360 YEARBOOK OF AGRICULTURE 1953 consist of chlor otic spots and streaks. The virus is transmitted by at least two species of leafhoppcrs, Nephoiettix apicalis (hipunctatus) var. cincticeps and Deltocephalus dor salis. Experiments with N. apicalis have shown that the virus The Smuts of passes through part of the eggs to the next generation, for as many as seven generations. Wheat, Oats, H. H. McKiNNEY holds degrees from Michigan State College and the University Barley of Wisconsin. In igig he joined the staff of the division of cereal crops and diseases of the Bureau of Plant Industry^ Soils, and C. S. Holton, V. F. Tapke Agricultural Engineering, where he has devoted most of his lim.e in research on Many millions of dollars' worth of viruses and virus diseases. grain are destroyed every year by the smuts of wheat, oats, and barley. For further reading: For purposes of study and control, //. H. McKinney: Evidence of Virus Muta- we can consider the smuts as being tion in the Common Mosaic of Tobcicco, seedling-infecting or floral-infecting. Journal of Agricultural Research, volume 5/, The seedling-infecting species come pages g^i-gSi, 1^33; Mosaic Diseases of Wheat and Related Cereals, U. S. D. A. Circular ^42, in contact with the host plants as ig37; Mosaic of Bromus inermis, Knih H. follows: The microscopic spores from Fellows and C. 0. Johnston, Phytopaihology, smutted plants are carried by wind, volume j2, page 331, ig42; Genera of the Plant rain, insects, and other agencies to Viruses, Journal oj the Washington Academy oj Sciences, volume 34, pages I3g-i54, 1944; De- the heads of healthy plants (as in scriptions and Revision of Several Species of loose smut of oats). Or, smutted Viruses in the Genera Marmor, Fractilinea, heads are crushed in threshing and and Galla, Journal of the Washington Academy spores are distributed to the clean of Sciences, volume 34, pages 322-329, 1944; Soil Factors in Relation to Incidence and Symp- seed or blown to fields, where later tom Expression of Virus Diseases, Soil Science, they come in contact with the host volume 6r, pages g3-ioo, 1946; Mosaics of at seeding time (as in the stinking Winter Oats Induced by Soil-borne Viruses, smuts of wheat). As the seed germi- Phytopathology, volume 36, pages 359-3^9, 1946; Stability of Labile Viruses in Desiccated nates and the seedling grows through Tissue, Phytopathology, volume 37, pages 139- the soil to the surface of the ground, 142, 1947; Wheats Immune From Soil-borne the smut inoculum develops thin Mosaic Viruses in the Field, Susceptible threads, which penetrate the seedling When Inoculated Manually, Phytopathology, volume 38, pages 1003-1013, 1948; Tests of and initiate infection. The fungus Varieties of Wheat, Barley, Oats, and Corn then grows internally in the plant for Reaction to Wheat Streak-Mosaic Viruses, and eventually forms spores in the Plant Disease Reporter, volume 33, pages 359—369, young heads. That completes the 1949; Mosaics of Winter Oats "and Their Control in the Southeastern States, with T. R. cycle and sets the stage for a new crap Staîiton, J, L. Seal, T. H. Rogers, W. R. Padcn, of smut. G. K. Middleton, and U. R. Gore, U. S. D. A. Circular 809, 1949', ^ Seed-borne Virus Caus- STINKING SMUT, or bunt, apparently ing False-Stripe Symptoms in Barley, Plant Disease Reporter, volume 35, page 48, ig5i; A has been a plague of wheat since wheat Method for Inoculating Varietal Test Nur- was first cultivated. It was prominent series With the Wheat Streak-Mosaic Virus, among the diseases studied by the with H. Fellows, Plant Disease Reporter, volume earliest plant scientists. It has been 35> P^g^^ 264-266, 1951; Wild and Forage Grasses Found To Be Susceptible to the Wheat widely investigated, and certain con- Streak-Mosaic Virus, with H. Fellows, Plant trol measures have long been known. Disease Reporter, volwne 35, pages 441-442, igji. Nevertheless bunt is still an economic THE SMUTS OF WHEAT, OATS, BARLEY 361 threat to the production of wheat, of the bunt fungus, the host variety, especially winter wheat, in all im- and the conditions of growth. Plants portant wheat regions of the world. with dwarf bunt are one-half to one- The average annual loss in the fourth the height of healthy plants. United States is estimated to be 1.3 Because of intergrading types, identifi- percent of the crop, or about 25 cation solely on the basis of stunting is million dollars. Its severity fluctuates sometimes difiiicult. from year to year and from region to The smutted wheat heads are bluish region. The heaviest losses occur in green when they emerge from the the winter wheat regions of the Mid- boot. The healthy heads are yellowish west and Pacific Northwest. Estimated green. Smutted wheat heads tend to annual losses in Kansas over 30 years be long and lax and to ripen sooner range from less than 200,000 dollars than healthy ones. The