94 Mciuldliin I iiln i'iUdh-c and Yilhnv Riisl in WIk tit 



APPENDIX. 



An allempl. lo estimate the Reduction of Yield due to . 

 Yellow Rust attack. 



Various estimates have been given as to the probable loss of fjrain 

 due to rust attack, but most of these appear to be little more than guesses. 

 It is, of course, certain that rust attack may be so severe as to destroy 

 the host plants altogether, but such super-susceptible races do not enter 

 into general cultivation in countries where they are Hable to attack. The 

 question to be answered is, What is the approximate reduction in yield 

 of those varieties which, though capable of being generally cultivated 

 are nevertheless subject to a moderate or occasionally a bad attack? 



A favourable opportunity occurred to test this point in 19'20. 



A variety of wheat known as "Jap" has been grown on small plots 

 at Cambridge for the last 15 years. It was formerly used as a parent, but 

 ill recent years has been discarded for tiiis i)ur|)ose owing partly to the 

 inferior nature of its straw and jiartly because of its susceptibility to 

 V'ellow Rust. A small culture has, however, been grown on each year, 

 and in 1918, when the parent stocks were examined as to their com- 

 parative susceptibility, it was noticed that the "Jap" culture consisted 

 of plants which varied widely in this respect. Looking at the plot as 

 a whole, it appeared to be suffering from a moderate rust attack, but 

 when examined in detail, certain plants were seen to be rust-free. A final 

 inspection of the culture in July showed that it contained 38 rusted and 

 14 rust-free individuals. Six of the rusted and four of the rust-free 

 plants were saved, and from these separate cidtures were grown in the 

 following year. Owing to the extremely favourable conditions for rust 

 attack in 1919 the cultures raised from the rusted plants were attacked 

 with great severity, and even those grown from the rust-free plants had 

 a slight attack. The relative difference in their power of resistance 

 appeared to be the same as in 1918. All the cultures were of the "Jap" 

 type as regards straw, ear-shape, etc., and if morphological diflerences 

 existed they were too slight to be apparent to the naked eye. The only 

 observed difference apart from rust resistance was in the period of "ear- 

 emergence," there being some 10 days between the earhest and latest in 

 this respect. Rust susceptible and resistant cultures were included in 

 both the early and late groups. 



One plant (3/16/29) was saved from a pure susceptible culture (3/16), 

 and also one plant (1/4/10) from a resistant culture (1/4), and a plot of 

 each was raised in 1920; a similar plot was grown from the original 



