998 
Journal of Agricultural Research 
Vol. XXIV, No. xa 
THE PRESENT STATUS OF THE PROBLEM 
For reviews of earlier studies which have led to the present mode of 
attack, the reader is referred to previous papers (j, 4). In the present 
paper it seems sufficient to sum up the facts now at hand without making 
an extended review of literature. 
Thirty-seven biologic forms of stem rust have been discovered (d), 
and of these 21 have been found in the upper Mississippi Valley. If the 
resistance of a wheat variety to each of these biologic forms was due to a 
separate set of independently inherited factors the problem would be a 
very difficult one. If the resistance to one biologic form was due to a 
genetic factor which was allelomorphic to a factor for resistance to a second 
form the problem of obtaining a wheat resistant to all biologic forms 
would be an impossibility. No variety of common wheat has been found 
which is resistant to all biologic forms of rust; but Khapli, an early 
maturing emmer, is resistant to all 37 forms. 
Crosses of Kdiapli with common varieties have given self-sterile F^ 
progeny. This has led to the attempt to transfer the resistance of Khapli 
to a durum variety, with the hope of then transferring this resistance to 
common wheats by a subsequent cross. 
That the resistance of durum wheats can be transferred to common 
wheats has been shown. Several years ago a cross was made between 
lumillo and Marquis. There was found to be a strong linkage between 
the durum characters and resistance although this was overcome by 
growing large numbers. Several plants were obtained from an Fg 
population of from 20,000 to 30,000 plants which appeared to have the 
spike characters of common wheat and which also appeared rust resistant. 
Selection has been practiced further and the wheats thus obtained 
appear of desirable field type and have shown high resistance in field 
experiments. 
Kamed, a rust-resistant winter wheat, produced at the Kansas sta¬ 
tion, proved immune from ii of the 21 biologic forms of rust which are 
found in the North-Central States. This immunity was transferred to 
spring wheats by crosses between Kanred and Marquis. The immunity 
from all ii biologic forms was apparently due to a single genetic factor 
(i, 2). _ 
Puttick (5), in a study of the Fg progeny of a cross between Mindum 
and Marquis, obtained seedling wheats which combined the resistance of 
both parents. In this study, two biologic forms of rust were used to 
which Marquis and Mindum reacted reciprocally, one parent being 
resistant and the other susceptible to the respective rust forms. Put- 
tick, however, did not grow the seedlings to maturity so it was impossible 
to say whether they were of common or durum types. 
In the paper preceding this, Harrington and Aamodt (2a) have pre¬ 
sented the results of a cross between Pentad, which is resistant to biologic 
Form XXXIV and susceptible to biologic Form I, and Mindum, which reacts 
in a reciprocal way to these rust forms. The resistance of Pentad to 
Form XXXIV and of Mindum to Form I appears to be due to a single main 
genetic factor in each case. These two factors appear to be independently 
inherited and in the production, from the cross Pentad X Mindum, of a 
wheat which is resistant to boUi biologic forms, it is only necessary to 
grow enough plants to obtain the necessary recombination of the two 
factors for resistance. 