smut balls to almost 20 million dollars; the protrude beyond the glumes as they average is almost 3 million dollars. enlarge. Dwarf bunt infected heads In recent years the Pacific Northwest usually are more compact than those has sufiTered the most. The loss there infected with common bunt, and the was about 10 million dollars in 1950. glumes are spread apart so that the The main loss comes from the smutted heads have a feathery look. reduction in yield. The percentage The smut balls vary considerably. of reduction in yield roughly equals The size depends on host variety, the percentage of smutted heads in location in the spikelet, physiologic the field. Losses in quality result when race, and climate. Common bunt balls market grain is discounted in price are elongate or round. Dwarf bunt because of the dark color and the balls are always round. Common bunt offensive odor of the smut spores that balls are more fragile and absorb water adhere to the kernels. Smut that is less readily than those of dwarf bunt. removed from the grain by cleaning Dwarf bunt stimulates excessive and washing is assessed as dockage tillering of infected plants. The effect against smutty grain. Stinking smut is more pronounced in the Turkey increases the cost of processing. It also variety than in others. Forty tillers on is a fire hazard to threshing equipment dwarf bunt infected plants are not and storage bins. uncommon. Bunt-infected flowers have longer Two KINDS OF BUNT occur on wheat., pistils and longer and broader ovaries The common bunt is caused by than do healthy flowers. Diseased Tilletia caries and T. Joetida, Dwarf ovaries are green; healthy ones are bunt is caused by T. caries. white. Stamens in diseased flowers are Common bunt is more prevalent reduced in length and breadth, and and better known than dwarf bunt. the anthers have a pale-yellow color Dwarf bunt stunts infected plants instead of a pronounced green, as in severely. It has been recognized as a healthy ones. The stamens in diseased distinct type only since 1925. Its heads fail to extrude at flowering time. prevalence and severity have increased Bunted plants are more susceptible steadily. Its principal region of distri- to seedling blights and to yellow stripe bution is the Pacific Northwest, but rust but more resistant to powdery it also occurs in Wyoming, Colorado, mildew than are healthy plants. and New York. Winter injury is more pronounced in Wheat plants infected with bunt are bunted plants. Increased sensitivity of recognized first by their reduced infected plants to gravity has been height. Common bunt shortens the reported. plants a few inches or as much as The spores of Tilleiia caries are uni- half the height of healthy plants, formly globose and have '^netted" depending on the physiologic race spore walls. Spores of T, Joetida are 362 YEARBOOK OF AGRICULTURE 1953 globose, elongate, or oval and have balls and rate and mode of spore smooth walls. The bunt balls of T, germination. In the dwarf bunt race foetida are larger and more elongate infection comes primarily from soil- than those of T, caries on the same borne inoculum. variety of wheat. In both, the ball Pathogenic specialization in the bunt shape tends to conform to the shape fungi was discovered in 1925 or so. of the wheat kernel. Different races have been identified The spore balls are broken in from various parts of the world. In the threshing. The grain becomes con- United States about 25 races are taminated with spores. If it is used for recognized by their reaction to a set of seed, the spores germinate and pene- eight differential wheat varieties. trate the young seedling in the course New or previously unrecognized of its subterranean emergence from the races frequently, appear, especially in seed to the surface of the soil and infec- places where varieties resistant to the tion is established in the growing point. recognized races have been introduced. The growth of the parasite keeps pace The new races attack and spread with with plant development; at maturity, the new varieties. Every smut-resistant bunt balls are formed in place of wheat variety introduced in the Pacific kernels. Northwest eventually has become the Wheat seedlings also may become distributor of one or more new races of infected by soil-borne spores. In less bunt. humid regions like the Pacific North- The two species as well as different west, airborne spores from the combine races of the bunt fungus have been harvester settle on summer fallow land intercrossed artificially and new patho- and remain dormant until moisture genic lines were selected from the and temperature are favorable for hybrid populations. The frequent germination in the fall. Usually that occurrence of intermediate spore forms occurs at the time winter wheat is in field collections indicates that seeded, so that smut spores and wheat natural