describing the range within a species. Clinton (61) 

 described the chlamydospores of U. maydis as ellip- 

 soidal to spherical, more or less irregular, prominently 

 but rather bluntly echinulated, and usually 8-11 u,, but 

 occasionally up to 15 u long. Fischer (97) considers 

 the spore mostly globose to sub-globose, light olive 

 brown, and chiefly 7-10 u diam. 



Christensen (55) gave the average size of 700 

 chlamydospores taken from 7 sources as 8.2 u ± 0.2 

 x 9.0 ± 0.2 u. The length varied, however, from 4.S u 

 to 30.7 u. Chlamydospores produced by different iso- 

 lates differed in average size from 7.8 yi x 8.5 u to 



9.1 (i x 10 u. 



Chilton (52) measured 100 chlamydospores from 

 each of 21 crosses and recorded big differences in size 

 of chlamydospores. He found an association between 

 abnormally large chlamydospores and their tendency 

 to germinate abnormally, i.e., by promycelia that were 

 distorted and often autolyzed. The width and length 

 of the abnormally germinating chlamydospores varied 

 from 9.1 u x 9.7 (.i to S.l ^i x 11.7 u; those from 

 normally germinating spores, from 7.4 u x 7.9 \i to 



7.2 u x S.9 u. On the basis of the above figures, the 

 size of chlamydospores of U. maydis may differ by 

 more than 30 c ' ( . 



Chlamydospores taken from very small galls were 

 frequently angular, very irregular, and sometimes 

 bizarre in shape. 



Sporidia. — The length of sporidia differs greatly, 

 especially when the lines are grown on artificial sub- 

 strates. Christensen (55) measured the sporidia from 

 3 diploid lines and 4 haploid lines. The average length 

 of sporidia of a given line varied from 13.4 [i to 28.9 u. 

 depending on their source. Although the -sporidia of 3 

 of the 4 haploid lines were shorter than those of the 

 3 diploid lines, the sporidia of 1 haploid line were 

 longer than 2 of the diploid lines. The size of sporidia 

 was no indication of their nuclear condition (55). The 

 primary sporidia. those produced on the promycelium. 

 were usually regular, hyaline, and fusiform, although 

 these also may differ considerably in size and shape. 



X umber of spores and dissemination. — U. maydis 

 produces an enormous number of both chlamydospores 

 and sporidia and is undoubtedly the most prolific 

 species of the smut fungi. The number of chlamydo- 

 spores 'cm 3 of gall tissue has been estimated at 2.5-6 

 billion (5. 56 i. Thus, in a single smut gall of medium 

 size, there may be more than 200 billion spores. Several 

 galls may occur on a single corn plant. When one con- 

 siders that there are over 70 million acres of corn in 

 the U.S.A. alone, the number of chlamydospores pro- 

 duced in a single season must be incomprehensibly 

 large, even if only a small percentage of the plants are 

 infected. 



The number of sporidia produced also must be very 

 great, for when chlamydospores germinate, the promy- 

 celium may give rise to 4, and often many more, 

 primary sporidia. These, in turn, may bud and continue 

 to produce secondary sporidia in chains in debris and in 

 the leaf whorls of corn plants. A single sporidium may 

 ''bud-off" another sporidium in about 3 hr. Further, as 

 previously indicated, developing smut gall, under humid 

 conditions, may produce huge masses of aerial sporidia 

 in chains (36. 55, 123). 



Dissemination of spores. — Both chlamydospores and 

 sporidia are readily disseminated by wind and have 

 been caught at high altitudes (318). Over a 2-year 

 period. Pady (244. 245) found that chlamydospores of 

 U. maydis were common in the air through the year in 

 the vicinity of Manhattan, Kans.. but never in large 

 numbers. C. M. Christensen (unpublished results. Uni- 

 versity of Minnesota) caught chlamydospores at St. 

 Paul. Minn., at intervals throughout the year. Wittich 

 (560i found that they were extremely abundant in the 

 fall. During a smut shower in 1957. he caught more 

 than 170.000 chlamydospores/1.8 cm- of surface area 

 of a slide. Unfortunately, the duration of the slides' 

 exposure was not given. 



C. M. Christensen (see 56) estimated the rate of fall 

 for chlamydospores of U. maydis at 3.5 mm/sec. a 

 much slower rate than for urediospores of Paccinia 

 graminis Pers.. which are sometimes carried hundreds 

 of miles by the wind. Therefore, it seems likely that 

 chlamydospores of U. maydis also may be carried long 

 distances by the wind. Smut inoculum and its dis- 

 tribution do not appear to be the limiting factor in 

 creating an epiphytotic of corn smut in the corn belt 

 of the U.S.A. 



Longevity of spores. — Chlamydospores of U. maydis 

 readily overwinter outside under different conditions 

 in Minnesota. Just how long they persist in a more 

 temperate climate, especially in regions with high 

 humidity and frequent rains, is not known. 



Brefeld in 1883 (34) germinated chlamydospores of 

 U. maydis that had been stored indoors for at least 

 8 years. Piemeisel (261) did the same with 5-year-old 

 spores. Benigni (18). in Italy, failed to germinate 

 spores that were more than 3 years old. Fischer (95) 

 obtained 90% germination of spores after 2 years of 

 storage. Chlamydospores of U. maydis from several 

 different countries also retained their viability for 

 several years in the laboratory at Minnesota. Chlamydo- 

 spores taken from smut galls stored over winter in bulk 

 lots in gunny sacks germinated readily the next summer. 

 Just how long chlamydospores can persist in nature 

 when mixed with soil is not known. 



Very little is known concerning the factors that affect 

 the survival of sporidia. Brefeld (34) concluded that 

 dry sporidia died in about 5 weeks, whereas Piemeisel 

 (261) desiccated sporidia for 5 months without ma- 

 terially impairing their viability. Alternate freezing and 

 thawing were very injurious to moist sporidia, but were 

 not so injurious to desiccated sporidia. Brefeld (35) 

 stated that sporidia lost viability within 8 months when 

 in continuous culture, whereas Piemeisel (261) ob- 

 tained good infection with sporidia maintained for 8 

 months in culture. We now know that sporidia can be 

 maintained in pure culture on nutrient media for an 

 indefinite period by transferring them periodically. 



Chlamydospores and sporidia of U. maydis are readily 

 killed by high temperatures. In water, they are killed 

 within 15 min at 52°C. In dry heat. 5 min at 103°C 

 usually is sufficient to kill them, although a greater 

 tolerance to high temperatures has been reported 

 (5. 327). In culture, the fungus growing in nutrient 

 solution is killed at 46 C C. There is no information on 

 the effect of high temperatures on spores or mycelium 

 in the living host. But the fungus can be stored at 



13 



