24 BITLLETI]Sr 1046, U. S. DEPARTMENT OF AGRICULTURE. 



under field conditions is of first importance from the agronomic and 

 plant-breeding viewpoint. The growing of wheat varieties in the 

 rust nursery places them under conditions at least as severe as those 

 to which a commercial field is subjected in a natural epidemic. 

 Field tests must finally determine the value of any variety. 



EVIDENCE OF SPECIFIC RUST RESISTANCE. 



In the studies made of the resistant varieties,, Kanred, PIO66, and 

 PIO68, it was noted that their reaction to rust infection was entirely 

 different from anything that had been observed in any other variety. 

 Such varieties as Khapli emmer (C. I. No. 4013), White Spring 

 emmers (C.I.Nos, 1524 and 1526 and Minnesota No. 1165), and the re- 

 sistant durums Kubanka (C. I. No. 2094) and Arnautka (C. I. No. 

 1493) are known to show resistance in the seedling stage in the 

 greenhouse as well as under field conditions. Their resistance in 

 the seedling stage is shown by the formation of relatively small 

 uredinia, surrounded by yellow or yellowish white areas, the occur- 

 rence of minute brown or yellowish dead areas, the presence of 

 yellowish islands, or other characters generally regarded as indicative 

 of resistance or hypersensitiveness. This evidence of resistance is 

 illustrated in Plate XL All the spring-wheat varieties which the 

 writers have studied and which are classed as highly resistant show 

 such reactions to rust infection, and almost always very distinct 

 uredinia, though frequently small, are formed in inoculated seedlings 

 of these varieties. 



Kanred, PIO66, and PIO68 are unique in their behavior toward 

 Puccinia graminis tritici, as hundreds of seedlings and of culms in 

 the heading stage have been inoculated with this strain of rust and 

 not a single uredinium ever has been observed. The enthely rust- 

 free and unflecked inoculated leaves of Kanred and P1066 are illus- 

 trated in Plate XI, fig. 1, A and B. These varieties may be said 

 to be immune ^° from this particular stem rust, if it be assumed that 

 the controlled conditions provided in the greenhouse are as favorable 

 and that exposure to infection is as severe as under natural field 

 conditions, and that seedling inoculations are as severe a test as can 

 be given to a variety. They are certainly more strikingly resistant 

 to Puccinia graminis tritici than any other varieties of common 

 wheat {Triticum vulgare) which have been studied by the writers. 



Because of this specific behavior these varieties have been used 

 as differential hosts in separating certain biologic strains of stem 

 rust of wheat. The inoculation studies with these varieties have 

 been carried over a long period, including every month in the year, 



10 The word "immune" is here used to mean freedoni from any macroscopic evidence of rust infection 

 or to designate the inability of the rust fungus to sporulate. 