hybridization also occurs. seeds germinate at the same time, thus Invariably the intermediate forms are exposing the seedlings to infection. found on highly susceptible varieties. Spores of common bunt perish in the These varieties, however, tend to soil in less than a year. Those of dwarf perpetuate the old, established races, bunt may remain viable for 7 years. despite their susceptibility to the new Even so, dwarf bunt does not attack races. spring wheat. The greatest infection occurs at soil EFFECTIVE CONTROL OF BUNT depends temperatures of 40° to 60° F., with on the use of clean seed of a smut- moisture content ranging from 15 to resistant variety properly treated with 60 percent of field carrying capacity. an appropriate fungicide. Also effec- Other factors affecting the develop- tive, when it is practical, is the seeding ment of bunt are soil fertility, depth of wheat when soil temperatures are of seeding, and length of day. The unfavorable to bunt development. spore load is important. About 0.5 Those control measures seem so gram of spores to 100 grams of seed simple that they give no hint of the are necessary for maximum infection. long and costly struggle of plant The greater the concentration of spore scientists to develop them. And the load around the embryo, the higher struggle continues today in an effort to the degree of infection. keep pace with the ever-changing Both species of the bunt fungus are problem, brought on by shifts in wheat highly specialized into distinct physio- varieties and by the adaptive nature of logic races. The races differ in their the smut fungi. ability to attack different varieties, The accidental discovery of seed also in the size and shape of their smut treatment as a means of controlling THE SMUTS OF WHEAT, OATS, BARLEY 363 wheat smut was made long before the dust treatments for cereal smut control parasitic nature of the disease was were not developed until several years known. later. They had some advantages over Some wheat seed salvaged from a copper carbonate, which they gradu- grounded sailing ship off the southern ally replaced after 1930. coast of England was sown by nearby Paradoxically, the elimination of the farmers. The crop they got had less wet feature of the formaldehyde and smut than that from local seed. That copper sulfate introduced the dust may have been the origin of the î oo- hazard of the dry treatments. Many ycar-old practice of brining the wheat workers were made ill when they seed. inhaled the dusts. A scientific approach to the problem Eflforts to eliminate this undesirable of smut control by seed treatment feature brought the development of the began about 1800 with the discovery slurry treatment, by which the fungi- of spore germination and the harmful cide is applied in a concentrated water effect of copper sulfate on their germi- suspension or slurry in such small nation. Fifty years later the parasitic amounts that the seed is not wetted nature of wheat smut was established, appreciably. It avoids the main dis- and the quest for more effective control advantages of both wet and dry treat- by seed treatment was begun. ments. It became the predominant Scientists recognized at the outset method of treating seed wheat in the that a good chemical for treating seed Pacific Northwest within 5 years. should be highly toxic to the smut The value of resistant varieties in the spores but harmless to the seed. Copper fight against wheat smut is illustrated sulfate did not always give good control by experience in the Pacific North- and often injured the seed. Seed had to west, where soil contamination occurs be soaked in the solution and dried and where seed treatment is practiced afterwards. It left much to be desired universally. Between 1931 and 1942, as a suitable fungicide for seed treat- when resistant varieties predominated, ment. It took almost 50 years of search the incidence of wheat smut declined to find a better treatment, but progress from more than 30 percent to less than was faster thereafter. 3 percent. In the succeeding years up The discovery of the value of to 1951, when susceptible varieties formaldehyde about 1895 was hailed predominated, the incidence of smut as the answer to smut control by seed again increased to 30 percent. treatment. After its adoption in the That difí^erent varieties of wheat re- United States, formaldehyde rapidly acted differently to smut was observed replaced copper sulfate because of its almost 200 years ago. Only 50 years several advantages. Even so, it still had ago did plant breeders begin to breed the disadvantage of being a wet treat- smut-resistant varieties systematically. ment and was likely to injure the seed At first they thought that complete unless used properly. The impelling control of bunt could be realized need for a dry treatment led to the through the use of resistant varieties. discovery