[newton] wheat stem RUST 167 



In Diagrams 3, 4, 5, 7, 9, 10, 11, and 12, it will be seen that the 

 rust collections in question proved to be composed of but one form. 

 It should be pointed out, however, that these cases have been de- \ 



liberately selected to simplify the presentation of these illustrative 

 data; the circumstance of a collection consisting of only one form 

 was unusual. The average condition is better represented in Dia- 

 grams 1, 6, 8, 13, and 14, where the collections consisted of 2 forms. 

 Occasionally a collection yielded 3 forms, as in the case shown in 

 Diagram 2. 



It will be seen in the latter group of diagrams that the trans- 

 ference of the rust of a given collection to the test wheats frequently 

 resulted in the appearance of 2 forms of pustules, small and large, 

 a " 1-4" infection. It was then assumed that a mixture of forms was 

 present, resistance of the host to one form being represented by "1" 

 and susceptibility to the other form by "4." The next step was to 

 separate the forms. Accordingly spores from the small and large 

 pustules were transferred separately to different plants of the same 

 host variety. If practically all the pustules resulting from this 

 inoculation were small in the case of the one host plant, and large in 

 the case of the other, the presence of at least 2 forms in the original 

 rust collection was practically confirmed. These results were always 

 checked by further inoculations on all the differential hosts given in 

 the key, a procedure which, as previously noted, served also for the 

 final identification of the forms. 



The reactions of Forms XXIX, XXX and XXXII are still 

 imperfectly understood. They always give two degrees of infection, 

 " 1-4," on the Arnautka, Mindum, Speltz Marz and Kubanka varieties 

 of durum wheat; whereas, on all the remaining differential hosts they 

 behave normally as pure forms. For two years every effort has been 

 made to separate these apparent mixtures, but in vain. Cultures 

 made from a single spore of each form still failed to resolve the com- 

 ponents.2 It has been suggested that these forms must be hetero- 

 zygous, or perhaps homozygous with a genetic composition resulting 

 in this type of infection. It seems to the writer at least equally 

 possible that the aberrant behaviour is connected with the physio- 

 logical relationship of the fungus and the host, perhaps even arising 

 from the genetic composition of the host itself with regard to rust 

 resistance. However, the problem must await further investigation, 

 and perhaps the development of more refined technique, before it can 

 be explained. 



'^Unpublished data from E. C. Stakman. 



