Introduction. — Corn smut, caused by Ustilago 

 maydis (DC.) Cda.. has been of tremendous interest 

 to biologists and to farmers for more than 250 years. 

 Smut galls are often large and conspicuous and corn 

 smut has been a subject of many observations, spec- 

 ulations, and superficial as well as intensive investiga- 

 tions. In spite of the interest which has been main- 

 tained for so long, the complete life history of U. 

 maydis was not known until 1927 when the sexual 

 stage was discovered. Even now it is not definitely 

 known when, where, and how most of the infection 

 occurs. Therefore, it is not surprising that much of 

 the literature dealing with etiology, ecological rela- 

 tionships, epiphytotics. and practical control contains a 

 vast array of confusing and often contradictory state- 

 ments. 



The fungus is nearly ideal as a tool in basic scien- 

 tific studies and for elucidating principles of mycology, 

 plant pathology, genetics, and plant physiology. This is 

 indicated by the large number of publications on muta- 

 tion, inheritance, and physiological studies on the 

 fungus and on the inheritance of resistance to corn 

 smut. 



U. maydis is of special genetic interest because it 

 is usually heterothallic or occasionally homothallic and 

 consists of an indefinite number of haploid and diploid 

 biotypes. Further, the organism is not static, but dy- 

 namic; new biotypes are constantly arising by hybrid- 

 ization and by mutation in both the haploid and diploid 

 stages; meiosis may occur in both the vegetative and 

 sexual stages. The magnitude of genetic variation is 

 indicated by the fact that in laboratories of the Uni- 

 versity of Minnesota alone more than 15.000 different 

 biotypes have been isolated and studied (282). 



History. — It is generally believed that U. maydis 

 was native to the Western Hemisphere and was carried 

 to southern Europe by the early Spanish explorers. 

 Although it was observed in Europe about 1750, it was 

 not reported from the U.S. until 1822 when Schweinitz 

 published his first list of American fungi (293). Ac- 

 cording to Arthur and Stuart (5), the earliest European 

 records of the fungus are by the French biologists 

 Bonnet (1754), Aymen (1760), and Tillet (1766). 



According to Hitchcock and Norton (143), Aymen 

 reported that all the flowers of the tassel were smutty 

 and that these were the source of the disease. He con- 

 cluded, therefore, that when a smutty ear also con- 

 tained sound kernels it had been fertilized by the pollen 

 of a healthy plant. 



In 1760-61. Tillet (335) started a detailed investiga- 

 tion of corn smut in fields in Agoumori, a province in 

 southern France. Five years previously in 1755, Tillet 

 had demonstrated conclusively that stinking smut of 

 wheat could be induced by inoculating the seed with 

 Tilletia sp. (see 98). Therefore, he dusted the kernels 

 of corn with chlamydospores of U. maydis before plant- 

 ing, but to his great surprise no smut developed. Con- 

 sequently, he concluded that corn smut was not a 

 contagious disease like stinking smut of wheat, but was 

 caused by an excessive amount of sap which accumu- 

 lated in certain parts of the plant and then induced 

 excessive dilation of cellular tissue. 



During the next 140 years, other attempts were made 

 to induce corn smut galls, but all were unsuccessful 



(5, 34, 335). Therefore, it is not surprising that some 

 of the leading biologists for many years accepted and 

 perpetuated the theory that stagnation of sap or exces- 

 sive sap was the cause of corn smut (25, 339). Others 

 attributed the smut galls to fogs, dews, rains, too rich 

 soil, or too much raw manure (5, 335). Still others 

 considered the galls spontaneous abnormalities, mon- 

 strosities. Bonnet (26) concluded that insects depos- 

 ited some irritating substance which caused the sap to 

 flow to injured parts thus producing the galls. 



Corn smut was not recognized as caused by a fungus 

 until about the early part of the nineteenth century 

 even though Beckmann (17) called it a puff ball and 

 named it Lycoperdon zeae in 1768. In 1836. Unger 

 (339) recognized it as a fungus and named it U. zeae; 

 but just 3 years previously, in 1833, he stated that a 

 young smut gall contained nothing but large cells filled 

 with raw sap. 



Bonafous (25), in 1836, also considered the smut a 

 plant and recorded some interesting accounts of inoc- 

 ulation experiments (Fig. 1). He stated that Bayle- 

 Barelle at the botanical garden in Pavia. Italy, obtained 

 no smut when the powder from tumors was used to 

 inoculate seed or the medullary body of stalks, nodes, 

 roots, and parenchyma of leaves. Apparently, the host 

 tissue was injured considerably in these tests because 

 when the spores were introduced into vigorous stalks, 

 without breaking the epidermis, smutty plants were 

 produced. Bonafous could not confirm these results. 



Beginning about 1850, de Bary (10), Tulasne and 

 Tulasne (337), Meyen (225), Wolff (364), Fischer von 

 Waldheim (99), Kiihn (190), and others attributed 

 the smut galls to fungus infection, but failed to prove 

 it. It was not until 1883-95 that Brefeld (34, 35. 36) 

 demonstrated beyond doubt the infective nature of V. 

 maydis. He, too, failed to get infection by inoculation 

 of seed; but induced infection by spraying the corn 

 plant with a suspension of sporidia and by dropping 

 sporidial suspensions into the spiral whorl of the corn 

 plant. Brefeld (34) also proved that any young meri- 

 stematic tissue above the ground was subject to infec- 

 tion. Previously, he had demonstrated for the first time 

 (35) that the corn smut fungus could be propagated 

 indefinitely on nutrient substrates, including manure 

 decoction. 



Bessey (20) in Nebraska. Hitchcock and Norton 

 (143) in Kansas, Arthur and Stuart (5) in Indiana. 

 Clinton (60) in Illinois, and Bessey (19) and Pammel 

 (246) in Iowa were among early workers in the U.S. 

 to make noteworthy contributions to our knowledge of 

 corn smut. At the end of the nineteenth century 

 numerous semipopular publications appeared on losses, 

 control practices, and on toxicity of smuts to animals. 

 The more recent publications dealt mostly with basic- 

 studies on the ecology, physiology, and genetics of I 

 maydis and on the inheritance of resistance of corn to 

 smut (55, 145, 158, 267, 283. 314, 317). 



Botanical Name. — Considerable diversity exists in 

 the literature in the use of botanical names for the 

 common corn smut fungus. Beckmann (17). in 1768. 

 called it Lycoperdon zeae because the smut spores re- 

 sembled those of a puff ball. Next, the smut fungus 

 was placed in the form genus Uredo or Caeoma along 

 with the rusts. 



