Recently, it has been established that most of the 

 infection apparently occurred during the leaf-whorl 

 stage of the corn plant, when the plants were 30 cm-1 m 

 tall (71, 267. 351. 35S). Then, the meristematic tissue 

 is only a few cm long and no appreciable elongation of 

 the internodes has taken place. Multiple galls may 

 arise on different parts of the plant from a single infec- 

 tion of the meristematic tissue in the spiral. In such 

 cases, the distribution of the fungus is actually brought 

 about by the host during the elongation of the stalk. 

 The relationship between the host and the fungus is not 

 one of true systemic development. 



The germ tubes from either the sporidia or promy- 

 celia usually gain entrance by direct penetration, al- 

 though sometimes they penetrate through stomata and 

 floral organs. The mycelium is mostly intracellular, 

 although intercellular hyphae are sometimes common. 

 The holes in cell walls through which the hyphae pass 

 are small and the bulging and curling of the enlarged 

 hyphae in the cell strongly suggest mechanical penetra- 

 tion (350). 



Haploid infection. — Although haploid hyphae can 

 penetrate the host tissue, dicaryotic or diploid hyphae 

 are essential for normal infection and development of 

 chlamydospores. Rawitcher (273) assumed that the 

 haploid phase was the parasitic stage of U. maydis, but 

 this is not supported by more recent work (59. 123). 

 Several workers found that haploid hyphae in the host 

 were rather limited in growth. The amount of haploid 

 growth in the host depends on the line of U. maydis 

 involved. 



Munnecke (236) found that certain haploid lines 

 induced curling and distortion of the host somewhat 

 similar to the hyperplastic phase of normal infection, 

 but no galls or chlamydospores were formed. Haploid 

 infection of the tissues was proven cytologically and 

 also by the reisolation of the original lines. 



Several investigators ( 234. 338. 363 ) have shown that 

 U. maydis synthesized indoleacetic acid and possibly 

 other auxins in culture and these substances most likely 

 are related to gall development. Therefore. DeVay (77) 

 attempted to supplement the missing stimuli essential 

 for gall formation by inoculating corn seedlings with 

 one haploid line plus the filtrate from a second line of 

 opposite sex. Although the degree of distortion was in- 

 creased by addition of the filtrate, no chlamydospores 

 were produced. 



Rowell and DeVay (28,2) demonstrated that smut 

 galls and chlamydospores were formed when they 

 paired certain haploid lines of U. maydis with specific 

 haploid lines of S. reiliana. Since the lines did not fuse 

 to form a dicaryon. some synergistic stimulus must have 

 been involved in this unique phenomenon. The haploid 

 smut galls in corn developed slower than those nor- 

 mally produced by compatible lines of U. maydis. No 

 evidence of hybridization was found for any characters 

 that were studied. The size, echinulations. and germina- 

 tion of the spores were all similar to those of the U. 

 maydis. All monosporidial isolates obtained from the 

 spores were identical to the original line of U. maydis 

 in morphology, cultural characters, and sex factors. 

 None of the isolates produced galls when paired with 

 one another or with the original I', maydis line. These 



isolates were mated to the same 5. reiliana lines and 

 again produced galls with mature haploid chlamydo- 

 spores (282). 



Composite infection. — There is no evidence that smut 

 infection may be materially increased by heavy applica- 

 tion of inoculum consisting of many biotypes. Some- 

 times, it may actually cause a decrease in severity. The 

 reason for this is not definitely known, but severe 

 chlorosis and necrosis often follow mass inoculation in 

 the field (351 I. 



Kernkamp and Martin (181 i compared the degree of 

 pathogenicity of 13 paired and compatible lines with 

 that of the composite inoculum. The severity of infec- 

 tion of the composite inoculum approached the average 

 severity of the single pairs of haploid lines. Likewise. 

 a mixture of diploid lines alone or in combination with 

 haploid lines did not increase the severity of infection. 

 Wilkinson and Kent (358) and Rowell and DeVay 

 (282) obtained similar results. 



Formation of galls. — The parasitic stage of U. maydis 

 is a dicaryon. Therefore, if the infection results from 

 fusion of compatible haploids of opposite sex. there is 

 formed a dicaryotic hypha with a stout and rapidly 

 growing mycelium. The mycelium penetrates only a 

 short distance, but ramifies freely in the infected area. 

 Then, the host cells in the invaded tissues begin to 

 multiply and enlarge at an extraordinary rate. Under 

 favorable conditions, the young galls may become 

 visible in a few days and mature chlamydospores ma- 

 ture in 7-9 days after inoculations. Scurti (296) states 

 that the swelling began in advance of hyphal invasion 

 by the fungus. The phenomenon involves both hyper- 

 trophy and hyperplasia ( 1S4. 297). In addition to the 

 smut mycelium and spores, the galls usually consist of 

 a considerable amount of modified tissue of the cortex, 

 xylem. phloem, parenchyma, and sclerenchyma strands. 



The abnormal activity of the infected tissue is due 

 to some active secretion by the fungus. Wolf (362). 

 Turian (338). and others believe that the production of 

 indoleacetic acid by U. maydis stimulates the develop- 

 ment of smut galls. They found that young galls had 

 about 20 times greater concentration of indoleacetic 

 acid than noninfected tissues. 



Types of hyphae. — There are several different types 

 of hyphae in the host and this may in part account for 

 some of the discrepancies between cytological studies 

 i 55. 60. 85 ). Haploid hyphae are rather fine, usually 

 uninucleate, and not extensively developed. Rawitscher 

 1 273. 2 74; was of the opinion that the parasitic 

 mycelium was uninucleate and haploid until about 

 chlamydospore formation. Ehrlich (85) suggested that 

 haploid hyphae sometimes may continue to grow in 

 the presence of dicaryotic mycelium. In culture, 

 sporidia and hyphal cells, whether haploid or diploid, 

 are almost universally uninucleate. 



There appear to be several types of dicaryotic hyphae 

 in the host tissue (55, 85). The most common one 

 is very irregular and variable in shape, frequently 

 branched, often contorted, and hence difficult to follow 

 in the host. The second type is rather stout, but usually 

 fairly uniform and rather extensive in growth. 



The third type of hyphal growth is a much contorted 

 cluster-like aggregation, somewhat suggestive of a grape 

 cluster. These spherical bodies are apparently composed 



17 



