46 



INFECTION OF INDIAN MILLET. 



In passing over now to the smut of Indian millet, to Ustilago sorghi (cruenta), we have 

 a different form of disease in host plants, which forms spore masses exclusively and only in 

 the inflorescences. Infection most undoubtedly takes place here generally in the germinating 

 seedlings of fresh seed, even when the spore masses appear first in the flower panicles of the 

 mature plants. In earlier experiments, when infections were carried out in sufficiently young 

 germinating seedlings, as high as 70% of smutted plants was obtained. In later experiments 

 these results were repeated in approximately the same way^. However, it was assumed earlier 

 that infected plants could not develop spore masses if they outgrew the smut germs by too rapid 

 development so that these could not reach the vegetative tips. However, in all cases it could be 

 proved that infection had taken place in those plants by the fact that fungus mycelia were 

 shown in the nodes of the grass and in the parenchyma cells and that it had been retarded and 

 had not reached the vegetative point nor passed over to the formation of spore masses^. 



In order to prove that infection had actually taken place, these plants were cut back to 

 two thirds of the height of the axis, as soon as it was seen that the panicle of the tip was 

 healthy. By this means the formation of axilla ry sprouts was caused and this formation takes 

 place, as described earlier in Part XI, at those points in which mycelia exist enclosed in the 

 parenchyma cells of the nodes. By this new formation of tissue for the axillary sprouts, paren- 

 chyma cells were affected which harbored the mycelia. These can here penetrate through the 

 young tissue and reach the vegetative points. It is now seen that in accord with the experi- 

 ments carried out previously with other plants, these axillary sprouts became smutted. There- 

 fore smut can be brought to development in apparently healthy plants as if by a stroke of magic, 

 if the apical healthy inflorescence is removed early enough, thereby causing the possibility of 

 an axillary sprout formation. In our climate this occurs very rarely in sorghum plants when 

 not artificially introduced. It can, however, take place, and with the result that the smut infec- 

 tion already existing will be subsequently proved by the smutted axillary sprouts. However, 

 these earlier experiments are capable of being multiplied from still another point of view. If 

 it is true that smut germs, which have already pressed their way in, are frustrated by too 

 rapid growth of the host plants and that thereby the decrease in the percentage of smutted 

 plants is brought about, the question arises whether this too rapid development of the host plant 

 cannot be restricted. This is most easily possible if seed older than that of the previous year 

 is used, the grains of which have weakened more or less in germinating energy with increasing 

 age. The young germinating embryos, whose development is retarded by delayed germination, are 

 most excellent material for further experimental infection of the young germinating seedlings. 

 This inoculation was carried on by means of an atomizer by the spraying with spores which had 

 stood a day in nutrient solution and which thus were brought to direct germination. It was 



(1) Compare the text of Part XI, pages 43-51. 



(2) Compare fig. 7, plate I, Part XI. 



