of uninucleate cells and are cut off into segments that 

 eventually become chlamydospores. 



Although clamp connections have been reported in 

 the host, their presence has been questioned (59). 

 Clamp-like structures are not uncommon in both 

 dicaryotic and diploid hyphae (55). These structures 

 usually develop into hyphal outgrowths and actual 

 fusion with adjoining cells has not been recorded (55). 



In general, the cells in the parasitic mycelium are 

 either binucleate or multinucleate, although the nuclei 

 are often loosely associated in pairs (54. 55. 60). 

 Ehrlich (85) observed 1-6 nuclei cell, although the 

 usual number was 2. Sleumer ('301 I also reported that 

 the binucleate condition predominated, whereas Lut- 

 man ( 204 | reported hyphal cells as multinucleate. 



Formation of chlamydospores. — Chlamydospore for- 

 mation has been described in detail by several workers 

 i 55, 60. 85. 301 I. The hyphae at the time of chlamydo- 

 spore formation gelatinize and appear as short, angular. 

 and irregular segments. These segments later round up 

 more or less and then a spore wall develops. The spore 

 wall is at first hyaline and smooth, but as the chla- 

 mydospores mature they become dark and echinulated. 

 There is no indication that the segments fuse as 

 suggested by Rawitscher (273). 



Sleumer (301) and Ehrlich (85) concluded that 

 karyogamy occurs shortly before the segmentation 

 of mycelium. Their observations were based primarily 

 on nuclear size. Most chlamydospores are uninucleate 

 and diploid: some, however, are uninucleate and 

 haploid: others occasionally are binucleate (55, 85). 

 Solopathogenic lines resemble dicaryotic lines in 

 respect to the development of the parasitic myceli- 

 um and formation of the chlamydospores. except that 

 no nuclear fusion occurs (55, 85). 



Environmental Factors. — There is no general 

 agreement on what weather conditions are most favor- 

 able for the development of an epiphytotic of corn 

 smut. It is usually assumed that rain and humid 

 weather are the most critical environmental factors 

 essential for infection. 



Moisture. — Arthur and Stuart I 5 1 believed that 

 the sporidia were delicate and easily desiccated and 

 therefore must be kept damp to remain viable, but 

 this is not in accord with the opinions of later workers 

 (261). Moisture is definitely needed for germination 

 and maintenance of germ-tube growth until the host 

 is penetrated. Coffman. Tisdale. and Brandon (63 1 

 believe that heavy rains may wash the sporidia out 

 of the air and off the corn plant and thus reduce 

 the amount of smut infection. They stated that early 

 spring rainfall induced the chlamydospores to germi- 

 nate before corn plants were large enough to become 

 infected: hence, large amounts of inoculum may be 

 destroyed. A number of workers (73. 267. 350) have 

 recorded the production of sporidia in the leaf whorl 

 of corn. According to Platz ('263 i. light rain is suffi- 

 cient to provide moisture for the multiplication of 

 sporidia in the infection courts. Arthur and Stuart 

 (5) believed that infection takes place during cloudy 

 days or dewy nights and Piemeisel C261 ) states that 

 cool damp weather favors infection. 



If most of the natural infection occurs in meri- 

 stematic regions of the leaf-whorl, then it would seem 



unlikely that humidity would be a limiting factor after 

 the inoculum has reached the infection court. For 

 instance. AValter (351 ) proved that dry chlamydo- 

 spores forced into the meristematic region germi- 

 nated and caused infection without the addition of 

 water. There is good evidence that free moisture 

 on the plant is not essential for smut development 

 once the infection is established. When plants are 

 inoculated by the hypodermic syringe method and 

 placed in a relatively dry greenhouse, excellent in- 

 fections have always resulted. 



Dry weather. — Several workers (159. 267) have 

 stated that corn smut is much more prevalent in dry 

 seasons. Potter and Melchers (267 > reported that 

 smut of corn was far more abundant and destructive 

 in the western dry and hot portion of the plains in 

 Kansas and Nebraska than in the more humid regions 

 of the U.S.A. They did not believe that lack of mois- 

 ture was the limiting factor in smut development, 

 whereas MacMillan (205 ) states that it is the limiting 

 factor in semiarid areas. 



At St. Paul. Minn., the effect of certain environ- 

 mental factors on prevalence of smut was studied from 

 1922-27. inclusive (159). About 140 selfed lines of 

 corn were inoculated artificially by repeated applica- 

 tion of chlamydospores. The results indicated that 

 dry weather, as expressed by a low number of days 

 of precipitation and a high percentage of sunshine, 

 was conducive to development of smut. During these 

 tests, temperature did not appear to be a very im- 

 portant factor (159 ). 



Temperature. — Relatively high temperatures favor 

 chlamydospore germination, budding of sporidia. 

 growth of mycelium, and formation of chlamydospores. 

 Most writers (281. 290. 336) agree that 25 C C or 

 above is conducive to development of the smut fungus 

 in the host. Rowell and DeVay (281 ) obtained much 

 better infection at 28' C than at 18 c or 21°C, whereas 

 Schmitt obtained no gall formation below 21 C C (290). 

 Tisdale and Johnston (336 > also found that tempera- 

 tures of 25 : C or above were conducive to infection, 

 whereas temperatures of 20 C C or below were one 

 of the chief limiting factors. It is well known that 

 smut galls develop and chlamydospores mature much 

 more rapidly at relatively high temperatures than 

 at low. 



Conditions that favor or prevent the initial entrance 

 of the smut fungus appear to be of much greater im- 

 portance in creating an epidemic of smut than those 

 that favor or hinder the fungus development after 

 infection. 



Host vigor. — Many workers (5, 193. 298) have 

 stated that vigor of the host was directly associated 

 with susceptibility. Contrary to the common belief. 

 corn smut appears to be least prevalent when the 

 growing season favors steady and normal growth of 

 corn (351). Walter (351) made a detailed and ex- 

 tensive study over a 4-year period on the vigor of 

 the host in relation to smut development. He concluded 

 that sustained rapid development of the corn plant 

 from the seedling stage to full-size plants enabled 

 them to resist or escape the smut. In Europe, similar 

 results were obtained by Maze and Maze (215). It 

 is true that, frequently, the galls are larger and usually 



II 



