62 REPORT OF THE AGRICULTURAL RESEARCH 



appeared early and spread uniformly, without much rela- 

 tion to position in the field or density of the crop. Later 

 on it became more intense where the crop was heaviest and 

 tallest, but was soon masked by yellow rust (Puccinia 

 glumarum). This did not appear in any quantity until the 

 crop was nearly full grown but then developed with great 

 intensity wherever the crop was tall and dense. Black rust 

 (Puccinia graminis) was late in appearing and did not 

 show any regularity in distribution. In February and 

 March, Dr. Leather kindly made some determinations of 

 soil moisture and air humidity in two plots, one with a 

 light, thin crop where rust was slight, and the other where 

 the crop was heavy and rust severe. The soil moisture was 

 much better in the latter, especially from the 3rd to the 

 6th foot in depth. The humidity was taken within the 

 crop, about 4 inches above ground level, the first test, lasting 

 for 7 to 8 hours, being made on February 14th, in dull and 

 humid weather. In the thin crop the relative humidity 

 was 57*3 per cent, and the mean air temperature was 26° C. 

 In the heavy, badly rusted crop, the relative humidity was 

 76-7 per cent, and the mean air temperature 23° C. A 

 second test was made on February 21st, the crop having 

 been much laid by a storm on the 15th and the day being 

 fine with a dry west wind blowing. No appreciable differ- 

 ence was found in the two plots. 



The intensity of the attack of yellow rust and the later 

 stages of orange rust thus varied directly with the soil 

 moisture and the air humidity within the crop. The early 

 stages of orange rust would not be exposed to the influence 

 of the latter factor, as at that period the crop is too small 

 to cover the ground closely. By the time black rust 

 appears, transpiration has much diminished, as the crop 

 is then almost ripe; the air temperature is higher and the 

 drying west winds have usually set in. Hence the late 

 rust is not much exposed to differences in air humidity. 

 Of the two factors the immediately important one is 

 apparently the humidity of the air within the crop, but 



