of 1 to several galls from a given locality certainly is 

 not representative of the smut population for a given 

 region. Even a single acre of corn may harbor many 

 thousand galls and the number of chlamydospores 

 produced would be virtually inconceivable. Then, 

 there may even be from several to many millions 

 of acres of corn within a single state or region. As both 

 chlamydospores and sporidia are readily wind-borne, 

 it seems likely that within a geographical region 

 bounded by natural barriers that the smut population 

 of U. maydis would consist of a vast mixture of 

 parasitic biotypes. Experiments made at St. Paul, 

 Minn., tend to prove this (57, 161). 



Local and nonlocal smut. — Christensen and Johnson 

 (57) made a comparative study for 5 successive years 

 of the response of 5 standard varieties and 95 inbred 

 lines of corn to local and nonlocal collections of smut. 

 The local inoculum consisted of a mixture of chla- 

 mydospores derived from several hundred smut galls 

 which were collected at St. Paul. The nonlocal inocu- 

 lum was obtained from 26 localities in 12 states east 

 of the Rocky Mountains. Each of the 26 collections 

 consisted of many galls and the chlamydospores in 

 the 26 collections were thoroughly mixed before 

 inoculation. The 2 experimental plots were about 

 ^ of a mile apart. At frequent intervals during the 

 growing season, the chlamydospores were dusted and 

 sprayed on the plants. 



The relative reactions of all the lines of corn to 

 smut were similar on the 2 fields. Lines of corn 

 resistant to local smut collections were equally re- 

 sistant to smut obtained from widely different sources. 

 There also was a high correlation between the 2 fields 

 in regard to the number of galls/infected plant and 

 to the location of galls on the plants. 



The close agreement in smut reaction of the inbred 

 lines to smut collections from widely different sources 

 should be very helpful in breeding for smut resistance 

 in corn. Because of vast numbers of biotypes in U. 

 maydis, it would be impractical and futile to breed 

 corn resistant to specific biotypes or races. Therefore, 

 varieties of corn must possess generalized smut resist- 

 ance or field resistance. This has long been the objec- 

 tive at Minnesota and other institutions where smut- 

 resistant corn hybrids have been produced ( 107, 129, 

 158, 328). 



Phenotypic Variability. — It is important to dis- 

 tinguish clearly between phenotypic modification or 

 variability and true genetic changes and differences. 

 Numerous experiments (55, 314) with monosporidial 

 cultures of U. maydis have shown that the rate of 

 growth, color, sporidial production of colonies, and 

 other cultural characters may be modified by en- 

 vironmental fluctuations due to temperature, nutrients, 

 vitamins, growth substances, chemicals, hydrogen-ion 

 concentrations, and presence of toxic substances, dyes, 

 and radiation. Phenotypic changes usually revert to 

 original characters when the stimulus is removed. 



Several workers (111, 140, 203, 290, 314) have 

 clearly demonstrated that standard nutrient media to 

 which various materials are added may have a pro- 

 found effect on the rate of growth, color, and general 

 topography of the smut colonies. Certain lines of smut 

 may be sporidial on one substrate, but mycelial on 



another; and sugar tends to increase sporidial growth 

 (179, 314). The temperature at which the lines are 

 grown may greatly alter the cultural characteristics 

 and the rate of growth (256, 314). The percentage 

 and purity of agar in a medium and kind of liquid 

 culture may also greatly modify cultural growth and 

 appearance of colonies. Even duration of steriliza- 

 tion of a medium may induce sharp modification in the 

 type of growth. Striking changes also occur in the 

 rate of growth and other characters by changing the 

 pH of the medium. The extent of some of these 

 phenotypic changes is illustrated in Fig. 9. 



Semipermanent modifications also apparently occur 

 in U. maydis. There is some evidence that some smut 

 lines may gradually acquire tolerance to toxic sub- 

 stances. Stakman, et al. (321) found that monospori- 

 dial lines of U. zeae increased their tolerance to sodium 

 arsenite in the medium. Certain lines originally toler- 

 ated only 2,400 ppm of this salt, but after 10 cul- 

 tural generations on this medium they tolerated 7,000 

 ppm (142, 256). The cultural characters of these 

 lines also were altered by the arsenic media, but 

 after several successive generations on arsenic-free 

 media, they gradually reverted to their normal types 

 and also lost their acquired tolerance for arsenic. 

 Later, Gattani (111) studied the adaptation of certain 

 lines to arsenic and malachite green and obtained 

 somewhat similar results. Hirschhorn and Munnecke 

 (142) not only proved that certain lines of U. maydis 

 developed tolerance to increasing concentration of 

 sodium arsenite, but the adapted lines also failed to 

 cause infection when paired in the usual way. These 

 lines regained their virulence, at least partially, after 

 a few asexual transfers on arsenite-free media, thus 

 suggesting adaptation rather than mutation. It has been 

 clearly demonstrated that there is a carry-over effect 

 from a physiological treatment or toxic substrate to 

 another medium (257). Naturally, apparent adaptation 

 to toxic materials can arise by mutations and use 

 of mass inoculum consisting of mixed biotypes. 



When Petty (256, 257) grew a line of U. maydis at 

 21°, 32°, and 35 C C for 5 cultural generations over a 

 period of 6-9 months, he obtained no proof of adapta- 

 tion to higher temperature. Although phenotypic 

 changes were frequent at 32° and 35°C. some of the 

 changes persisted for only 1 or 2 cultural generations 

 and others did not persist beyond the generation of 

 their origin. Mutations in cultural characters were 

 common at the higher temperatures; none of the 

 mutants grew faster than their parents. 



Since mutations are rather frequent in U. maydis 

 and since toxic materials induce mutation, the possibil- 

 ity of mutation as a cause of adaptation must not be 

 overlooked. Further, because certain mutants may 

 be hidden by the parental culture through many 

 transfers, there is always the possibility of using 

 mixed biotypes as inoculum. Then, too, genetic changes 

 affecting the physiology of the fungus may occur 

 without any apparent change in colony morphology. 

 Obviously, these facts must be given due consideration 

 in any study on adaptation (53, 203, 314). Unfortu- 

 nately, detailed genetic analyses were seldom made on 

 the smut populations involved at the end of the ex- 

 periments. 



21 



