pigment, and produce normal smut galls. The chla- 

 mydospores derived from single solopathogenic lines 

 are of normal shape and size and they germinate 

 and develop normal promycelia. Factors for sexual 

 compatibility, color, topography, virulence, and tend- 

 ency to mutate segregate in the promycelia exactly 

 as in promycelia of chlamydospores produced by 2 

 unisexual lines of opposite sex. The sporidia are pre- 

 dominantly uninucleate and hence are apparently 

 diploid in the strict sense of the word. 



Solopathogenic lines are as stable as unisexual lines; 

 some mutate frequently on certain substrates, others 

 seldom (55). The stability in virulence is not in- 

 fluenced by the passage through the living host in the 

 mycelial stage nor is it associated with the length 

 of time a given line has grown in culture (55). Some 

 monosporidial lines have maintained their parasitic 

 capabilities for about 30 years, others gave rise to 

 nonvirulent variants within 1 year or a few weeks 

 after isolation (54. 111. 203). 



Some lines may fail to mutate on one medium, but 

 develop many mutants on another one and sometimes 

 certain media may aid in selecting mutants. When 

 solopathogenic lines are grown on media containing 

 arsenic or lithium, the frequency of mutation and 

 perhaps also vegetative segregation are increased when 

 exposed to radiation. The mutants or the segregates 

 are occasionally avirulent, most likely unisexual, at 

 least some of them cause normal infection when paired 

 with certain unisexual lines (58, 111 J. 



Studies by several investigators have shown that 

 solopathogenic lines differ strikingly in parasitic 

 capabilities (54. 55. 145, 203. 358). Some are virulent 

 and produce large galls, others are weakly pathogenic 

 and produce very small galls and still others are 

 intermediate in virulence. Some induced the produc- 

 tion of an abundance of anthocyanin in certain selfed 

 lines of corn, others did not (54. 203). The great range 

 in parasitism among solopathogenic lines especially 

 among mutants derived from the same monosporidial 

 lines is an indication that pathogenicity is not neces- 

 sarily linked with factors for sexual compatibility (55. 

 279). 



Physiologic Specialization'. — The term "physio- 

 logic race" was originally used to designate groups 

 within species that differed consistently in patho- 

 genicity. In corn smut, the term ''physiologic race" 

 is usually restricted to a collection of chlamydospores 

 that are relatively consistent in pathogenicity on 

 certain lines or varieties of corn. From the practical 

 standpoint, there is little value in dividing U. maydis 

 into physiologic races and assigning them specific 

 numbers. Now. it is generally recognized that phys- 

 iologic races also may differ in cultural characters 

 on artificial media, physiological and ecological char- 

 acters, biochemical responses, and to a limited extent 

 also in morphology. U. maydis comprises an indefinite 

 number of lines or biotypes (310, 314;. Thousands 

 have been isolated and studied (282, 309). These 

 differ in so many physiologic characteristics and so 

 much in pathogenicity that they could be considered 

 physiologic races. Of course, describing or numbering 

 such races would not be feasible or practical. 



Further. U. maydis is predominantly heterothallic, 

 requiring the pairing of 2 compatible haploid lines 

 of opposite sex for normal infection and the production 

 of chlamydospores. New biotypes arise by hybridiza- 

 tion: as chlamydospores are diploid and meiosis oc- 

 curs at the time of germination and new recombinants 

 occur, consequently a changing population of biotypes 

 results in each new sexual generation. New biotypes 

 also arise by mutation. Further, there are excellent 

 opportunities for outbreeding as the chlamydospores 

 and aerial sporidia are disseminated long distances by 

 the wind. Consequently, field infections are usually the 

 results of dicaryons of a diverse genetic constitution. 

 Thus, from a single gall, it is often possible to isolate 

 a vast array of haploid lines and some diploid lines 

 that differ greatly in parasitism. In spite of the 

 heterogeneity of chlamydospores in a smut collection, 

 fairly consistent differences in pathogenicity have 

 been secured in the field with chlamydospores derived 

 from different sources (59). Whenever basic studies 

 are made on parasitism, however, it would be prefer- 

 able to use dicaryon or solopathogenic lines rather 

 than a collection of chlamydospores which may con- 

 sist of a mixture of many biotypes. 



Parasitic differences. — Melchers (220), in 1921, 

 found that cultures of U. maydis seemed to differ in 

 their ability to cause infection when the plants were 

 artificially inoculated. He also indicated that inbred 

 lines and hybrids differed in resistance when grown 

 in different localities in Kansas, an indication of 

 physiologic differences in U. maydis. Tisdale and 

 Johnston ('336), in 1926. noted slight differences in 

 the parasitic behavior of smut from different sources. 

 They also recorded cultural differences in mass isolates 

 of U. maydis. 



About this time. Stakman and Christensen (312) 

 demonstrated striking parasitic differences among 12 

 chlamydospore collections of U. maydis. Eleven of 

 these originated from widely separated states and 2 

 were from Minnesota. Mass isolations were made from 

 chlamydospores and these were used to inoculate 

 10 varieties of corn that differed greatly in suscep- 

 tibility to smut. All the cultures caused infection 

 on all the varieties, but differences in the severity 

 of infection on certain lines of corn were sometimes 

 very large. Certain races produced only minute galls 

 on some selfed lines, others produced large smut 

 galls on nearly all lines. Some races differed in their 

 parasitic effect on all 10 lines of corn whereas others 

 differed only on a few of them. 



Some races also induced chlorosis of the leaves, 

 often followed by necrosis. The degree of chlorosis 

 and necrosis depended on the race and also on the 

 line of corn. There are similar differences among races 

 and lines of corn in respect to the anthocyanin pro- 

 duction. Other investigators found somewhat similar 

 differences in parasitism among 10 other chlamydo- 

 spore collections of U. maydis on 6 varieties of 

 corn (203, 310;. 



The fact that the collections of smut from different 

 states possessed different parasitic capabilities does 

 not necessarily indicate the prevalence of distinct 

 parasitic races in specific regions. A sample consisting 



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