RUST-RESISTANT VARIETIES OF WHEAT 31 



Variety Incubation Period 



Minnesota No.' 163 7 days 



lumillo 1736 9 days 



Emmer 1522 11 days 



Arnautka 288 12 days 



Khapli 14 days 



It will thus be seen that the incubation period is longer on the more 

 resistant varieties. These figures are, of course, not absolute, but vary 

 with the temperature, and, to some extent, with soil conditions. All 

 however, vary in nearly, but not absolutely, the same proportion. 



The character of infection is distinctly different on the different 

 varieties. It is quite noticeable that the same phenomena are observed 

 as appear on various biologic forms. On Minnesota No. 163 the pus- 

 tules are large, varying from 2 to 6 millimeters in length. They rup- 

 ture the epiderm very readily and shed spores in great abundance. 

 Very rarely are small, unruptured pustules developed. The host tissues 

 nearly always remain fairly healthy, a yellowing which gradually ap- 

 pears furnishing the external evidence that the fungous hyphae are in 

 the plant. On einkorn and lumillo, which in the greenhouse are only 

 fairly resistant, the pustules are usually somewhat smaller. There is 

 a tendency in these two forms toward the development of small dead 

 areas. These areas are either very distinctly yellowed or sometimes 

 killed outright. The general appearance is, however, usually not 

 sharply different from that of infected Minnesota No. 163, except in 

 degree of infection as evidenced by smaller pustules on einkorn and 

 lumillo. On emmer there were often long infected, yellow areas in 

 which there was a fairly large number of very small pustules, usually 

 less than one millimeter in length, many of which never ruptured. 

 Then again fairly large areas of host tissue were practically killed and 

 only a few small green islands developed. On both Arnautka and 

 Khapli, areas from one to*two centimeters long were killed, the leaf 

 appearing white and dead (see Plate VI, A). In these areas there was 

 often a moderately large number of "green islands" with very small, 

 unruptured pustules in the centers. When the pustules did rupture they 

 were always very small, seldom, if ever, exceeding one millimeter in 

 length and often being mere dots. The large areas involved can be 

 explained rather on the basis of multiple infection than on the basis of 

 the spreading of the mycelium from a few infections. Histological 

 examination of diseased areas verifies this supposition. 



The spores on the resistant varieties were smaller than on Minne- 

 sota No. 163. Spores of Minnesota No. 163 averaged 35.38x21.39 /*. 

 while those of emmer were 33.04 x 21.30 //. The Khapli spores were 

 smallest, being only 29.69 x 20.68 >w. It was found that spores from 

 different pustules varied somewhat in average size. Therefore spores 



