994 
Journal of Agricultural Research 
Vol. XXIV, No. 12 
Fg families to Form XXXIV gave some indication that a single factor 
was present, but dominance was not apparent. No evidence of a correla¬ 
tion between seed color and rust reaction was found. 
As the action of each biologic form in general was fairly uniform on 
individual plants of the differential varieties and also on the plants of 
Kubanka No. 8, Mindum and Pentad and the Fg families, barring the 
X— and x+ types of infection, it may be concluded that the uredi- 
niospores of each form probably were of the same genetic constitution. 
Therefore, the factors concerned with susceptibility and resistance must 
have been located in the plants themselves. Furthermore, as different 
biologic forms differ in their parasitizing capabilities on the same varie¬ 
ties of wheat, their genetic constitution can not be the same. It is prob¬ 
able, then, that factors governing susceptibility and resistance to differ¬ 
ent biologic forms used in this experiment are different in nature and 
are located in different chromosome pairs. The results of the present in¬ 
vestigation show that when two varieties of wheat react reciprocally to 
two biologic forms of P, graminis tritici, one variety being resistant to one 
form and the other to the other form, it is possible by means of crossing 
these varieties to obtain progeny resistant to both forms of rust. These 
results furnish further evidence for the belief that eventually a variety 
of wheat can be produced which will be resistant to all biologic forms 
of this rust. 
SUMMARY 
(1) A study was made of the parasitic capabilities of two biologic 
forms of Puccinia graminis tritici on the Fg progeny from crosses between 
three varieties of Triticum durum. 
(2) Two of the parental varieties, Mindum and Pentad, reacted 
reciprocally to two of the biologic forms used. 
(3) The constancy of the parasitic capabilities of the biologic forms 
had been determined previously and, as additional evidence, complete 
sets of differential varieties were inoculated at the commencement and 
at the completion of the work with each form. Form XXXIV attacked 
Kubanka No. 8 but had little effect on Pentad. Form I produced no 
uredinia on Mindum but developed vigorously on Pentad. On the other 
hand. Form XXXIV infected Mindum severely but developed weakly on 
Pentad. 
(4) A separate set of 8 to 20 plants from each Fg family was grown 
for use with each form of rust. Only first leaves were inoculated. 
(5) With respect to the reaction of the Fg families to each of the two 
biologic forms of rust there were some families that were as resistant 
as the resistant parent, some as susceptible as the susceptible parent, 
and others which were heterozygous. 
(6) The results of using Form XXXIV on Kubanka 8 X Pentad Fg 
families indicated the presence of two differential factors. 
(7) A single main factor difference explained fairly satisfactorily the 
results obtained for Mindum X Pentad Fg families inoculated with 
Form I. 
(8) The results obtained when Mindum X Pentad Fg families vrere in¬ 
fected with Form XXXIV, gave some indication of the presence of a 
single factor. The data would not permit, however, a definite genetic 
analysis. 
(9) No relation was found between seed color and rust resistance. 
