[newton] wheat stem RUST 195 



Another possible v/ay in whicii an observer may be misled with 

 regard to the constancy of biologic forms is in the interpretation of 

 morphological variations in the urediniospores. Resistant host 

 plants, and unfavourable cultural conditions affecting the develop- 

 ment and vigour of the fungus, may cause the urediniospores formed 

 to be appreciably smaller. However, as soon as the rust is returned 

 to a congenial host the spores developed are normal in size from the 

 outset. No more significance is to be attached to such variations in 

 size than can be attached to variations in the size of the wheat plant 

 itself when grown in different soils of varying degrees of fertility. 

 These variations within a given form are not to be confused with true 

 morphological distinctions between different biologic forms, of which 

 reports have been published by Stakman and Levine (55) and Melchers 

 and Parker (45). 



The 6 biologic forms of rust collected in the area mapped in 

 Fig. 1, B. deserve the first attention of Canadian plant breeders. 

 They cover practically the whole of the main wheat-growing areas 

 of the West. All 6 forms cause heavy infection, "3-4," upon all the 

 bread wheats, except upon Kanred, a winter wheat. There is, in 

 fact, a tendency for all the hard spring wheats to act as a group with 

 reference to resistance or susceptibility to any biologic form, or group 

 of forms. Their common susceptibility to this important group of 

 biologic forms is, of course, unfortunate; but, on the other hand, the 

 tendency to a common reaction towards a group of forms gives reason 

 to hope that when a spring wheat is produced which is resistant to 

 one of these 6 forms, it will prove resistant likewise to the others. 

 This hope appears the more reasonable in that Kanred is consistently 

 immune to all members of this group. Since these forms included 

 70 per cent, of all the rust collections made, and since the bread 

 wheats comprise practically the whole of the Canadian wheat crop, 

 it is apparent that the production of a spring wheat resistant to this 

 group, and satisfactory in its agronomic and milling qualities, must 

 potentially effect a tremendous reduction of the annual losses from 

 wheat rust. 



That rust resistance is an inherited character was conclusively 

 proved by Biffen (4). Recently, Puttick (47) attempted an analysis, 

 from the genetic standpoint, of the reaction of the F2 generation of a 

 cross between a common and a durum wheat to two of the biologic; 

 forms of Puccinia graminis isolated by Stakman and Levine. The 

 parental plants were in each case resistant to one of the biologic forms 

 and susceptible to the other, reacting reciprocally in this respect. 

 The author in his summary states that, "All combinations of sus- 