of the value of copper They emphasized the development of carbonate dust in Australia. Copper varieties with smut resistance. The carbonate was first tested in the United first of the new varieties carried the States about 1918. It gradually dis- recommendation that they could be placed formaldehyde for the control of grown without seed treatment. In- wheat smut but was inefí'ective against variably, however, they became sus- oat and barley smuts, for which ceptible to new races of smut. Breed- formaldehyde continued to be used. ing for smut resistance consequently is We had organic mercury materials recognized as a perpetual contest for treating seed before we had copper between the directed processes of man carbonate, but the organic mercury to produce resistant varieties of wheat 364 YEARBOOK OF AGRICULTURE 1953 and the processes of nature to develop smut was not found outside the original new races of bunt. area of infestation until 1941, when it The breeding of smut-resistant vari- was reported in central Washington. eties has been faciUtated by the Up to 1953 it had not become widely systematic identification of different prevalent or intensively severe in any genetic factors for resistance. At least locality, but its persistence in the two major and numerous minor major wheat areas posed a constant factors are recognized. Probably others threat. exist. Thus far, however, the two Infected plants usually are dwarfed major factors (Martin MM and Tur- and have twisted leaf blades and key TT) control the entire gamut of sheaths marked with grayish-l^lack physiologic races now recognized, but stripes. The stems also may be invaded. this simple situation may not always Infected plants rarely produce heads, remain. Consequently it is advisable but smutted heads occasionally appear. to develop additional varieties with At maturity the diseased tissues dry up maximum resistance to smut while and become shredded. The stripes continuing to search for new sources of rupture and expose the black spore resistances. masses. Symptoms may appear on in- The best tem.perature for bunt in- fected plants at any time from the fection is 37° to 57° F. Seeding early fourth leaf stage, or i month after (before the soil temperature drops that germination, up to the heading stage low) or late (after it has fallen below One or all culms of a plant may be in- that point) tends to reduce the amount fected. The oldest culms of partly of smut. Both early and very late seed- smutted plants are smut-free. Partial ing, however, hurt the wheat crop. A smutting is more common in the green- reasonable delay in seeding should aid house than in the field and in some in control where soil contamination varieties more than in others. Infected prevails, as in the Pacific Northwest. plants may die and dry prematurely, Seeding practices have never been thus obscuring much of the loss from widely adopted as a means of smut the disease. control-— they have olwious limitations, Flag smut of wheat is caused by the particularly that sowing must be done fungus Urocystis iriiici. Its spores occur w^hen the soil is moist enough to ger- singly or in balls, which usually are minate the wheat. covered by a layer of sterile cells and contain three spores. Individual spores FLAG SMUT occurs on wheat in most are smooth, oval to spherical, dark of the world's major wheat countries brown, and 12 to 16 microns ])y 9 to 12 but has caused heavy losses only in microns in size. Germinated spores Australia. Its presence in the United have a short promycelium on which 2 States was established first in 1919 in to 6 sporidia are borne. The sporidia Illinois. It was found in Missouri and remain attached to the promycelium, Kansas shortly thereafter. The source fuse, and give rise to infecting of infection was never discovered, but mycelium. imports of wheat for feed from Aus- Infection occurs in the seedling stage tralia were suspected. from spores carried on the seed or in Considerable apprehension followed the soil. Systemic development inside the discovery of flag smut in the United the plant follows. The cycle is com- States. Quarantines were established to plete with the production of spore prevent further importation of wheat masses in the diseased tissues. from Australia and to prevent the The number of races of f/. tritici is spread of the smut outside the known relatively small. Five races have been infested area. Those measures—or identified in China. Only two races are natural factors—may have operated known in the United States. One oc- against the spread of the disease; flag curs in the Midwest. The other, more THE SMUTS OF WHEAT, OATS, BARLEY 365 widely virulent, is in the Pacific glumes. The branches are shortened, so Northwest. that the panicle has a compact appear- Germination of spores and infection cuice. Covered smut panicles ripen pre- take place