marked differences in susceptibility among open-pol- 

 linated varieties have been reported by many investi- 

 gators (57. 107, 129, 158). It is well known that most 

 field varieties of corn now grown in the U.S.A. are 

 much more resistant than the old open-pollinated 

 varieties, whereas most varieties of sweet corn are still 

 quite susceptible. 



Jones (173). in 1918, was the first to demonstrate 

 that inbred or selfed lines of corn derived from the 

 same parental variety differed greatly in their suscepti- 

 bility to smut. Since then, it has been shown many 

 times that inbred lines of corn can be produced easily 

 with every degree of resistance to smut (129. 158, 

 174). 



Breeding for resistance. — Several workers have 

 studied the inheritance of resistance to smut in corn. 

 Jones was the first one to make crosses betwen long- 

 time inbred lines that differed in smut reaction. Resist- 

 ance to smut was dominant in F x and segregation for 

 resistance occurred in F 2 . Hayes, et al. (129) found 

 crosses between resistant and susceptible lines inter- 

 mediate in reaction, whereas resistant X resistant 

 lines gave an F x slightly more resistant than either 

 parent. Immer and Christensen (158), Immer (157), 

 and Hoover (148) also found that F T 's from crosses 

 between selfed lines that differed in smut reaction 

 were usually intermediate in reaction. Immer (157) 

 did find a few cases of apparent dominance of resist- 

 ance to susceptibility. Immer and Christensen (161) 

 found that the Fv progeny from a composite cross 

 made of 7 low-smut lines and 1 medium-smut line 

 were predominantly less susceptible to smut than the 

 most susceptible parent. 



Linkage groups have been used by Immer (157). 

 Hoover (148), and Saboe and Hayes (283) to study 

 inheritance of smut reaction in maize. Burnham and 

 Cartledge (40) used chromosomal translocation as a 

 means of studying inheritance of resistance to smut. 

 Hayes, et al. (128) by back-crossing a smut-susceptible 

 line with smut-resistant line obtained improved lines 

 that were highly resistant to smut. Further studies 

 are necessary before concluding how many factors are 

 involved. Genetic studies, plus the rapidity with which 

 inbred lines become homozygous for smut reaction, 

 indicate that a relatively small number of factors 

 determine smut reaction. Naturally, the breeding 

 of smut-resistant varieties would be helped materially 

 if the genotypes of the host for smut resistance and 

 the characters involved in resistance were known. 



Nature of Resistance. — Little is known regarding 

 the basic nature of resistance of corn to smut and 

 the interaction between the pathogen and the host. 

 Smut resistance appears to be associated with func- 

 tional, physiological, and morphological characters of 

 corn plants. 



There are many conflicting statements about the 

 relative susceptibility of corn at different stages. Some 

 say the plants are most susceptible at 25 cm high: 

 others, at 1 m; still others observed no difference 

 ( 5. 34, 35, 260). Although young seedlings usually 

 escape infection in the field, all varieties and lines of 

 corn are extremely susceptible when inoculated by the 

 partial vacuum method or by the hypodermic syringe 



method. Young seedlings that become infected, whether 

 in the field or in the greenhouse, usually are killed 

 (118, 157, 170). They appear to be less susceptible 

 as they grow older. By the time the plants become 

 about 15 or more cm high, they react very differently 

 to different dicaryons of the smut fungus, whether 

 grown in the field or greenhouse (161). This is also 

 the stage of growth most efficient in distinguishing 

 physiologic races (59). Obviously, these differences in 

 smut reaction among selfed lines, when artificially 

 inoculated by the hypodermic syringe method, must be 

 considered physiological. Ranker (270. 2 71) believed 

 that smut resistance was physiological. He found that 

 U. maydis grew better in extract of leaves, husks, and 

 stalks from susceptible selfed lines than from resistant 

 lines. He called the toxic agent a smut inhibitor. 



It seems likely that young seedlings in nature tend 

 to escape infection because the inoculum cannot readily 

 reach the meristematic tissue. Some believe that corn 

 plants in the field continue to escape infection until 

 just before tasseling because of the tightness of the 

 leaf whorl. Most investigators (5, 25, 351) have not 

 secured good or consistent infection by spraying of 

 seedlings with a suspension of sporidia or chlamydo- 

 spores. This type of resistance may be considered 

 functional and morphological. 



Griffiths (118) and Platz (263) believed that the 

 relative accessibility of the growing tip when the plants 

 are about 1/3 m tall determines their reaction to smut. 

 Walter (351 ) found no association between the relative 

 heights of growing tips in the leaf whorls of 81 lines 

 of corn and their reaction to smut. 



Morphology no doubt plays an important role in 

 factors influencing both infection and development of 

 smut. Immer (157) and Hoover (148) suggested that 

 certain morphological characters, such as lack of 

 ligules on the leaves, may be associated with the 

 location of smut on the plant. Kyle (193) found an 

 association between tightness of the husk and resist- 

 ance to ear smut. According to Davis (72. 73) and 

 Platz (262). latent smut infection of bud shoots is 

 not uncommon in corn, because of retardation of the 

 growth tissues. Penetration of the host tissues that 

 are too old for smut development has been observed. 

 Therefore, rapidity by which certain tissues mature 

 also may influence gall formation. Walter (351) found 

 no association between the resistance of leaves to 

 puncture as measured by the Joly balance and smut 

 reaction in selfed lines of corn. 



Since there are so many parasitic races and biotypes 

 of U. maydis, future studies should continue to em- 

 phasize generalized field resistance. Lines and varieties 

 of corn should be selected on the basis of their ability 

 to withstand natural infection in the field. 



References and Bibliography. — There is a vast 

 amount of literature on the subject of corn smut. 

 I excluded many papers which contributed little or 

 nothing of importance concerning the history or biology 

 of the pathogen such as popular and semipopular 

 articles, especially those on distribution and prevalence 

 of the smut, on control methods, and on the toxicity 

 of smut spores to animals. The papers are given in 

 alphabetical order. 



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