Methods in Rust Investigations 251 



cies or varieties of plants, it is very important that precautions be taken 

 to prevent contamination of the cultures. Plants upon which these cul- 

 tures are to be sown are grown in a greenhouse or compartment where 

 the rust under investigation is not present. If the rust is very prevalent 

 out of doors, as is the case with Puccinia tHticina in the summer, it is 

 necessary to grow such plants under very confined conditions, such as 

 in a Wardian case (fig. 3), in order to protect them from wind-blown 

 spores from without. 



It would be highly desirable in such studies if methods as refined 

 as those used in bacteriology could be employed. However, no nutrient 

 medium has been devised upon which a rust can be grown. It is neces- 

 sary to grow it on its living host and under the conditions which are 

 favorable for the growth of such a host. Usually the confined condi- 

 tions necessary to be absolutely sure there is no chance for contamina- 

 tion are not favorable for the best development of the host. In conse- 

 quence, various expedients have to be employed. As the rusts primarily 

 are wind-borne diseases, it is essential that direct air currents be avoided. 



For this purpose, the partitions on the greenhouse bench made of 

 a double thickness of muslin, and the cages of similar material used by 

 Stakman, Piemeisel, and Levine (1918) have been found good. How- 

 ever, for Puccinia triticina the plant covers (fig. 2) described above 

 have been found better, there being less chance of contamination. Where 

 larger numbers of plants are being inoculated than these plant covers 

 will hold, Wardian chambers (fig. 3) are used. These are made entirely 

 of glass, the top being on the order of a hotbed sash so that a spray of 

 water can be run on the chambers in the summer in order to lower the 

 temperature. During the winter the sash usually is raised, the indi- 

 vidual compartments being curtained off with muslin. 



In the study of Puccinia triticina for the presence of specialized 

 strains, some method was sought which would permit the growing of a 

 fairly large number of single-spore cultures of the rust organism within 

 a small space. The plan finally adopted was to sow wheat in four-inch 

 pots filled with soil to the top. A lantei'n globe, covered over the top with 

 a thin layer of cotton held in place with cheesecloth, was then pressed 

 down into the soil as deeply as possible (fig. 4, D). The wheat coming 

 up within this lantern globe is protected from wind-blown spores. Cul- 

 tures of rust in such lantern globes can be carried from place to place 

 without distributing spores or contaminating the cultures themselves. 



During the winter months such leaf-rust cultures are maintained 

 in good condition and transfers are made about every three or four 

 weeks. During the summer, however, the high h midity and tempera- 

 tures in such lantern globes are very favorable f;r the development of 

 other fungi, especially Helminthosporium, which often kills the wheat 

 and makes it difficult to carry the cultures through the summer. If 

 only a few pots of seedlings are to be inoculated the spores developed 

 in these lantern globes are sufficient. Where it is desirable to inoculate 

 a large number of varieties at one time, so as to have all under as nearly 

 the same set of conditions as possible, it is necc -oary to multiply the 

 culture. The rust spores are first sown on wheat under another lantern 

 globe in order that a pure culture may always be available. The remain- 