in a wide range of tempera- maturely, l)ut the spore balls remain ture and humidity. Spores germinate intact until !)roken up in harvesting. best at temperatures of 40° to 80° F. A buff-colored type of covered smut Soil temperatures of 57° to 70^ are was collected from experimental ma- favora})le for infection. The highest terial, but it has not been observed in infection occurs at 66° to 70°. Optimum commercial fields of oats. moisture content apparently shifts with Scientists in experiments have dem- prevailing temperatures. With soil mois- onstrated, hybridization between spe- ture at 40 percent, best infection occurs cies and races of the smuts and the at 50° to 59°. At 60 percent soil-mois- segregation and recombination of ture infection is best at 50°. At tempera- pathogenic and other characters to tures above 59°, best infection occurs produce intergrading types. The oc- when soil moisture is below 40 percent. currence of intergrading types in the Spore viability is best retained at rela- field is evidence that hybridization tive humidities of 50 to 75 percent. occurs in nature. New types of smut Seed treatment with a suitable fun- are produced in that way. gicide destroys spores carried on the Both oat smuts infect the seedlings, seed. The use of resistant varieties pre- develop systemically in the host, and vents infection by soil-borne spores. finally destroy the oat kernel. Because the spores of flag smut may Spores of loose smut are disseminated remain viable in the soil more than by wind immediately after the diseased one season, crop rotation helps control panicle emerges from the leaf sheath. the smut. Infection thus is spread to flowers of healthy panicles in the same field or SMUTS ON OATS seem to be among the other fields. The spores germinate in first cereal diseases to come under the the flowers and produce mycelium, scrutiny of early writers on plant dis- which establishes itself between the eases. Their prevalence and economic developing kernel ¿md the glumes or importance are world-wide. They chaff. The mycelium becomes dormant destroy 40 to 50 million bushels of oats as the kernel matures. The smut in the United States each year. Total mycelium becomes active again when losses aggregating more than 27 million the oats germinate and the seedling is bushels were reported in 1945 from infected. Iowa, Minnesota, Illinois, and Wis- The covered smut, on the other consin, the leaders in the production hand, emerges from the host with the of oats. The heaviest infections occur in spores enclosed in the glumes. The the South Atlantic region, where losses spore balls remain intact until broken up to 35 percent have been reported in harvesting and threshing. The in som_e States. The least damage spores are then distributed over the occurs in the West. surface of the threshed seed. They The two kinds of smuts on oats de- remain dormant until the seed germi- stroy the panicles by producing masses nates, when they also germinate and of spores in place of oat kernels. infect the seedling. Loose smut, caused by Ustilago The dissemination of the loose smut avenae^ destroys the enveloping glumes spores by wind makes it ten times more as well. The spores, borne in a loose prevalent than covered smut, which mass, are quickly disseminated by spreads only by contact with infested wind currents, leaving behind a seed. denuded panicle skeleton. Covered smut is rare in the South Covered smut, caused by U. kolleri^ Atlantic region. Loose smut occurs produces spores in balls enclosed by the least often in the western region. 201500°—58 25 366 YEARBOOK OF AGRICULTURE 1953 The degree of infection by the smuts The membranes begin to split a few is determined chiefly by soil tempera- days after the smutted heads emerge. ture and moisture when the scedHngs The spores scatter and inoculate emerge. Infection may occur at tem- developing seed in healthy heads. The peratures between 41° and 86° F., but spores may spread until the grain is usually it is highest between 59*^ and threshed. Some that reach the seed lie 68°. Infection may occur when soils dormant on the surface of the grain. have a moisture content between 5 Others are carried under the hulls or percent and 60 percent, but chiefly send infection hyphae beneath the hulls at about 35 to 40 percent. Soil reaction before or after threshing. The spores near the neutral point or slightly acid and mycelium beneath the hulls pro- seems to favor infection. Response to duce most of the infection. environmental factors is highly vari- In experimental cultures a good able, possibly because the smuts have infection is obtained by inoculating the such a high degree of specialization. seed with a suspension of spores. The About 25 races of the oat smut fungus seed is washed and dried to loosen the species are recognized in the United hulls around the kernel, the suspension