July 8,1918 Can Biologic Forms of Stemrust Change Rapidly 9 
121 
Observations in the rust nursery extending over a period of seven 
years also serve to strengthen the idea that rust resistance is a definite 
character present in a greater or less degree in particular varieties and 
not greatly influenced by cultural conditions. It has not been possible 
to build up resistance within a susceptible line by continuous selection 
nor to isolate resistant plants from such a line. Since 1915 several 
hundred varieties of all degrees of susceptibility and resistance have 
been grown in the same nursery with hybrids of the F 1? F 2 , and later 
generations and always under optimum conditions for infection—that is, 
in a severe epiphytotic of stemrust. No instances have been observed 
of a resistant variety being attacked by a form of stemrust able to cause 
severe infection, as would be expected if the hybrid plants near by 
could produce such a rust form. There are seasonal fluctuations in the 
severity of the rust attack, but the greenhouse experiments here reported, 
the experimental work in the rust nursery, and the field observations in 
several States with resistant varieties of spring and winter wheats, and 
with oats, all point to the conclusion that both rust and host are rela¬ 
tively stable. It seems more likely that resistant varieties will be only 
of regional value because of the occurrence of different biologic forms 
in various regions. 
There seems, however, no basis, from the facts now at hand, for the fear 
expressed by Pole Evans that these hybrids once produced will not only 
gradually lose their own power to resist attacks of the rust, but will 
also give the parasite new infection capabilities, enabling it to cause greater 
injury in susceptible varieties and even to attack previously resistant 
varieties. 
SUMMARY 
(1) Neither barley nor club wheat enabled Puccinia graminis tritici - 
compacti to attack resistant common wheats or other resistant cereals 
more vigorously than normally. 
(2) P. graminis iritici-compacti was confined to barley and resistant 
wheat for a number of successive generations, but it did not acquire 
increased virulence for these hosts. 
(3) The parasitism of P. graminis iritici-compacti was not changed by 
bridging hosts nor by association with a given host. 
(4) P . graminis tritici was used to determine the possible action of 
hybrids as bridging forms. Studies were made of F^ F 2 , and F 9 hybrids 
in comparison with their resistant and susceptible parent varieties. 
(5) Susceptible plants of the F t generation of the cross Haynes 
Bluestem (Minnesota 169) X Kubanka (Cl 2094) did not enable the 
rust to infect seedlings of the resistant parent normally, nor to infect 
the susceptible parent more virulently. 
(6) The culture of stemrust on susceptible plants of the F 2 generation 
of the cross White Spring emmer (Minnesota 1165) X Marquis had no 
appreciable effect on the parasite. 
