60 THE RUSTS OP GEAINS IN THE UNITED 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 

 fi-esh spores. Plants at all stages of development, from the time 

 when the head was still m the boot to the time when the grams were 

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

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

 they are in full l)loom 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 ])articular physiological weakness due 

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

 and the susceptibility of the grain may be mcreased 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 lengihened, the number of uredospores falling on each plant is 

 very considera])ly increased, infections have a longer time in which to 

 develop, and the danger of an epidemic is imminent. 



CLIMATOLOGICAL CONDITIONS IN RELATION TO RUSTS IN 1903, 1904, 



AND 1905. 



To determine how closely the conditions favorable for rust epi- 

 demics have been approximated in 3^ears of severe rust, a study has 

 been made of the climatological conditions over the important wheat 

 States in the Mississippi VaUey 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 North 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 North Dakota and South Dakota. 



Wheat heads out in April in southern Texas; m May m northern 

 Texas and 'Oklalioma, Kansas, and Missouri; in June m Nebraska 

 and Iowa; and in July in South Dakota, ]\Iinnesota, 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 i)lace would normally not extend over 10 days or two 

 weeks. 



PRECiriTATION. 



Table VI summarizes the precipitation records for several periods 

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

 above. 



216 