States by the reaction of 10 host testers is mixed with the grain, and the moist of the species Avena sativa and A. inoculated seed is stored for 16 to 20 byzantina. All the major commercial hours to promote spore germination varieties grown in 1953 were and the spread of mycelium under the susceptible to one or more races of hull. smut. The Victoria oat was highly Higher infection has been obtained smut-resistant for at least 15 years and in plants kept under mild temperatures was used as a source of smut resistance for 2 to 4 weeks after the seedlings in breeding new varieties. Races of emerged than when the plants were smut that attack varieties derived from grown continuously under cool condi- Victoria are widely prevalent through- tions like those of fall and winter. out the southern half of the United During the initial infection, between States. That experience with the seed germination and the emergence formerly smut-resistant Victoria oat of the seedling, the highest percentages emphasizes the perpetual nature of the of smutted plants are obtained at soil problem. temperatures of 50° to 70° F. Smuts on oats can be controlled by The amounts of smut tend to be growing resistant varieties and by seed higher in plants grown in acid soil than treatment—positive measure-s that in neutral or alkaline soil. Soil type, should eliminate most of the loss if they soil compaction, depth of seeding, and are applied diligently everywhere. But the rate of seedling growth also may seed treatment is not universal, and afí'ect development of smut. smut continues to reduce the crop. Thirteen physiologic races of the New smut races that attack formerly barley covered smut fungus have been resistant varieties appear at times. isolated by their reactions on eight varieties of spring barley. One prevails THE COVERED SMUT OF BARLEY is in the winter barley region, another in found the world over. In the United the Mississippi Valley, and a third race States it destroys half a million to 4 along the Pacific coast. Several vari^ million bushels annually. eties have shown high resistance to The disease first becomes noticeable covered smut. Two varieties, Ogalitsu at heading time, when smutted heads and Jet, are resistant to 13 races of emerge from the boot. The hard, black covered smut, 9 races of nigra loose masses of smut along the axis of the smut, and 6 races of nuda loose smut. afí'ected heads are covered with a Covered smut may be controlled by grayish membrane. Each smutted head treating the seed with formaldehyde or contains millions of microscopic spores. organic mercury dusts. THE SMUTS OF WHEAT, OATS, BARLEY 367 NiGRA LOOSE SMUT of barlcy, Ustilago At least four varieties of barley are nigra^ was fírst discovered in 1932 by highly resistant to all races of the V. F. Tapke, who separated it from pathogen thus far found in the United ordinary loose smut. The two loose States. None is grown commercially, smuts, which often were mixed to- but all are useful in breeding produc- gether in a field, previously had Î3een tive resistant varieties. regarded as a single type that infected barley only through the flowers. The ONLY TWO of the small grain smuts two loose smuts look alike, but the are floral-infecting. The smutted heads nigra pathogen is a seedling-infecting appear at heading time. They shed smut and is controlled by treating the their spores during the bloom period seed with surface disinfectants. When of the normal heads. The loosely held the two loose smuts occur together spores are distributed by wind, rain, only partial control can be obtained insects, or other agents. Initial infec- by surface disinfection because the tion rarely occurs after the fertilized deep-borne loose smut is not affected ovary has attained one-third of its by such treatment. The fungi causing mature size. Spores that come in con- barley covered smut and nigra loose tact with healthy flowers germinate by smut have been hybridized artificially. forming infection threads, which grow Nigra loose smut causes an average down the pistil or through the ovary annual loss of i million bushels of wall into the young, developing seeds. barley in the United States. A deep infection of the seed follows, The disease usually first becomes so that surface seed disinfectants do noticeable at heading time, when little good. The fungi also are unable dusty, smutted heads appear. Each to produce infection v/hen spores are contains millions of loosely held, dark- applied to the surface of ripe seed. brown microscopic spores. Wind car- The two floral-infecting loose smuts, ries the spores. Spores that come in one of wheat and one of barley, are contact with the flowers and young practically identical except that one developing seeds of healthy heads then attacks v/heat but not barley and the behave like the spores of covered smut. other attacks barley but not