68 Mendellaii Inheritance and Yellov Rust in Win ni 



(15) Spread of ru.sl during 1!U9. 



Yellow Rust W.1S observed for the first time on May 8tli, a luontli 

 later than in tlie previous year. During May, and up to about the 20th 

 of June, its s])rea(l was very slow, except in the case of very susceptible 

 varieties and cultures. This difference in regard to the "|ieriod of in- 

 fection" was very remarkable. For example, a variety of wheat which 

 may be referred to as "B"" was grown alongside tlie f\ cultures, and 

 proved to be extremely susceptible to Yellow Rust. Se\'eral of the 

 plants were rusted by May 16th, i.e. fairly early in the period of drought. 

 By May 24th almost every plant bore numerous pustules, and on the 

 31st of May, out of 136 plants in the plot, 135 were excessively rusted. 

 A large plot of another variety (Sudanese wheat) was grown in tlie 

 neighbourhood, and this was also attacked very early by Yellow Rust. 

 This attack was so severe that, by the middle of .Tune, all the plants were 

 prematurely destroyed, and no grain was formed. 



These cases are here referi'ed to because the attacks developed with 

 great intensity durivg n period of drought, and afforded a striking contrast 

 to the scarcely perceptible attacks made at the same time on moderately 

 susceptible varieties. They demonstrate that the more susceptible varieties 

 are liable to an earlier successful attack than less susceptible kinds; and 

 also that, on very susceptible wheats at least, Yellow Hu.st is quite 

 capable of making rapid progress in the tissues of tlie liost during the 

 hottest weather we are likely to experience in this country. 



Similar differences as regai-ds the period of infection were observed 

 among the Fg cultures. By June 14th rust was spreading rapidly on all 

 those cultures which finally proved to be pure susceptibles; on the others, 

 there was either no rust at all, or its progress was very slow (see Table XI). 



In order that all the plants might be exposed to an equal chance of 

 infection, on the evening of June 13th all the cultures were sprayed with 

 water teeming with fresh uredospores of Yellow Rust obtained from 

 a neighbouring crop. This procedure was {>robal)ly quite unnecessary, 

 since the pure susceptible cultures (already containing many attacked 

 plants) were well scattered over the entire experimental area. Almo.st 

 immediately after the fall of rain on June 2()th, a distinct increase in 

 the rate of rust-spread was noticed, and during the cool, moist weather 

 which prevailed in July tiie attack developed into an epidemic of llie 

 greatest severitv the writei' has ever seen. 



