60 THE RUSTS OF GEAIXS IK THE UXTTED STATES. 



Puccinia graminis both in Texas and Minnesota. Hundreds of 

 wheat plants in field conditions were inoculated with spores of 

 P. graminis by pouring over the head and culm water filled with 

 fresh spores. Plants at all stages of development, from the time 

 when the head was still in the boot to the time when the grains were 

 half filled, were used for the inoculations. It was found that plants 

 inoculated from the time when the heads emerge froru the boot until 

 thev are in full bloom rusted far more than plants inoculated either 

 before or after this stage of development. Just why the wheat 

 should be very susceptible to a rust attack at this time requires fur- 

 ther study. There may be a particular physiological weakness due 

 to the rapid growth and abundant elaboration of starch at this period 

 and the susceptibility of the gram may be increased on that account. 

 Whatever may be the cause, the critical period for wheat with regard 

 to attacks of P. graminis is during the heading time, a period of about 

 10 days for any one locality. If for any reason this period is delayed 

 or lengthened, the number of uredospores falling on each plant is 

 very considerably increased, infections have a longer time hi which to 

 develop, and the danger of an epidemic is imminent. 



CLIMATOLOGICAL COXDITIOXS IX RELATIOX TO RUSTS IX 1903, 1904, 



AND 1905. 



To determine how closely the conditions favorable for rust epi- 

 demics have been approximated in years of severe rust, a study has 

 been made of the cliniatologieal conditions over the important wheat 

 States in the Mississippi Valley from the Gulf to Canada for the 

 years 1903, 1904, and 1905. Rusts were fairly abundant in 1903, 

 though not strikingly, so. In 1904 an epidemic occurred which was 

 particularly severe over Xorth and South Dakota, Minnesota, and 

 parts of Iowa, while in 1905 the rust, though not epidemical, was 

 present in great abundance, causing considerable damage in certain 

 localities, particularly in Xorth Dakota and South Dakota, 



Wheat heads out hi April in southern Texas: in May in northern 

 Texas and Oklahoma, Kansas, and Missouri; in June in Xebraska 

 and Iowa; and in July in South Dakota, Minnesota, and parts of Wis- 

 consin. These three months, then, include the critical period for the 

 several States, that is, the period when rust infection develops and 

 an epidemic, if it occurs, gets its initial impulse. The critical period 

 at any one place would normally not extend over 10 days or two 

 weeks. 



PRECIPITATION. 



Table YI summarizes the precipitation records for several periods 

 in 1903, 1904, and 1905 in the important wheat States mentioned 

 above. 



216 



