()4 A N Nl'AL HKPOUTS OF DKPARTM KXT OF AOllUTLTl'ItE, 1940 
reseiitative Localities within cadi geographic area of the United 
States and Mexico and 3tudies made to determine the particular races 
of the fungus present. 
Stem rust caused only slight aggregate damage to any of the 
email-grain crops in VXY.). The red, or summer, stage of the fungus 
did not survive the winter in Texas as abundantly as in some years. 
In nori hern Mexico rusl persisted throughout the winter in occa- 
sional early-sown fields of wheat: however, there was less rust than 
usual in the spring in this area. Inoculum of southern origin was 
not so abundant as in some years, and the spread northward was 
much later than usual. Crops ripened over wide areas in eastern 
Kansas and Missouri about June 15, thus preventing further increase 
of inoculum in 1 1 1 i > section. In the spring-wheat area there was an 
increase in acreage of varieties of grain resistant to the more preva- 
lent races of stem rust, and hot dry weather caused premature 
ripening of crops in many localities. 
Numerous local epidemics of stem rust were traceable to barberries. 
These were particularly noticeable because of the absence of a huge 
amount of infection in grain fields some distance away from bushes. 
Examination of slides exposed in representative sections of the 
United States indicated that urediospores of stem rust were de- 
posited later than usual and that spore showers were comparatively 
li<xht. No stem rust spores were found on slides exposed during 
April and the first half of May and only small numbers appeared 
during the last half of May. The amount of infection reaching the 
State- from Nebraska northward, as indicated by spore counts, com- 
pared in quantity with that of 1936 rather than with certain years 
when rust has become epidemic over wide areas. 
The identification of physiologic races of stem rust in 1939 indi- 
cated a high degree of similarity in the population of races in 
northern Mexico and the United States. Most of the races found in 
northern Mexico in the spring were collected later in the season in 
the United States. Five races were most prevalent and widespread, 
constituting over 95 percent of all isolates. These were, in the order 
of their prevalence, races 56, 38, 17, 19, and 11. Race 5C ranked 
first in prevalence for the sixth consecutive year: however, it de- 
creased from 66 percent of all isolates in L938 to 59 percent in 1939. 
Although widely distributed, it occurred in greatest concentration 
in the West Central State-, including Iowa. Nebraska. South Dakota, 
North Dakota, and Minnesota. Race 5(5 was isolated from all samples 
of Ceres wheat collected in North Dakota. 
Tt is significant that the tritici. or wheat, variety of stem rust was 
more prevalent this year in barberry collections than either the rye 
or oat varieties. In addition, aeciaj material from barberries con- 
tinue, I to yield more physiologic races of rusl than did the grain 
collections. A different race of rust was found in approximately every 
fifth barberry collection, while a different race was found only in 
every fifty-second collection of rust from wheat. 
POPl LATION TRENDS IN PARASITIC STRAINS OK TUB WHEAT STEM R1 ST 
FUNGUS 
Ii is evident from results of the past year and those of previous 
years that there are decided population shifts in parasitic races 
of the wheal -inn rust fi M ilo IS, / HtCt h<ia <ji <im inis trith i. A sununarv 