wheat. They may lie dormant or germinate Both are widely distributed in humid and form a subhull mycelium. Mois- and subhumid areas but are less com- ture, temperature, and other condi- mon in dry areas. Infection threads tions aff*ect germination of spores. develop too slowly in dry air to pene- Temperatures of 60^ to 70° and a trate the ovary during its brief period relatively dry soil are most favorable of susceptibility. to infection of the seedling during the Wheat loose smut caused annual period of emergence. losses ranging from 3 million to 18 Immediately after the seedlings have million bushels in the United States emerged, temperatures of 60° to 70° from 1917 to 1939. Loose smut in for 10 to 30 days give higher percent- barley caused annual losses of 750,000 ages of smut than do constant low to 4.5 million bushels in those years. temperatures. High humidity favors infection by Nine distinct pathogenic strains or loose smut in wheat and barley, but physiologic races of the nigra loose the ranges of temperature and hu- smut fungus from this country have midity most favorable to infection have been isolated by their reaction on eight not been determined exactly. Certain varieties of spring barley. Four are wheats that rarely develop loose smut rather widespread in the United when grown under the dry conditions States. At least four races that do not of western United States are extremely occur in the United States have been susceptible when grown in humid found in smutted barley from various climates. parts of Israel. The experiments in the Netherlands 368 YEARBOOK OF AGRICULTURE 1953 show that wheat loose smut spores may be disseminated and cause infection up to at least 100 yards from their point of origin. Infection diminished regu- larly with distance from the source of inoculum where high winds prevailed Four Enemies during the flowering season; it was spotty when air currents were ir- regular. of Sorghum A warm soil when seedlings emerge seems more conducive to smut than does a cold soil. In Japan, for example, Crops early seedings of winter wheats and ])arleys often show higher amounts of loose smut than do later seedings. R, W. Leukel, J^hn H. Martin Grains that follow rice show little smut because seeding then is delayed The sorghum crops include grain for at least 6 weeks. sorghum, sorgo (or sweet sorghum), Soil fertility seems to have little in- broomcorn, Sudangrass, and Johnson- fluence on infection. Wheat plants grass. grown from infected seed are more Four general types of diseases attack susceptible to winterkilling, especially them: Those that rot the seed or kill under severe conditions, than are the seedlings; those that attack the similar healthy plants. leaves and lower the value of the The deep-infecting loose smuts of plants for forage; those that attack wheat and barley can be controlled by only the heads and so prevent the hot-water seed treatment. The usual normal formation of grain; and those method involves immersing the seed that cause root or stalk rots and pre- 4 to 12 hours in unheated water, vent the normal development of the followed by a lo-minute dip in water plant. at 129° F. A single immersion in water Seed rot is most severe when the soil held at 120° for 95 minutes also is is cold and wet after planting—a com- eflf'ective in control. Such treatments mon condition in the North and in are so laborious that individual growers other areas when seed is planted early. rarely use them. The treated seed is Much of the seed then fails to germi- soft, sv/ollen, and hard to dry before nate and rots because it is attacked by seeding. The treatment often injures various seed-borne and soil-inhabiting germination. Occasionally central fungi. To germinate promptly, sor- treating plants, established where ghum seed requires a relatively warm steam or hot water are available, treat soil, above 70° F. Most seed-rotting the seed for local growers. Control also fungi thrive at lower temperatures has been obtained by soaking the seed that retard the germination of the for 6 hours in water at room tempera- seed and give the harmful fungi an ture followed by a soak of 40 hours in opportunity to attack it. a 0.2-percent solution of Spergon. Some fungi (chiefly species of Fusari- urn, Aspergillus, Rhizopus, Rhizoctonia, C. S. HoLTON holds degrees from Pénicillium., and Helminthosporium) in- Louisiana State University and the Uni- vade and destroy the endosperm, the versity of Mhinesota. He is in charge of starchy tissue of the seed, thus robbing the wheat smut program in the Northwest, it of the food necessary to produce a V. F. TAPKE has conducted research on strong seedling. Cracks in the seed cereal diseases since joining the Bureau of coats of the kernels give the fungi Plant Industry^ Soils, and Agricultural ready access to the interior and thus Engineering in igi8. aggravate the trouble.