much more conspicuous on vigorous than on un- 

 thrifty corn plants. Vigorous plants may sometimes 

 have fewer concealed galls, those more or less en- 

 closed or hidden by the leaf sheath, hence these plants 

 may appear more susceptible. Further, according to 

 Walter (351). when plants are extremely low in 

 vigor, the number of galls plant may be reduced be- 

 cause of a decrease in the development of nodal buds. 



Date of planting. — There are considerable differences 

 of opinion on the prevalence of smut in corn planted 

 on different dates. Many tests have been made; some 

 workers reported more smut in early planting, others 

 less, others obtained no differences in amount of smut 

 in corn planted at different dates. 



Arthur and Stuart ( 5 ) said that corn planted early 

 developed more smut because it had a greater number 

 of chances to become inoculated and infected. Hitch- 

 cock and Norton (143) stated that although smut 

 infection becomes visible first on early planted corn, 

 late in the season, irrespective of date of planting, 

 they are all equally smutted regardless of age. Potter 

 and Melchers (266) observed no differences in amount 

 of smut between early or late plantings. In general. 

 Walter (351) found that the smut was more destruc- 

 tive in the late plantings than in the early plantings, 

 but there were exceptions. Before a definite statement 

 can be made on effect of time of planting, experiments 

 should be made on many varieties or lines of corn 

 under many different environmental conditions in 

 different regions of the country. 



Injuries to the host. — Aymen (6), in 1760, reported 

 that removal of tassels from plants did not increase 

 smut infection. Since then, there have been several 

 contradictory reports (73, 86. 143, 351). This probably 

 can best be explained in terms of the stage of de- 

 velopment of the host at the time of detasseling. 

 This factor also seems to influence the effect of other 

 mechanical injuries, since smut infection seldom 

 develops on injured parts. Multilation appears to have 

 an indirect effect, as it stimulates the development of 

 smut galls in lateral and internodal meristem. 



Hitchcock and Norton (143 ) and Davis (73) se- 

 cured marked increases in numbers of smut galls by 

 the removal of tassels and ears. Clinton (60) obtained 

 no increase in smut infection by mutilating plants 

 that were a few cm to 1 m high, whereas injuries to 

 very young tassels and ears increased infection, es- 

 pecially if the plants were inoculated following the 

 mutilations. 



MacMillan ( 205 ) recorded a severe epidemic of 

 corn smut following a severe hail storm. Walter (351 ) 

 failed to increase the severity of smut by puncturing 

 the growing tip of corn plants with a needle or nails. 

 Platz ( 263 ) also failed to increase smut infection 

 by mutilating the plants, even when they were sprayed 

 with a suspension of sporidia or dusted with chla- 

 mydospores following the injury. 



Eldredge (86) showed that there was an increase 

 in the prevalence of smut only if the stalks were 

 injured about 2 weeks preceding the appearance of 

 the tassel. Walter (351) also concluded that injuries 

 induced by mechanical means and by detasseling 

 markedly increased the prevalence of smut provided 



the damage occurred when the corn plant was in an 

 intermediate stage of rapid elongation. 



Rate of planting. — The density or rate of planting 

 of corn in relation to prevalence of smut has long 

 been a subject of much interest. Burger (38), in 1809, 

 reported that thick planting of corn favored the devel- 

 opment of smut. Arthur and Stuart ( 5 ) stated that 

 thickly planted corn promoted smut development be- 

 cause this maintained the moist air longer. Piemeisel 

 (261 ) believed that close planting increased the amount 

 of smut because the plants remained succulent for 

 a longer period of time. Kornfeld (187), in Germany, 

 also reported that corn grown close together had a 

 higher percentage of infected plants than corn grown 

 farther apart. 



Wilcoxson and Covey (357) obtained the opposite 

 results; the more spacious planting not only gave a 

 higher percentage of smutty plants, but the severity 

 was 3 times greater on the corn with the less dense 

 population. Their results were based on standard 

 field plot techniques and involved 3 years' data. 

 Davis' ( 73 ) results, based on 4 years of extensive 

 field tests, indicated less smut when there were 2 or 3 

 plants hill than in hills with fewer or more plants. 

 Obviously, it would be desirable to make additional 

 tests on plant populations in relation to smut under 

 different environments using several varieties of corn. 



Solopathogens. — As previously indicated, U. may- 

 dis is predominantly heterothallic. but occasionally 

 monosporidial lines are solopathogenic. causing in- 

 fection when inoculated singly into the corn plant 

 (54, 84, 301, 358). The frequency of solopathogenic 

 lines is often associated with particular galls and 

 crosses (55). Thus, Chilton (50, 51) concluded that 

 tendency for production of diploid lines was asso- 

 ciated with lysis of the promycelium, whereas Chris- 

 tensen observed them arising from normal promycelia 

 (55). In any case, solopathogens are not uncommon, 

 because they have been reported from material of 

 diverse origins (54, 55, 84, 111, 301, 358). The 

 pairing of 2 solopathogenic lines from the same or 

 different chlamydospores did not increase their para- 

 sitic capabilities. Likewise, the pairing of a solo- 

 pathogenic line with unisexual lines from divergent 

 sources does not change the pathogenicity of the 

 former (55). 



The solopathogenic lines resemble the unisexual 

 lines in certain characteristics. They grow and multiply 

 like the unisexual lines and the colonies of solo- 

 pathogenic lines cannot be distinguished from the 

 haploid colonies. Their range in color, consistency, 

 growth, topography, etc., is as great and as variable 

 as unisexual lines (55). 



Gattani (111) found that 7 diploid lines differed 

 from one another in rate of growth, ability to digest 

 casein, ability to liquefy gelatin, and growth characters 

 on standard bacteriological media. They also differed in 

 their ability to produce anthocyanin and in their path- 

 ogenicity on 4 selfed lines of corn. One line had 

 much greater tendency to mutate than the other 6. 



In certain respects, the solopathogenic lines behave 

 like the dicaryophytic stage of 2 unisexual lines. They 

 are parasitic, stimulate production of anthocyanin 



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