde solu- tion are on the market. These machines may be used to wash the formaldehyde-treated seed when it is to be stored for a period of time, or planted in dry soil. When formaldehyde-treated seed is stored for a period longer than 48 hours injury to seed germination may result. It appears that the formaldehyde solution on the seed rapidly evaporates, leaving behind a residue of para-formaldehyde which is very destructive to seed germs. If formaldehyde-treated seed is stored and allowed to dry, or when it is sown in soil too dr}^ to sprout, seed injury frequently may occur. This injury often results in the death of the seed germ. Many of the remaining seedlings may appear contorted and bent, producing poor plants. These injuries do not occur when the for- maldehyde-treated seed is planted in soil sufficiently moist to sprout the seed promptly. Bluestone and Formaldehyde Solutions Compared. — Bluestone and formaldehyde are both effective in destroying smut spores. "Wheat which is visibly blackened with smut spores or which con- tains smut balls in large amounts, should not be used for seed purposes because no method of seed treatment has been found wholly effective with such badly infested lots. Solutions of bluestone and formaldehyde do not deteriorate on standing but become more concentrated through evaporation. 8 Both fungicides will destroy or injure the germs of seed if the solutions are concentrated beyond the strength given in the formulas. When the seed coat is perfect, bluestone cannot injure the germ if the recommended formula is used. Formaldehyde, on the contrary, appears to cause injury even when the seed coats are perfect if the solution is too strong. With slightly injured seed coats, formaldehyde causes less injury than bluestone. Bluestone injury is almost entirely eliminated when the seed is dipped in a lime solution after the bluestone dip. Delayed germination is caused by both solutions. This is accentu- ated when the seeding occurs in cold wet weather. Formaldehyde injury to cereal seedlings is shown in the delayed growth of the plumule which may appear curved and distorted. Instead of leaving the germ at nearest contact with the endosperm, the plumule may pass under the seed coat escaping at the brush end of the kernel. The bluestone solution, on the contrary, causes delayed germination of the primary rootlets, while the plumule behaves almost normally. This delayed root germination is corrected by the lime dip. Both wheat and barley seed treated with the bluestone-lime solu- tions and thoroughly dried, may be stored and kept for at least two years without deterioration greater than that for untreated seed of the same lot. Formaldehyde-treated seed cannot be stored for a period longer than 48 hours without liability to severe loss in seed germination. Seed treated by the bluestone-lime method may be planted in dry soil where it remains in as good condition as untreated seed, awaiting the rain. Formaldehyde-treated seed, on the contrary, may be badly injured by the deposits of para-formaldehyde which appear immedi- ately on the evaporation of the formaldehyde solution. With badly smutted lots of seed the effectiveness of both formalde- hyde and bluestone as fungicides is greatly enhanced by a previous washing of the smutty seed to remove smut balls and to wash away and disperse masses of spores. Washing the seed in water previous to dipping in either fungicidal solution, prevents a great amount of seed germ injury, especially when bluestone is used alone. Washing formaldehyde-treated seed immediately after removing the covering, two hours after dipping, prevents seed injury when the seed is to be stored or planted in dry soil. Where stinking smut of wheat comes from soil infestation the bluestone-lime or bluestone formulas are recommended, as formalde- hyde evaporates rapidly and has no lasting residual effect against smut infection. The bluestone-lime method is more expensive than formaldehyde, in cost of materials and labor, but is indispensable in regions where soil infestation exists or dry seeding occurs. This is especially true in the interior valleys where summer fallowing is the common practice. For covered smut of barley formaldehyde is preferred, but the bluestone-lime solution may be successfully used. For hidden smut of oats, kernel smut of sorghum, and Sudan smut, formaldehyde alone is recommended.