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94 BULLETIN 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, P1066, and 
P1068, 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 XI. 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. 3 
Kanred, P1066, and P1068 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 entirely 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 Puccima 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 freedom from any macroscopic evidence of rust infection 
or to designate the inability of the rust fungus to sporulate. 
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